Separate Apple Clang from LLVM Clang (#17110)

* Separate Apple Clang from LLVM Clang

Apple Clang is a compiler of its own. All places
referring to "-apple" suffix have been updated.

* Hack to use a dash in 'apple-clang'

To be able to use autodoc from Sphinx we need
a valid Python name for the module that contains
Apple's Clang code.

* Updated packages to account for the existence of apple-clang

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

* Added unit test for XCode related functions

Co-authored-by: Gregory Becker <becker33@llnl.gov>
Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

80 files changed, 537 insertions, 351 deletions

diff --git a/etc/spack/defaults/darwin/packages.yaml b/etc/spack/defaults/darwin/packages.yaml
index 1150f6bab7..b2bcd560c6 100644
--- a/etc/spack/defaults/darwin/packages.yaml
+++ b/etc/spack/defaults/darwin/packages.yaml
@@ -15,7 +15,11 @@
 # -------------------------------------------------------------------------
 packages:
   all:
-    compiler: [clang, gcc, intel]
+    compiler:
+    - apple-clang
+    - clang
+    - gcc
+    - intel
     providers:
       elf: [libelf]
       unwind: [apple-libunwind]
diff --git a/lib/spack/docs/basic_usage.rst b/lib/spack/docs/basic_usage.rst
index 8acebeb0e6..93c5858d93 100644
--- a/lib/spack/docs/basic_usage.rst
+++ b/lib/spack/docs/basic_usage.rst
@@ -627,8 +627,8 @@ output metadata on specs and all dependencies as json:
        "target": "x86_64"
       },
       "compiler": {
-       "name": "clang",
-       "version": "10.0.0-apple"
+       "name": "apple-clang",
+       "version": "10.0.0"
       },
       "namespace": "builtin",
       "parameters": {
diff --git a/lib/spack/docs/getting_started.rst b/lib/spack/docs/getting_started.rst
index 1fbddbd4a9..226b1f0883 100644
--- a/lib/spack/docs/getting_started.rst
+++ b/lib/spack/docs/getting_started.rst
@@ -478,7 +478,7 @@ Fortran.
           cxx: /usr/bin/clang++
           f77: /path/to/bin/gfortran
           fc: /path/to/bin/gfortran
-        spec: clang@11.0.0-apple
+        spec: apple-clang@11.0.0
 
 
    If you used Spack to install GCC, you can get the installation prefix by
diff --git a/lib/spack/docs/packaging_guide.rst b/lib/spack/docs/packaging_guide.rst
index 533036852b..d3a888b1fc 100644
--- a/lib/spack/docs/packaging_guide.rst
+++ b/lib/spack/docs/packaging_guide.rst
@@ -1675,15 +1675,15 @@ can see the patches that would be applied to ``m4``::
 
   Concretized
   --------------------------------
-  m4@1.4.18%clang@9.0.0-apple patches=3877ab548f88597ab2327a2230ee048d2d07ace1062efe81fc92e91b7f39cd00,c0a408fbffb7255fcc75e26bd8edab116fc81d216bfd18b473668b7739a4158e,fc9b61654a3ba1a8d6cd78ce087e7c96366c290bc8d2c299f09828d793b853c8 +sigsegv arch=darwin-highsierra-x86_64
-      ^libsigsegv@2.11%clang@9.0.0-apple arch=darwin-highsierra-x86_64
+  m4@1.4.18%apple-clang@9.0.0 patches=3877ab548f88597ab2327a2230ee048d2d07ace1062efe81fc92e91b7f39cd00,c0a408fbffb7255fcc75e26bd8edab116fc81d216bfd18b473668b7739a4158e,fc9b61654a3ba1a8d6cd78ce087e7c96366c290bc8d2c299f09828d793b853c8 +sigsegv arch=darwin-highsierra-x86_64
+      ^libsigsegv@2.11%apple-clang@9.0.0 arch=darwin-highsierra-x86_64
 
 You can also see patches that have been applied to installed packages
 with ``spack find -v``::
 
   $ spack find -v m4
   ==> 1 installed package
-  -- darwin-highsierra-x86_64 / clang@9.0.0-apple -----------------
+  -- darwin-highsierra-x86_64 / apple-clang@9.0.0 -----------------
   m4@1.4.18 patches=3877ab548f88597ab2327a2230ee048d2d07ace1062efe81fc92e91b7f39cd00,c0a408fbffb7255fcc75e26bd8edab116fc81d216bfd18b473668b7739a4158e,fc9b61654a3ba1a8d6cd78ce087e7c96366c290bc8d2c299f09828d793b853c8 +sigsegv
 
 .. _cmd-spack-resource:
@@ -1713,7 +1713,7 @@ wonder where the extra boost patches are coming from::
 
   $ spack spec dealii ^boost@1.68.0 ^hdf5+fortran | grep '\^boost'
       ^boost@1.68.0
-          ^boost@1.68.0%clang@9.0.0-apple+atomic+chrono~clanglibcpp cxxstd=default +date_time~debug+exception+filesystem+graph~icu+iostreams+locale+log+math~mpi+multithreaded~numpy patches=2ab6c72d03dec6a4ae20220a9dfd5c8c572c5294252155b85c6874d97c323199,b37164268f34f7133cbc9a4066ae98fda08adf51e1172223f6a969909216870f ~pic+program_options~python+random+regex+serialization+shared+signals~singlethreaded+system~taggedlayout+test+thread+timer~versionedlayout+wave arch=darwin-highsierra-x86_64
+          ^boost@1.68.0%apple-clang@9.0.0+atomic+chrono~clanglibcpp cxxstd=default +date_time~debug+exception+filesystem+graph~icu+iostreams+locale+log+math~mpi+multithreaded~numpy patches=2ab6c72d03dec6a4ae20220a9dfd5c8c572c5294252155b85c6874d97c323199,b37164268f34f7133cbc9a4066ae98fda08adf51e1172223f6a969909216870f ~pic+program_options~python+random+regex+serialization+shared+signals~singlethreaded+system~taggedlayout+test+thread+timer~versionedlayout+wave arch=darwin-highsierra-x86_64
   $ spack resource show b37164268
   b37164268f34f7133cbc9a4066ae98fda08adf51e1172223f6a969909216870f
       path:       /home/spackuser/src/spack/var/spack/repos/builtin/packages/dealii/boost_1.68.0.patch
diff --git a/lib/spack/docs/repositories.rst b/lib/spack/docs/repositories.rst
index 6c2050a4be..4a2c163886 100644
--- a/lib/spack/docs/repositories.rst
+++ b/lib/spack/docs/repositories.rst
@@ -280,16 +280,16 @@ you install it, you can use ``spack spec -N``:
 
    Concretized
    --------------------------------
-   builtin.hdf5@1.10.0-patch1%clang@7.0.2-apple+cxx~debug+fortran+mpi+shared~szip~threadsafe arch=darwin-elcapitan-x86_64
-       ^builtin.openmpi@2.0.1%clang@7.0.2-apple~mxm~pmi~psm~psm2~slurm~sqlite3~thread_multiple~tm~verbs+vt arch=darwin-elcapitan-x86_64
-           ^builtin.hwloc@1.11.4%clang@7.0.2-apple arch=darwin-elcapitan-x86_64
-               ^builtin.libpciaccess@0.13.4%clang@7.0.2-apple arch=darwin-elcapitan-x86_64
-                   ^builtin.libtool@2.4.6%clang@7.0.2-apple arch=darwin-elcapitan-x86_64
-                       ^builtin.m4@1.4.17%clang@7.0.2-apple+sigsegv arch=darwin-elcapitan-x86_64
-                           ^builtin.libsigsegv@2.10%clang@7.0.2-apple arch=darwin-elcapitan-x86_64
-                   ^builtin.pkg-config@0.29.1%clang@7.0.2-apple+internal_glib arch=darwin-elcapitan-x86_64
-                   ^builtin.util-macros@1.19.0%clang@7.0.2-apple arch=darwin-elcapitan-x86_64
-       ^builtin.zlib@1.2.8%clang@7.0.2-apple+pic arch=darwin-elcapitan-x86_64
+   builtin.hdf5@1.10.0-patch1%apple-clang@7.0.2+cxx~debug+fortran+mpi+shared~szip~threadsafe arch=darwin-elcapitan-x86_64
+       ^builtin.openmpi@2.0.1%apple-clang@7.0.2~mxm~pmi~psm~psm2~slurm~sqlite3~thread_multiple~tm~verbs+vt arch=darwin-elcapitan-x86_64
+           ^builtin.hwloc@1.11.4%apple-clang@7.0.2 arch=darwin-elcapitan-x86_64
+               ^builtin.libpciaccess@0.13.4%apple-clang@7.0.2 arch=darwin-elcapitan-x86_64
+                   ^builtin.libtool@2.4.6%apple-clang@7.0.2 arch=darwin-elcapitan-x86_64
+                       ^builtin.m4@1.4.17%apple-clang@7.0.2+sigsegv arch=darwin-elcapitan-x86_64
+                           ^builtin.libsigsegv@2.10%apple-clang@7.0.2 arch=darwin-elcapitan-x86_64
+                   ^builtin.pkg-config@0.29.1%apple-clang@7.0.2+internal_glib arch=darwin-elcapitan-x86_64
+                   ^builtin.util-macros@1.19.0%apple-clang@7.0.2 arch=darwin-elcapitan-x86_64
+       ^builtin.zlib@1.2.8%apple-clang@7.0.2+pic arch=darwin-elcapitan-x86_64
 
 .. warning::
 
diff --git a/lib/spack/llnl/util/cpu/microarchitecture.py b/lib/spack/llnl/util/cpu/microarchitecture.py
index f507837f89..284bfebb7e 100644
--- a/lib/spack/llnl/util/cpu/microarchitecture.py
+++ b/lib/spack/llnl/util/cpu/microarchitecture.py
@@ -204,10 +204,22 @@ class Microarchitecture(object):
             compiler (str): name of the compiler to be used
             version (str): version of the compiler to be used
         """
-        # If we don't have information on compiler return an empty string
-        if compiler not in self.compilers:
+        # If we don't have information on compiler at all
+        # return an empty string
+        if compiler not in self.family.compilers:
             return ''
 
+        # If we have information but it stops before this
+        # microarchitecture, fall back to the best known target
+        if compiler not in self.compilers:
+            best_target = [
+                x for x in self.ancestors if compiler in x.compilers
+            ][0]
+            msg = ("'{0}' compiler is known to optimize up to the '{1}'"
+                   " microarchitecture in the '{2}' architecture family")
+            msg = msg.format(compiler, best_target, best_target.family)
+            raise UnsupportedMicroarchitecture(msg)
+
         # If we have information on this compiler we need to check the
         # version being used
         compiler_info = self.compilers[compiler]
@@ -219,15 +231,10 @@ class Microarchitecture(object):
         def satisfies_constraint(entry, version):
             min_version, max_version = entry['versions'].split(':')
 
-            # Check version suffixes
-            min_version, min_suffix = version_components(min_version)
-            max_version, max_suffix = version_components(max_version)
-            version, suffix = version_components(version)
-
-            # If the suffixes are not all equal there's no match
-            if ((suffix != min_suffix and min_version) or
-                (suffix != max_suffix and max_version)):
-                return False
+            # Extract numeric part of the version
+            min_version, _ = version_components(min_version)
+            max_version, _ = version_components(max_version)
+            version, _ = version_components(version)
 
             # Assume compiler versions fit into semver
             tuplify = lambda x: tuple(int(y) for y in x.split('.'))
diff --git a/lib/spack/llnl/util/cpu/microarchitectures.json b/lib/spack/llnl/util/cpu/microarchitectures.json
index 51411d4a24..e94146bf85 100644
--- a/lib/spack/llnl/util/cpu/microarchitectures.json
+++ b/lib/spack/llnl/util/cpu/microarchitectures.json
@@ -61,12 +61,14 @@
             "flags": "-march={name} -mtune={name}"
           }
         ],
-        "clang": [
+        "apple-clang": [
           {
-            "versions": "0.0.0-apple:",
+            "versions": ":",
             "name": "x86-64",
             "flags": "-march={name}"
-          },
+          }
+        ],
+        "clang": [
           {
             "versions": ":",
             "name": "x86-64",
diff --git a/lib/spack/spack/ci.py b/lib/spack/spack/ci.py
index ce74abf29c..230f25ccee 100644
--- a/lib/spack/spack/ci.py
+++ b/lib/spack/spack/ci.py
@@ -345,18 +345,18 @@ def compute_spec_deps(spec_list):
            ],
            "specs": [
                {
-                 "root_spec": "readline@7.0%clang@9.1.0-apple arch=darwin-...",
-                 "spec": "readline@7.0%clang@9.1.0-apple arch=darwin-highs...",
+                 "root_spec": "readline@7.0%apple-clang@9.1.0 arch=darwin-...",
+                 "spec": "readline@7.0%apple-clang@9.1.0 arch=darwin-highs...",
                  "label": "readline/ip6aiun"
                },
                {
-                 "root_spec": "readline@7.0%clang@9.1.0-apple arch=darwin-...",
-                 "spec": "ncurses@6.1%clang@9.1.0-apple arch=darwin-highsi...",
+                 "root_spec": "readline@7.0%apple-clang@9.1.0 arch=darwin-...",
+                 "spec": "ncurses@6.1%apple-clang@9.1.0 arch=darwin-highsi...",
                  "label": "ncurses/y43rifz"
                },
                {
-                 "root_spec": "readline@7.0%clang@9.1.0-apple arch=darwin-...",
-                 "spec": "pkgconf@1.5.4%clang@9.1.0-apple arch=darwin-high...",
+                 "root_spec": "readline@7.0%apple-clang@9.1.0 arch=darwin-...",
+                 "spec": "pkgconf@1.5.4%apple-clang@9.1.0 arch=darwin-high...",
                  "label": "pkgconf/eg355zb"
                }
            ]
diff --git a/lib/spack/spack/compilers/__init__.py b/lib/spack/spack/compilers/__init__.py
index 771459f7f3..dfa750cc4d 100644
--- a/lib/spack/spack/compilers/__init__.py
+++ b/lib/spack/spack/compilers/__init__.py
@@ -245,7 +245,9 @@ def supported_compilers():
        See available_compilers() to get a list of all the available
        versions of supported compilers.
     """
-    return sorted(name for name in
+    # Hack to be able to call the compiler `apple-clang` while still
+    # using a valid python name for the module
+    return sorted(name if name != 'apple_clang' else 'apple-clang' for name in
                   llnl.util.lang.list_modules(spack.paths.compilers_path))
 
 
@@ -469,7 +471,13 @@ def class_for_compiler_name(compiler_name):
     """Given a compiler module name, get the corresponding Compiler class."""
     assert(supported(compiler_name))
 
-    file_path = os.path.join(spack.paths.compilers_path, compiler_name + ".py")
+    # Hack to be able to call the compiler `apple-clang` while still
+    # using a valid python name for the module
+    module_name = compiler_name
+    if compiler_name == 'apple-clang':
+        module_name = compiler_name.replace('-', '_')
+
+    file_path = os.path.join(spack.paths.compilers_path, module_name + ".py")
     compiler_mod = simp.load_source(_imported_compilers_module, file_path)
     cls = getattr(compiler_mod, mod_to_class(compiler_name))
 
@@ -662,7 +670,7 @@ def make_compiler_list(detected_versions):
         operating_system, compiler_name, version = cmp_id
         compiler_cls = spack.compilers.class_for_compiler_name(compiler_name)
         spec = spack.spec.CompilerSpec(compiler_cls.name, version)
-        paths = [paths.get(l, None) for l in ('cc', 'cxx', 'f77', 'fc')]
+        paths = [paths.get(x, None) for x in ('cc', 'cxx', 'f77', 'fc')]
         target = cpu.host()
         compiler = compiler_cls(
             spec, operating_system, str(target.family), paths
@@ -716,6 +724,8 @@ def is_mixed_toolchain(compiler):
             toolchains.add(compiler_cls.__name__)
 
     if len(toolchains) > 1:
+        if toolchains == set(['Clang', 'AppleClang']):
+            return False
         tty.debug("[TOOLCHAINS] {0}".format(toolchains))
         return True
 
diff --git a/lib/spack/spack/compilers/apple_clang.py b/lib/spack/spack/compilers/apple_clang.py
new file mode 100644
index 0000000000..8ef58550ea
--- /dev/null
+++ b/lib/spack/spack/compilers/apple_clang.py
@@ -0,0 +1,165 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+import os.path
+import re
+import shutil
+
+import llnl.util.tty as tty
+import llnl.util.lang
+import spack.compiler
+import spack.compilers.clang
+import spack.util.executable
+import spack.version
+
+
+class AppleClang(spack.compilers.clang.Clang):
+    openmp_flag = "-Xpreprocessor -fopenmp"
+
+    @classmethod
+    @llnl.util.lang.memoized
+    def extract_version_from_output(cls, output):
+        ver = 'unknown'
+        match = re.search(
+            # Apple's LLVM compiler has its own versions, so suffix them.
+            r'^Apple (?:LLVM|clang) version ([^ )]+)', output
+        )
+        if match:
+            ver = match.group(match.lastindex)
+        return ver
+
+    @property
+    def cxx11_flag(self):
+        # Adapted from CMake's AppleClang-CXX rules
+        # Spack's AppleClang detection only valid from Xcode >= 4.6
+        if self.version < spack.version.ver('4.0.0'):
+            raise spack.compiler.UnsupportedCompilerFlag(
+                self, "the C++11 standard", "cxx11_flag", "Xcode < 4.0.0"
+            )
+        return "-std=c++11"
+
+    @property
+    def cxx14_flag(self):
+        # Adapted from CMake's rules for AppleClang
+        if self.version < spack.version.ver('5.1.0'):
+            raise spack.compiler.UnsupportedCompilerFlag(
+                self, "the C++14 standard", "cxx14_flag", "Xcode < 5.1.0"
+            )
+        elif self.version < spack.version.ver('6.1.0'):
+            return "-std=c++1y"
+
+        return "-std=c++14"
+
+    @property
+    def cxx17_flag(self):
+        # Adapted from CMake's rules for AppleClang
+        if self.version < spack.version.ver('6.1.0'):
+            raise spack.compiler.UnsupportedCompilerFlag(
+                self, "the C++17 standard", "cxx17_flag", "Xcode < 6.1.0"
+            )
+        return "-std=c++1z"
+
+    def setup_custom_environment(self, pkg, env):
+        """Set the DEVELOPER_DIR environment for the Xcode toolchain.
+
+        On macOS, not all buildsystems support querying CC and CXX for the
+        compilers to use and instead query the Xcode toolchain for what
+        compiler to run. This side-steps the spack wrappers. In order to inject
+        spack into this setup, we need to copy (a subset of) Xcode.app and
+        replace the compiler executables with symlinks to the spack wrapper.
+        Currently, the stage is used to store the Xcode.app copies. We then set
+        the 'DEVELOPER_DIR' environment variables to cause the xcrun and
+        related tools to use this Xcode.app.
+        """
+        super(AppleClang, self).setup_custom_environment(pkg, env)
+
+        if not pkg.use_xcode:
+            # if we do it for all packages, we get into big troubles with MPI:
+            # filter_compilers(self) will use mockup XCode compilers on macOS
+            # with Clang. Those point to Spack's compiler wrappers and
+            # consequently render MPI non-functional outside of Spack.
+            return
+
+        # Use special XCode versions of compiler wrappers when using XCode
+        # Overwrites build_environment's setting of SPACK_CC and SPACK_CXX
+        xcrun = spack.util.executable.Executable('xcrun')
+        xcode_clang = xcrun('-f', 'clang', output=str).strip()
+        xcode_clangpp = xcrun('-f', 'clang++', output=str).strip()
+        env.set('SPACK_CC', xcode_clang, force=True)
+        env.set('SPACK_CXX', xcode_clangpp, force=True)
+
+        xcode_select = spack.util.executable.Executable('xcode-select')
+
+        # Get the path of the active developer directory
+        real_root = xcode_select('--print-path', output=str).strip()
+
+        # The path name can be used to determine whether the full Xcode suite
+        # or just the command-line tools are installed
+        if real_root.endswith('Developer'):
+            # The full Xcode suite is installed
+            pass
+        else:
+            if real_root.endswith('CommandLineTools'):
+                # Only the command-line tools are installed
+                msg  = 'It appears that you have the Xcode command-line tools '
+                msg += 'but not the full Xcode suite installed.\n'
+
+            else:
+                # Xcode is not installed
+                msg  = 'It appears that you do not have Xcode installed.\n'
+
+            msg += 'In order to use Spack to build the requested application, '
+            msg += 'you need the full Xcode suite. It can be installed '
+            msg += 'through the App Store. Make sure you launch the '
+            msg += 'application and accept the license agreement.\n'
+
+            raise OSError(msg)
+
+        real_root = os.path.dirname(os.path.dirname(real_root))
+        developer_root = os.path.join(spack.stage.get_stage_root(),
+                                      'xcode-select',
+                                      self.name,
+                                      str(self.version))
+        xcode_link = os.path.join(developer_root, 'Xcode.app')
+
+        if not os.path.exists(developer_root):
+            tty.warn('Copying Xcode from %s to %s in order to add spack '
+                     'wrappers to it. Please do not interrupt.'
+                     % (real_root, developer_root))
+
+            # We need to make a new Xcode.app instance, but with symlinks to
+            # the spack wrappers for the compilers it ships. This is necessary
+            # because some projects insist on just asking xcrun and related
+            # tools where the compiler runs. These tools are very hard to trick
+            # as they do realpath and end up ignoring the symlinks in a
+            # "softer" tree of nothing but symlinks in the right places.
+            shutil.copytree(
+                real_root, developer_root, symlinks=True,
+                ignore=shutil.ignore_patterns(
+                    'AppleTV*.platform', 'Watch*.platform', 'iPhone*.platform',
+                    'Documentation', 'swift*'
+                ))
+
+            real_dirs = [
+                'Toolchains/XcodeDefault.xctoolchain/usr/bin',
+                'usr/bin',
+            ]
+
+            bins = ['c++', 'c89', 'c99', 'cc', 'clang', 'clang++', 'cpp']
+
+            for real_dir in real_dirs:
+                dev_dir = os.path.join(developer_root,
+                                       'Contents',
+                                       'Developer',
+                                       real_dir)
+                for fname in os.listdir(dev_dir):
+                    if fname in bins:
+                        os.unlink(os.path.join(dev_dir, fname))
+                        os.symlink(
+                            os.path.join(spack.paths.build_env_path, 'cc'),
+                            os.path.join(dev_dir, fname))
+
+            os.symlink(developer_root, xcode_link)
+
+        env.set('DEVELOPER_DIR', xcode_link)
diff --git a/lib/spack/spack/compilers/clang.py b/lib/spack/spack/compilers/clang.py
index 846c3609dc..f158ff3276 100644
--- a/lib/spack/spack/compilers/clang.py
+++ b/lib/spack/spack/compilers/clang.py
@@ -4,17 +4,11 @@
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
 import re
-import os
 import sys
-from shutil import copytree, ignore_patterns
 
 import llnl.util.lang
-import llnl.util.tty as tty
 
-import spack.paths
-import spack.stage
 from spack.compiler import Compiler, UnsupportedCompilerFlag
-from spack.util.executable import Executable
 from spack.version import ver
 
 
@@ -89,87 +83,40 @@ class Clang(Compiler):
         return link_paths
 
     @property
-    def is_apple(self):
-        ver_string = str(self.version)
-        return ver_string.endswith('-apple')
-
-    @property
     def verbose_flag(self):
         return "-v"
 
-    @property
-    def openmp_flag(self):
-        if self.is_apple:
-            return "-Xpreprocessor -fopenmp"
-        else:
-            return "-fopenmp"
+    openmp_flag = "-fopenmp"
 
     @property
     def cxx11_flag(self):
-        if self.is_apple:
-            # Adapted from CMake's AppleClang-CXX rules
-            # Spack's AppleClang detection only valid from Xcode >= 4.6
-            if self.version < ver('4.0.0'):
-                raise UnsupportedCompilerFlag(self,
-                                              "the C++11 standard",
-                                              "cxx11_flag",
-                                              "Xcode < 4.0.0")
-            else:
-                return "-std=c++11"
-        else:
-            if self.version < ver('3.3'):
-                raise UnsupportedCompilerFlag(self,
-                                              "the C++11 standard",
-                                              "cxx11_flag",
-                                              "< 3.3")
-            else:
-                return "-std=c++11"
+        if self.version < ver('3.3'):
+            raise UnsupportedCompilerFlag(
+                self, "the C++11 standard", "cxx11_flag", "< 3.3"
+            )
+        return "-std=c++11"
 
     @property
     def cxx14_flag(self):
-        if self.is_apple:
-            # Adapted from CMake's rules for AppleClang
-            if self.version < ver('5.1.0'):
-                raise UnsupportedCompilerFlag(self,
-                                              "the C++14 standard",
-                                              "cxx14_flag",
-                                              "Xcode < 5.1.0")
-            elif self.version < ver('6.1.0'):
-                return "-std=c++1y"
-            else:
-                return "-std=c++14"
-        else:
-            if self.version < ver('3.4'):
-                raise UnsupportedCompilerFlag(self,
-                                              "the C++14 standard",
-                                              "cxx14_flag",
-                                              "< 3.5")
-            elif self.version < ver('3.5'):
-                return "-std=c++1y"
-            else:
-                return "-std=c++14"
+        if self.version < ver('3.4'):
+            raise UnsupportedCompilerFlag(
+                self, "the C++14 standard", "cxx14_flag", "< 3.5"
+            )
+        elif self.version < ver('3.5'):
+            return "-std=c++1y"
+
+        return "-std=c++14"
 
     @property
     def cxx17_flag(self):
-        if self.is_apple:
-            # Adapted from CMake's rules for AppleClang
-            if self.version < ver('6.1.0'):
-                raise UnsupportedCompilerFlag(self,
-                                              "the C++17 standard",
-                                              "cxx17_flag",
-                                              "Xcode < 6.1.0")
-            else:
-                return "-std=c++1z"
-        else:
-            if self.version < ver('3.5'):
-                raise UnsupportedCompilerFlag(self,
-                                              "the C++17 standard",
-                                              "cxx17_flag",
-                                              "< 3.5")
-            elif self.version < ver('5.0'):
-                return "-std=c++1z"
-            else:
-                return "-std=c++17"
+        if self.version < ver('3.5'):
+            raise UnsupportedCompilerFlag(
+                self, "the C++17 standard", "cxx17_flag", "< 3.5"
+            )
+        elif self.version < ver('5.0'):
+            return "-std=c++1z"
+
+        return "-std=c++17"
 
     @property
     def c99_flag(self):
@@ -207,9 +154,10 @@ class Clang(Compiler):
     @llnl.util.lang.memoized
     def extract_version_from_output(cls, output):
         ver = 'unknown'
+        if 'Apple' in output:
+            return ver
+
         match = re.search(
-            # Apple's LLVM compiler has its own versions, so suffix them.
-            r'^Apple (?:LLVM|clang) version ([^ )]+)|'
             # Normal clang compiler versions are left as-is
             r'clang version ([^ )]+)-svn[~.\w\d-]*|'
             # Don't include hyphenated patch numbers in the version
@@ -219,8 +167,7 @@ class Clang(Compiler):
             output
         )
         if match:
-            suffix = '-apple' if match.lastindex == 1 else ''
-            ver = match.group(match.lastindex) + suffix
+            ver = match.group(match.lastindex)
         return ver
 
     @classmethod
@@ -235,107 +182,3 @@ class Clang(Compiler):
     @classmethod
     def f77_version(cls, f77):
         return cls.fc_version(f77)
-
-    def setup_custom_environment(self, pkg, env):
-        """Set the DEVELOPER_DIR environment for the Xcode toolchain.
-
-        On macOS, not all buildsystems support querying CC and CXX for the
-        compilers to use and instead query the Xcode toolchain for what
-        compiler to run. This side-steps the spack wrappers. In order to inject
-        spack into this setup, we need to copy (a subset of) Xcode.app and
-        replace the compiler executables with symlinks to the spack wrapper.
-        Currently, the stage is used to store the Xcode.app copies. We then set
-        the 'DEVELOPER_DIR' environment variables to cause the xcrun and
-        related tools to use this Xcode.app.
-        """
-        super(Clang, self).setup_custom_environment(pkg, env)
-
-        if not self.is_apple or not pkg.use_xcode:
-            # if we do it for all packages, we get into big troubles with MPI:
-            # filter_compilers(self) will use mockup XCode compilers on macOS
-            # with Clang. Those point to Spack's compiler wrappers and
-            # consequently render MPI non-functional outside of Spack.
-            return
-
-        # Use special XCode versions of compiler wrappers when using XCode
-        # Overwrites build_environment's setting of SPACK_CC and SPACK_CXX
-        xcrun = Executable('xcrun')
-        xcode_clang = xcrun('-f', 'clang', output=str).strip()
-        xcode_clangpp = xcrun('-f', 'clang++', output=str).strip()
-        env.set('SPACK_CC', xcode_clang, force=True)
-        env.set('SPACK_CXX', xcode_clangpp, force=True)
-
-        xcode_select = Executable('xcode-select')
-
-        # Get the path of the active developer directory
-        real_root = xcode_select('--print-path', output=str).strip()
-
-        # The path name can be used to determine whether the full Xcode suite
-        # or just the command-line tools are installed
-        if real_root.endswith('Developer'):
-            # The full Xcode suite is installed
-            pass
-        else:
-            if real_root.endswith('CommandLineTools'):
-                # Only the command-line tools are installed
-                msg  = 'It appears that you have the Xcode command-line tools '
-                msg += 'but not the full Xcode suite installed.\n'
-
-            else:
-                # Xcode is not installed
-                msg  = 'It appears that you do not have Xcode installed.\n'
-
-            msg += 'In order to use Spack to build the requested application, '
-            msg += 'you need the full Xcode suite. It can be installed '
-            msg += 'through the App Store. Make sure you launch the '
-            msg += 'application and accept the license agreement.\n'
-
-            raise OSError(msg)
-
-        real_root = os.path.dirname(os.path.dirname(real_root))
-        developer_root = os.path.join(spack.stage.get_stage_root(),
-                                      'xcode-select',
-                                      self.name,
-                                      str(self.version))
-        xcode_link = os.path.join(developer_root, 'Xcode.app')
-
-        if not os.path.exists(developer_root):
-            tty.warn('Copying Xcode from %s to %s in order to add spack '
-                     'wrappers to it. Please do not interrupt.'
-                     % (real_root, developer_root))
-
-            # We need to make a new Xcode.app instance, but with symlinks to
-            # the spack wrappers for the compilers it ships. This is necessary
-            # because some projects insist on just asking xcrun and related
-            # tools where the compiler runs. These tools are very hard to trick
-            # as they do realpath and end up ignoring the symlinks in a
-            # "softer" tree of nothing but symlinks in the right places.
-            copytree(real_root, developer_root, symlinks=True,
-                     ignore=ignore_patterns('AppleTV*.platform',
-                                            'Watch*.platform',
-                                            'iPhone*.platform',
-                                            'Documentation',
-                                            'swift*'))
-
-            real_dirs = [
-                'Toolchains/XcodeDefault.xctoolchain/usr/bin',
-                'usr/bin',
-            ]
-
-            bins = ['c++', 'c89', 'c99', 'cc', 'clang', 'clang++', 'cpp']
-
-            for real_dir in real_dirs:
-                dev_dir = os.path.join(developer_root,
-                                       'Contents',
-                                       'Developer',
-                                       real_dir)
-                for fname in os.listdir(dev_dir):
-                    if fname in bins:
-                        os.unlink(os.path.join(dev_dir, fname))
-                        os.symlink(
-                            os.path.join(spack.paths.build_env_path, 'cc'),
-                            os.path.join(dev_dir, fname))
-
-            os.symlink(developer_root, xcode_link)
-
-        env.set('DEVELOPER_DIR', xcode_link)
diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index b09d72003e..b23142e101 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -1448,8 +1448,8 @@ class Spec(object):
                         'target': 'x86_64',
                     },
                     'compiler': {
-                        'name': 'clang',
-                        'version': '10.0.0-apple',
+                        'name': 'apple-clang',
+                        'version': '10.0.0',
                     },
                     'namespace': 'builtin',
                     'parameters': {
@@ -1554,8 +1554,8 @@ class Spec(object):
                                 'target': 'x86_64',
                             },
                             'compiler': {
-                                'name': 'clang',
-                                'version': '10.0.0-apple',
+                                'name': 'apple-clang',
+                                'version': '10.0.0',
                             },
                             'namespace': 'builtin',
                             'parameters': {
diff --git a/lib/spack/spack/test/architecture.py b/lib/spack/spack/test/architecture.py
index 48cec134d2..80d3fc72c0 100644
--- a/lib/spack/spack/test/architecture.py
+++ b/lib/spack/spack/test/architecture.py
@@ -176,8 +176,8 @@ def test_arch_spec_container_semantic(item, architecture_str):
     # Check mixed toolchains
     ('clang@8.0.0', 'broadwell', ''),
     ('clang@3.5', 'x86_64', '-march=x86-64 -mtune=generic'),
-    # Check clang compilers with 'apple' suffix
-    ('clang@9.1.0-apple', 'x86_64', '-march=x86-64')
+    # Check Apple's Clang compilers
+    ('apple-clang@9.1.0', 'x86_64', '-march=x86-64')
 ])
 @pytest.mark.filterwarnings("ignore:microarchitecture specific")
 def test_optimization_flags(
@@ -200,7 +200,7 @@ def test_optimization_flags(
      '-march=icelake-client -mtune=icelake-client'),
     # Check that the special case for Apple's clang is treated correctly
     # i.e. it won't try to detect the version again
-    (spack.spec.CompilerSpec('clang@9.1.0-apple'), None, 'x86_64',
+    (spack.spec.CompilerSpec('apple-clang@9.1.0'), None, 'x86_64',
      '-march=x86-64'),
 ])
 def test_optimization_flags_with_custom_versions(
diff --git a/lib/spack/spack/test/compilers/basics.py b/lib/spack/spack/test/compilers/basics.py
index 32f298ae2b..825975b21a 100644
--- a/lib/spack/spack/test/compilers/basics.py
+++ b/lib/spack/spack/test/compilers/basics.py
@@ -7,6 +7,7 @@ import pytest
 
 import sys
 import os
+import shutil
 
 from copy import copy
 from six import iteritems
@@ -16,6 +17,8 @@ import llnl.util.filesystem as fs
 import spack.spec
 import spack.compiler
 import spack.compilers as compilers
+import spack.spec
+import spack.util.environment
 
 from spack.compiler import Compiler
 from spack.util.executable import ProcessError
@@ -130,7 +133,7 @@ def test_compiler_flags_from_config_are_grouped():
 
 # Utility function to test most flags.
 default_compiler_entry = {
-    'spec': 'clang@2.0.0-apple',
+    'spec': 'apple-clang@2.0.0',
     'operating_system': 'foo-os',
     'paths': {
         'cc': 'cc-path',
@@ -370,26 +373,27 @@ def test_cce_flags():
                         'cce@1.0')
 
 
-def test_clang_flags():
-    # Apple Clang.
+def test_apple_clang_flags():
     supported_flag_test(
-        "openmp_flag", "-Xpreprocessor -fopenmp", "clang@2.0.0-apple")
-    unsupported_flag_test("cxx11_flag", "clang@2.0.0-apple")
-    supported_flag_test("cxx11_flag", "-std=c++11", "clang@4.0.0-apple")
-    unsupported_flag_test("cxx14_flag", "clang@5.0.0-apple")
-    supported_flag_test("cxx14_flag", "-std=c++1y", "clang@5.1.0-apple")
-    supported_flag_test("cxx14_flag", "-std=c++14", "clang@6.1.0-apple")
-    unsupported_flag_test("cxx17_flag", "clang@6.0.0-apple")
-    supported_flag_test("cxx17_flag", "-std=c++1z", "clang@6.1.0-apple")
-    supported_flag_test("c99_flag", "-std=c99", "clang@6.1.0-apple")
-    unsupported_flag_test("c11_flag", "clang@6.0.0-apple")
-    supported_flag_test("c11_flag", "-std=c11", "clang@6.1.0-apple")
-    supported_flag_test("cc_pic_flag",  "-fPIC", "clang@2.0.0-apple")
-    supported_flag_test("cxx_pic_flag", "-fPIC", "clang@2.0.0-apple")
-    supported_flag_test("f77_pic_flag", "-fPIC", "clang@2.0.0-apple")
-    supported_flag_test("fc_pic_flag",  "-fPIC", "clang@2.0.0-apple")
-
-    # non-Apple Clang.
+        "openmp_flag", "-Xpreprocessor -fopenmp", "apple-clang@2.0.0"
+    )
+    unsupported_flag_test("cxx11_flag", "apple-clang@2.0.0")
+    supported_flag_test("cxx11_flag", "-std=c++11", "apple-clang@4.0.0")
+    unsupported_flag_test("cxx14_flag", "apple-clang@5.0.0")
+    supported_flag_test("cxx14_flag", "-std=c++1y", "apple-clang@5.1.0")
+    supported_flag_test("cxx14_flag", "-std=c++14", "apple-clang@6.1.0")
+    unsupported_flag_test("cxx17_flag", "apple-clang@6.0.0")
+    supported_flag_test("cxx17_flag", "-std=c++1z", "apple-clang@6.1.0")
+    supported_flag_test("c99_flag", "-std=c99", "apple-clang@6.1.0")
+    unsupported_flag_test("c11_flag", "apple-clang@6.0.0")
+    supported_flag_test("c11_flag", "-std=c11", "apple-clang@6.1.0")
+    supported_flag_test("cc_pic_flag", "-fPIC", "apple-clang@2.0.0")
+    supported_flag_test("cxx_pic_flag", "-fPIC", "apple-clang@2.0.0")
+    supported_flag_test("f77_pic_flag", "-fPIC", "apple-clang@2.0.0")
+    supported_flag_test("fc_pic_flag", "-fPIC", "apple-clang@2.0.0")
+
+
+def test_clang_flags():
     supported_flag_test("version_argument", "--version", "clang@foo.bar")
     supported_flag_test("openmp_flag", "-fopenmp", "clang@3.3")
     unsupported_flag_test("cxx11_flag", "clang@3.2")
@@ -713,3 +717,101 @@ fi
     except ProcessError:
         # Confirm environment does not change after failed call
         assert 'SPACK_TEST_CMP_ON' not in os.environ
+
+
+def test_apple_clang_setup_environment(mock_executable, monkeypatch):
+    """Test a code path that is taken only if the package uses
+    Xcode on MacOS.
+    """
+    class MockPackage(object):
+        use_xcode = False
+
+    apple_clang_cls = spack.compilers.class_for_compiler_name('apple-clang')
+    compiler = apple_clang_cls(
+        spack.spec.CompilerSpec('apple-clang@11.0.0'), 'catalina', 'x86_64', [
+            '/usr/bin/clang', '/usr/bin/clang++', None, None
+        ]
+    )
+    env = spack.util.environment.EnvironmentModifications()
+    # Check a package that doesn't use xcode and ensure we don't add changes
+    # to the environment
+    pkg = MockPackage()
+    compiler.setup_custom_environment(pkg, env)
+    assert not env
+
+    # Prepare mock executables to fake the Xcode environment
+    xcrun = mock_executable('xcrun', """
+if [[ "$2" == "clang" ]] ; then
+  echo "/Library/Developer/CommandLineTools/usr/bin/clang"
+fi
+if [[ "$2" == "clang++" ]] ; then
+  echo "/Library/Developer/CommandLineTools/usr/bin/clang++"
+fi
+""")
+    mock_executable('xcode-select', """
+echo "/Library/Developer"
+""")
+    bin_dir = os.path.dirname(xcrun)
+    monkeypatch.setenv('PATH', bin_dir, prepend=os.pathsep)
+
+    def noop(*args, **kwargs):
+        pass
+
+    real_listdir = os.listdir
+
+    def _listdir(path):
+        if not os.path.exists(path):
+            return []
+        return real_listdir(path)
+
+    # Set a few operations to noop
+    monkeypatch.setattr(shutil, 'copytree', noop)
+    monkeypatch.setattr(os, 'unlink', noop)
+    monkeypatch.setattr(os, 'symlink', noop)
+    monkeypatch.setattr(os, 'listdir', _listdir)
+
+    # Qt is so far the only package that uses this code path, change
+    # introduced in https://github.com/spack/spack/pull/1832
+    pkg.use_xcode = True
+    compiler.setup_custom_environment(pkg, env)
+    assert len(env) == 3
+    assert env.env_modifications[0].name == 'SPACK_CC'
+    assert env.env_modifications[1].name == 'SPACK_CXX'
+    assert env.env_modifications[2].name == 'DEVELOPER_DIR'
+
+
+@pytest.mark.parametrize('xcode_select_output', [
+    '', '/Library/Developer/CommandLineTools'
+])
+def test_xcode_not_available(
+        xcode_select_output, mock_executable, monkeypatch
+):
+    # Prepare mock executables to fake the Xcode environment
+    xcrun = mock_executable('xcrun', """
+    if [[ "$2" == "clang" ]] ; then
+      echo "/Library/Developer/CommandLineTools/usr/bin/clang"
+    fi
+    if [[ "$2" == "clang++" ]] ; then
+      echo "/Library/Developer/CommandLineTools/usr/bin/clang++"
+    fi
+    """)
+    mock_executable('xcode-select', """
+    echo "{0}"
+    """.format(xcode_select_output))
+    bin_dir = os.path.dirname(xcrun)
+    monkeypatch.setenv('PATH', bin_dir, prepend=os.pathsep)
+    # Prepare compiler
+    apple_clang_cls = spack.compilers.class_for_compiler_name('apple-clang')
+    compiler = apple_clang_cls(
+        spack.spec.CompilerSpec('apple-clang@11.0.0'), 'catalina', 'x86_64', [
+            '/usr/bin/clang', '/usr/bin/clang++', None, None
+        ]
+    )
+    env = spack.util.environment.EnvironmentModifications()
+
+    class MockPackage(object):
+        use_xcode = True
+
+    pkg = MockPackage()
+    with pytest.raises(OSError):
+        compiler.setup_custom_environment(pkg, env)
diff --git a/lib/spack/spack/test/compilers/detection.py b/lib/spack/spack/test/compilers/detection.py
index 90311ad2d3..b38cf89b5d 100644
--- a/lib/spack/spack/test/compilers/detection.py
+++ b/lib/spack/spack/test/compilers/detection.py
@@ -53,11 +53,22 @@ def test_cce_version_detection(version_str, expected_version):
      'Target: x86_64-apple-darwin18.7.0\n'
      'Thread model: posix\n'
      'InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin\n',  # noqa
-     '11.0.0-apple'),
+     '11.0.0'),
     ('Apple LLVM version 7.0.2 (clang-700.1.81)\n'
      'Target: x86_64-apple-darwin15.2.0\n'
-     'Thread model: posix\n', '7.0.2-apple'),
-    # Other platforms
+     'Thread model: posix\n', '7.0.2'),
+])
+def test_apple_clang_version_detection(
+        version_str, expected_version
+):
+    cls = spack.compilers.class_for_compiler_name('apple-clang')
+    version = cls.extract_version_from_output(version_str)
+    assert version == expected_version
+
+
+@pytest.mark.regression('10191')
+@pytest.mark.parametrize('version_str,expected_version', [
+    # LLVM Clang
     ('clang version 6.0.1-svn334776-1~exp1~20181018152737.116 (branches/release_60)\n'  # noqa
      'Target: x86_64-pc-linux-gnu\n'
      'Thread model: posix\n'
diff --git a/lib/spack/spack/test/concretize.py b/lib/spack/spack/test/concretize.py
index fd0e185168..cfe0748c4b 100644
--- a/lib/spack/spack/test/concretize.py
+++ b/lib/spack/spack/test/concretize.py
@@ -608,7 +608,7 @@ class TestConcretize(object):
         ('mpileaks%gcc@4.8', 'haswell'),
         ('mpileaks%gcc@5.3.0', 'broadwell'),
         # Apple's clang always falls back to x86-64 for now
-        ('mpileaks%clang@9.1.0-apple', 'x86_64')
+        ('mpileaks%apple-clang@9.1.0', 'x86_64')
     ])
     @pytest.mark.regression('13361')
     def test_adjusting_default_target_based_on_compiler(
diff --git a/lib/spack/spack/test/conftest.py b/lib/spack/spack/test/conftest.py
index 1efdd6ba95..9a292821f5 100644
--- a/lib/spack/spack/test/conftest.py
+++ b/lib/spack/spack/test/conftest.py
@@ -1145,7 +1145,7 @@ def mock_executable(tmpdir):
     import jinja2
 
     def _factory(name, output, subdir=('bin',)):
-        f = tmpdir.mkdir(*subdir).join(name)
+        f = tmpdir.ensure(*subdir, dir=True).join(name)
         t = jinja2.Template('#!/bin/bash\n{{ output }}\n')
         f.write(t.render(output=output))
         f.chmod(0o755)
diff --git a/lib/spack/spack/test/data/config/compilers.yaml b/lib/spack/spack/test/data/config/compilers.yaml
index 7aec138473..3a2db05e72 100644
--- a/lib/spack/spack/test/data/config/compilers.yaml
+++ b/lib/spack/spack/test/data/config/compilers.yaml
@@ -127,7 +127,7 @@ compilers:
       cxxflags: -O3
     modules: 'None'
 - compiler:
-    spec: clang@9.1.0-apple
+    spec: apple-clang@9.1.0
     operating_system: elcapitan
     paths:
       cc: /path/to/clang
diff --git a/var/spack/repos/builtin/packages/allpaths-lg/package.py b/var/spack/repos/builtin/packages/allpaths-lg/package.py
index 6ebca2eee6..b0bb49b3a1 100644
--- a/var/spack/repos/builtin/packages/allpaths-lg/package.py
+++ b/var/spack/repos/builtin/packages/allpaths-lg/package.py
@@ -18,6 +18,7 @@ class AllpathsLg(AutotoolsPackage):
     # compiles with gcc 4.7.0 to 4.9.4)
     conflicts('%gcc@:4.6.4,5.1.0:')
     conflicts('%cce')
+    conflicts('%apple-clang')
     conflicts('%clang')
     conflicts('%intel')
     conflicts('%nag')
diff --git a/var/spack/repos/builtin/packages/aluminum/package.py b/var/spack/repos/builtin/packages/aluminum/package.py
index 5edc4c3792..c5d6a45ed8 100644
--- a/var/spack/repos/builtin/packages/aluminum/package.py
+++ b/var/spack/repos/builtin/packages/aluminum/package.py
@@ -47,8 +47,8 @@ class Aluminum(CMakePackage):
             '-DALUMINUM_ENABLE_MPI_CUDA:BOOL=%s' % ('+mpi_cuda' in spec),
             '-DALUMINUM_ENABLE_NCCL:BOOL=%s' % ('+nccl' in spec)]
 
-        # Add support for OS X to find OpenMP
-        if (self.spec.satisfies('%clang platform=darwin')):
+        # Add support for OS X to find OpenMP (LLVM installed via brew)
+        if self.spec.satisfies('%clang platform=darwin'):
             clang = self.compiler.cc
             clang_bin = os.path.dirname(clang)
             clang_root = os.path.dirname(clang_bin)
diff --git a/var/spack/repos/builtin/packages/amber/package.py b/var/spack/repos/builtin/packages/amber/package.py
index d2ec13fb3e..481377047c 100644
--- a/var/spack/repos/builtin/packages/amber/package.py
+++ b/var/spack/repos/builtin/packages/amber/package.py
@@ -100,12 +100,10 @@ class Amber(Package, CudaPackage):
     depends_on('cuda@7.5.18', when='@:16+cuda')
 
     # conflicts
-    conflicts('+x11', when='platform=cray',
-              msg='x11 amber applications not available for cray')
-    conflicts('+openmp', when='%clang',
-              msg='openmp optimizations not available for the clang compiler')
-    conflicts('+openmp', when='%pgi',
-              msg='openmp optimizations not available for the pgi compiler')
+    conflicts('+x11', when='platform=cray', msg='x11 amber applications not available for cray')
+    conflicts('+openmp', when='%clang', msg='OpenMP optimizations not available for the clang compiler')
+    conflicts('+openmp', when='%apple-clang', msg='OpenMP optimizations not available for the Apple clang compiler')
+    conflicts('+openmp', when='%pgi', msg='OpenMP optimizations not available for the pgi compiler')
 
     def setup_build_environment(self, env):
         amber_src = self.stage.source_path
diff --git a/var/spack/repos/builtin/packages/amrex/package.py b/var/spack/repos/builtin/packages/amrex/package.py
index 32da808f1d..f6cb778f4b 100644
--- a/var/spack/repos/builtin/packages/amrex/package.py
+++ b/var/spack/repos/builtin/packages/amrex/package.py
@@ -65,6 +65,7 @@ class Amrex(CMakePackage):
     depends_on('cmake@3.5:',  type='build', when='@:18.10.99')
     depends_on('cmake@3.13:', type='build', when='@18.11:')
     depends_on('cmake@3.14:', type='build', when='@19.04:')
+    conflicts('%apple-clang')
     conflicts('%clang')
 
     def url_for_version(self, version):
diff --git a/var/spack/repos/builtin/packages/ape/package.py b/var/spack/repos/builtin/packages/ape/package.py
index 114185aaa4..bf9f895e68 100644
--- a/var/spack/repos/builtin/packages/ape/package.py
+++ b/var/spack/repos/builtin/packages/ape/package.py
@@ -29,11 +29,13 @@ class Ape(Package):
         # When preprocessor expands macros (i.e. CFLAGS) defined as quoted
         # strings the result may be > 132 chars and is terminated.
         # This will look to a compiler as an Unterminated character constant
-        # and produce Line truncated errors. To vercome this, add flags to
+        # and produce Line truncated errors. To overcome this, add flags to
         # let compiler know that the entire line is meaningful.
         # TODO: For the lack of better approach, assume that clang is mixed
-        # with GNU fortran.
-        if spec.satisfies('%clang') or spec.satisfies('%gcc'):
+        # TODO: with GNU fortran.
+        if (spec.satisfies('%apple-clang') or
+                spec.satisfies('%clang') or
+                spec.satisfies('%gcc')):
             args.extend([
                 'FCFLAGS=-O2 -ffree-line-length-none'
             ])
diff --git a/var/spack/repos/builtin/packages/atlas/package.py b/var/spack/repos/builtin/packages/atlas/package.py
index 4d56d0016d..2a355dfca7 100644
--- a/var/spack/repos/builtin/packages/atlas/package.py
+++ b/var/spack/repos/builtin/packages/atlas/package.py
@@ -102,7 +102,7 @@ class Atlas(Package):
 
         # Workaround for macOS Clang:
         # http://math-atlas.sourceforge.net/atlas_install/node66.html
-        if spec.satisfies('@3.10.3: %clang platform=darwin'):
+        if spec.satisfies('@3.10.3: %apple-clang'):
             options.append('--force-clang=' + spack_cc)
 
         # Lapack resource to provide full lapack build. Note that
diff --git a/var/spack/repos/builtin/packages/binutils/package.py b/var/spack/repos/builtin/packages/binutils/package.py
index 791606f564..96fdcc067b 100644
--- a/var/spack/repos/builtin/packages/binutils/package.py
+++ b/var/spack/repos/builtin/packages/binutils/package.py
@@ -120,9 +120,10 @@ class Binutils(AutotoolsPackage, GNUMirrorPackage):
     def flag_handler(self, name, flags):
         # To ignore the errors of narrowing conversions for
         # the Fujitsu compiler
-        if name == 'cxxflags'\
-           and (self.compiler.name == 'fj' or self.compiler.name == 'clang')\
-           and self.version <= ver('2.31.1'):
+        if name == 'cxxflags' and (
+            self.spec.satisfies('@:2.31.1') and
+            self.compiler.name in ('fj', 'clang', 'apple-clang')
+        ):
             flags.append('-Wno-narrowing')
         elif name == 'cflags':
             if self.spec.satisfies('@:2.34 %gcc@10:'):
diff --git a/var/spack/repos/builtin/packages/boost/package.py b/var/spack/repos/builtin/packages/boost/package.py
index 1e5bd00c2b..d0d8691c33 100644
--- a/var/spack/repos/builtin/packages/boost/package.py
+++ b/var/spack/repos/builtin/packages/boost/package.py
@@ -398,7 +398,9 @@ class Boost(Package):
         # and at least in clang 3.9 still fails to build
         #   http://www.boost.org/build/doc/html/bbv2/reference/precompiled_headers.html
         #   https://svn.boost.org/trac/boost/ticket/12496
-        if spec.satisfies('%clang') or spec.satisfies('%fj'):
+        if (spec.satisfies('%apple-clang') or
+                spec.satisfies('%clang') or
+                spec.satisfies('%fj')):
             options.extend(['pch=off'])
             if '+clanglibcpp' in spec:
                 cxxflags.append('-stdlib=libc++')
diff --git a/var/spack/repos/builtin/packages/bucky/package.py b/var/spack/repos/builtin/packages/bucky/package.py
index 859fabde34..41337e7e84 100644
--- a/var/spack/repos/builtin/packages/bucky/package.py
+++ b/var/spack/repos/builtin/packages/bucky/package.py
@@ -19,6 +19,7 @@ class Bucky(MakefilePackage):
 
     # Compilation requires gcc
     conflicts('%cce')
+    conflicts('%apple-clang')
     conflicts('%clang')
     conflicts('%intel')
     conflicts('%nag')
diff --git a/var/spack/repos/builtin/packages/cbench/package.py b/var/spack/repos/builtin/packages/cbench/package.py
index ceeec3b468..8a24a54d6e 100644
--- a/var/spack/repos/builtin/packages/cbench/package.py
+++ b/var/spack/repos/builtin/packages/cbench/package.py
@@ -26,6 +26,7 @@ class Cbench(MakefilePackage):
 
     # The following compilers are not supported by Cbench:
     conflicts('%cce')
+    conflicts('%apple-clang')
     conflicts('%clang')
     conflicts('%nag')
     conflicts('%xl')
diff --git a/var/spack/repos/builtin/packages/cp2k/package.py b/var/spack/repos/builtin/packages/cp2k/package.py
index 0a172d025f..1fba619ffc 100644
--- a/var/spack/repos/builtin/packages/cp2k/package.py
+++ b/var/spack/repos/builtin/packages/cp2k/package.py
@@ -141,6 +141,7 @@ class Cp2k(MakefilePackage, CudaPackage):
     depends_on('wannier90', when='@3.0+mpi', type='build')
 
     # CP2K needs compiler specific compilation flags, e.g. optflags
+    conflicts('%apple-clang')
     conflicts('%clang')
     conflicts('%nag')
 
diff --git a/var/spack/repos/builtin/packages/dealii/package.py b/var/spack/repos/builtin/packages/dealii/package.py
index 0f46c318f6..8d7bd2be81 100644
--- a/var/spack/repos/builtin/packages/dealii/package.py
+++ b/var/spack/repos/builtin/packages/dealii/package.py
@@ -301,7 +301,7 @@ class Dealii(CMakePackage, CudaPackage):
             cxx_flags_release.extend(['-O3'])
         elif spec.satisfies('%intel'):
             cxx_flags_release.extend(['-O3'])
-        elif spec.satisfies('%clang'):
+        elif spec.satisfies('%clang') or spec.satisfies('%apple-clang'):
             cxx_flags_release.extend(['-O3', '-ffp-contract=fast'])
 
         # Python bindings
diff --git a/var/spack/repos/builtin/packages/elfutils/package.py b/var/spack/repos/builtin/packages/elfutils/package.py
index 651caf3d76..cd6b961227 100644
--- a/var/spack/repos/builtin/packages/elfutils/package.py
+++ b/var/spack/repos/builtin/packages/elfutils/package.py
@@ -56,6 +56,7 @@ class Elfutils(AutotoolsPackage, SourcewarePackage):
     # in gcc, but not in clang. C code compiled with gcc is
     # binary-compatible with clang, so it should be possible to build
     # elfutils with gcc, and then link it to clang-built libraries.
+    conflicts('%apple-clang')
     conflicts('%clang')
 
     # Elfutils uses -Wall and we don't want to fail the build over a
diff --git a/var/spack/repos/builtin/packages/f18/package.py b/var/spack/repos/builtin/packages/f18/package.py
index 699cfde6c6..235210c142 100644
--- a/var/spack/repos/builtin/packages/f18/package.py
+++ b/var/spack/repos/builtin/packages/f18/package.py
@@ -30,6 +30,9 @@ class F18(CMakePackage):
 
     # Conflicts
     compiler_warning = 'F18 requires a compiler with support for C++17'
+    # See https://en.wikipedia.org/wiki/Xcode#Latest_versions for a
+    # conversion table from LLVM versions to Apple's Clang
+    conflicts('%apple-clang@:10.1', msg=compiler_warning)
     conflicts('%clang@:6', msg=compiler_warning)
     conflicts('%gcc@:7.1', msg=compiler_warning)
     conflicts('%intel', msg=compiler_warning)
diff --git a/var/spack/repos/builtin/packages/fftw/package.py b/var/spack/repos/builtin/packages/fftw/package.py
index 961c4cfc66..b644677fd1 100644
--- a/var/spack/repos/builtin/packages/fftw/package.py
+++ b/var/spack/repos/builtin/packages/fftw/package.py
@@ -57,6 +57,7 @@ class Fftw(AutotoolsPackage):
               msg='Long double precision is not supported in FFTW 2')
     conflicts('precision=quad', when='@2.1.5',
               msg='Quad precision is not supported in FFTW 2')
+    conflicts('+openmp', when='%apple-clang', msg="Apple's clang does not support OpenMP")
 
     provides('fftw-api@2', when='@2.1.5')
     provides('fftw-api@3', when='@3:')
@@ -124,11 +125,6 @@ class Fftw(AutotoolsPackage):
 
         # Variants that affect every precision
         if '+openmp' in spec:
-            # Note: Apple's Clang does not support OpenMP.
-            if spec.satisfies('%clang'):
-                ver = str(self.compiler.version)
-                if ver.endswith('-apple'):
-                    raise InstallError("Apple's clang does not support OpenMP")
             options.append('--enable-openmp')
             if spec.satisfies('@:2'):
                 # TODO: libtool strips CFLAGS, so 2.x libxfftw_threads
diff --git a/var/spack/repos/builtin/packages/fujitsu-mpi/package.py b/var/spack/repos/builtin/packages/fujitsu-mpi/package.py
index c0a882ec4c..fcf4dc8e94 100644
--- a/var/spack/repos/builtin/packages/fujitsu-mpi/package.py
+++ b/var/spack/repos/builtin/packages/fujitsu-mpi/package.py
@@ -13,6 +13,7 @@ class FujitsuMpi(Package):
 
     conflicts('%arm')
     conflicts('%cce')
+    conflicts('%apple-clang')
     conflicts('%clang')
     conflicts('%gcc')
     conflicts('%intel')
diff --git a/var/spack/repos/builtin/packages/gitconddb/package.py b/var/spack/repos/builtin/packages/gitconddb/package.py
index 9400ea549c..e2808e91c0 100644
--- a/var/spack/repos/builtin/packages/gitconddb/package.py
+++ b/var/spack/repos/builtin/packages/gitconddb/package.py
@@ -40,4 +40,5 @@ class Gitconddb(CMakePackage):
 
     # Known conflicts on C++17 compatibility (aggressive for now)
     conflicts('%gcc@:7.9.999', msg="GitCondDB requires GCC 8 or newer for C++17 support")
+    conflicts('%apple-clang', when="@:0.1.99", msg="No Darwin support for clang in older versions")
     conflicts('%clang platform=darwin', when="@:0.1.99", msg="No Darwin support for clang in older versions")
diff --git a/var/spack/repos/builtin/packages/graphviz/package.py b/var/spack/repos/builtin/packages/graphviz/package.py
index 4f8e772f19..34b2d8dd2e 100644
--- a/var/spack/repos/builtin/packages/graphviz/package.py
+++ b/var/spack/repos/builtin/packages/graphviz/package.py
@@ -137,6 +137,13 @@ class Graphviz(AutotoolsPackage):
         filter_file(r'-lstdc\+\+', '-lc++', 'configure.ac',
                     *(d + '/Makefile.am' for d in mkdirs))
 
+    @when('%apple-clang')
+    def patch(self):
+        # When using Clang, replace GCC's libstdc++ with LLVM's libc++
+        mkdirs = ['cmd/dot', 'cmd/edgepaint', 'cmd/mingle', 'plugin/gdiplus']
+        filter_file(r'-lstdc\+\+', '-lc++', 'configure.ac',
+                    *(d + '/Makefile.am' for d in mkdirs))
+
     def configure_args(self):
         spec = self.spec
         args = ['--disable-silent-rules']
diff --git a/var/spack/repos/builtin/packages/hpgmg/package.py b/var/spack/repos/builtin/packages/hpgmg/package.py
index e701d1c212..f5567ba2c9 100644
--- a/var/spack/repos/builtin/packages/hpgmg/package.py
+++ b/var/spack/repos/builtin/packages/hpgmg/package.py
@@ -59,13 +59,11 @@ class Hpgmg(Package):
             args.append('--no-fv')
         else:
             # Apple's Clang doesn't support OpenMP
-            if not (self.spec.satisfies('%clang') and self.compiler.is_apple):
+            if not self.spec.satisfies('%apple-clang'):
                 cflags.append(self.compiler.openmp_flag)
 
         if '+debug' in self.spec:
             cflags.append('-g')
-        elif any(map(self.spec.satisfies, ['%gcc', '%clang', '%intel'])):
-            cflags.append('-O3')
         else:
             cflags.append('-O3')
 
diff --git a/var/spack/repos/builtin/packages/hydrogen/package.py b/var/spack/repos/builtin/packages/hydrogen/package.py
index 449fe046dc..9d12235d72 100644
--- a/var/spack/repos/builtin/packages/hydrogen/package.py
+++ b/var/spack/repos/builtin/packages/hydrogen/package.py
@@ -131,8 +131,8 @@ class Hydrogen(CMakePackage):
             '-DHydrogen_ENABLE_HALF=%s' % ('+half' in spec),
         ]
 
-        # Add support for OS X to find OpenMP
-        if (self.spec.satisfies('%clang platform=darwin')):
+        # Add support for OS X to find OpenMP (LLVM installed via brew)
+        if self.spec.satisfies('%clang platform=darwin'):
             clang = self.compiler.cc
             clang_bin = os.path.dirname(clang)
             clang_root = os.path.dirname(clang_bin)
diff --git a/var/spack/repos/builtin/packages/intel-tbb/package.py b/var/spack/repos/builtin/packages/intel-tbb/package.py
index c470e8933b..4c652f6fce 100644
--- a/var/spack/repos/builtin/packages/intel-tbb/package.py
+++ b/var/spack/repos/builtin/packages/intel-tbb/package.py
@@ -69,6 +69,8 @@ class IntelTbb(Package):
     #
     #    See https://github.com/intel/tbb/pull/147 for details.
     #
+    conflicts('%apple-clang', when='@:2019.6',
+              msg='2019.7 or later required for clang')
     conflicts('%clang', when='@:2019.6',
               msg='2019.7 or later required for clang')
 
@@ -163,7 +165,7 @@ class IntelTbb(Package):
         #
         self.coerce_to_spack("build")
 
-        if spec.satisfies('%clang'):
+        if spec.satisfies('%clang') or spec.satisfies('%apple-clang'):
             tbb_compiler = "clang"
         elif spec.satisfies('%intel'):
             tbb_compiler = "icc"
diff --git a/var/spack/repos/builtin/packages/kahip/package.py b/var/spack/repos/builtin/packages/kahip/package.py
index 7d12d36bc2..e2ca1aeaa1 100644
--- a/var/spack/repos/builtin/packages/kahip/package.py
+++ b/var/spack/repos/builtin/packages/kahip/package.py
@@ -32,6 +32,7 @@ class Kahip(SConsPackage):
     depends_on('argtable')
     depends_on('mpi')  # Note: upstream package only tested on openmpi
 
+    conflicts('%apple-clang')
     conflicts('%clang')
 
     def patch(self):
diff --git a/var/spack/repos/builtin/packages/kentutils/package.py b/var/spack/repos/builtin/packages/kentutils/package.py
index 379d3486c0..f0b4ee8322 100644
--- a/var/spack/repos/builtin/packages/kentutils/package.py
+++ b/var/spack/repos/builtin/packages/kentutils/package.py
@@ -22,6 +22,7 @@ class Kentutils(MakefilePackage):
     depends_on('mariadb')
 
     conflicts('%cce')
+    conflicts('%apple-clang')
     conflicts('%clang')
     conflicts('%intel')
     conflicts('%nag')
diff --git a/var/spack/repos/builtin/packages/lbann/package.py b/var/spack/repos/builtin/packages/lbann/package.py
index 04b044610f..bc0da109ec 100644
--- a/var/spack/repos/builtin/packages/lbann/package.py
+++ b/var/spack/repos/builtin/packages/lbann/package.py
@@ -164,16 +164,16 @@ class Lbann(CMakePackage):
             # protobuf is included by py-protobuf+cpp
             '-DProtobuf_DIR={0}'.format(spec['protobuf'].prefix)])
 
-        if self.spec.satisfies('@:0.90') or self.spec.satisfies('@0.95:'):
+        if spec.satisfies('@:0.90') or spec.satisfies('@0.95:'):
             args.extend([
                 '-DHydrogen_DIR={0}/CMake/hydrogen'.format(
                     spec['hydrogen'].prefix)])
-        elif self.spec.satisfies('@0.94'):
+        elif spec.satisfies('@0.94'):
             args.extend([
                 '-DElemental_DIR={0}/CMake/elemental'.format(
                     spec['elemental'].prefix)])
 
-        if self.spec.satisfies('@0.94:0.98.2'):
+        if spec.satisfies('@0.94:0.98.2'):
             args.extend(['-DLBANN_WITH_NCCL:BOOL=%s' % ('+gpu +nccl' in spec)])
 
         if '+vtune' in spec:
@@ -188,8 +188,8 @@ class Lbann(CMakePackage):
                 '-DConduit_DIR={0}'.format(spec['conduit'].prefix)])
 
         # Add support for OpenMP
-        if (self.spec.satisfies('%clang')):
-            if (sys.platform == 'darwin'):
+        if spec.satisfies('%clang') or spec.satisfies('%apple-clang'):
+            if sys.platform == 'darwin':
                 clang = self.compiler.cc
                 clang_bin = os.path.dirname(clang)
                 clang_root = os.path.dirname(clang_bin)
@@ -210,7 +210,7 @@ class Lbann(CMakePackage):
             args.extend([
                 '-DcuDNN_DIR={0}'.format(
                     spec['cudnn'].prefix)])
-            if self.spec.satisfies('@0.94:0.98.2'):
+            if spec.satisfies('@0.94:0.98.2'):
                 args.extend(['-DCUB_DIR={0}'.format(
                     spec['cub'].prefix)])
                 if '+nccl' in spec:
diff --git a/var/spack/repos/builtin/packages/libceed/package.py b/var/spack/repos/builtin/packages/libceed/package.py
index 36d46dae0b..a7ae5b21f8 100644
--- a/var/spack/repos/builtin/packages/libceed/package.py
+++ b/var/spack/repos/builtin/packages/libceed/package.py
@@ -71,14 +71,14 @@ class Libceed(Package):
                 opt = '-O3 -g -ffp-contract=fast'
                 if compiler.version >= ver(4.9):
                     opt += ' -fopenmp-simd'
+            elif compiler.name == 'apple-clang':
+                opt = '-O3 -g -march=native -ffp-contract=fast'
+                if compiler.version >= ver(10):
+                    opt += ' -fopenmp-simd'
             elif compiler.name == 'clang':
                 opt = '-O3 -g -march=native -ffp-contract=fast'
-                if compiler.version.string.endswith('-apple'):
-                    if compiler.version >= ver(10):
-                        opt += ' -fopenmp-simd'
-                else:  # not apple clang
-                    if compiler.version >= ver(6):
-                        opt += ' -fopenmp-simd'
+                if compiler.version >= ver(6):
+                    opt += ' -fopenmp-simd'
             elif compiler.name in ['xl', 'xl_r']:
                 opt = '-O -g -qsimd=auto'
             else:
diff --git a/var/spack/repos/builtin/packages/libcerf/package.py b/var/spack/repos/builtin/packages/libcerf/package.py
index e37e7f6339..a5c00dabaf 100644
--- a/var/spack/repos/builtin/packages/libcerf/package.py
+++ b/var/spack/repos/builtin/packages/libcerf/package.py
@@ -23,7 +23,7 @@ class Libcerf(AutotoolsPackage, SourceforgePackage):
         options = []
         # Clang reports unused functions as errors, see
         # http://clang.debian.net/status.php?version=3.8.1&key=UNUSED_FUNCTION
-        if spec.satisfies('%clang'):
+        if spec.satisfies('%clang') or spec.satisfies('%apple-clang'):
             options.append('CFLAGS=-Wno-unused-function')
         # fujitsu compiler has a error about unused functions too.
         if spec.satisfies('%fj'):
diff --git a/var/spack/repos/builtin/packages/librsb/package.py b/var/spack/repos/builtin/packages/librsb/package.py
index 90cc5b67a8..2a292b9479 100644
--- a/var/spack/repos/builtin/packages/librsb/package.py
+++ b/var/spack/repos/builtin/packages/librsb/package.py
@@ -17,6 +17,7 @@ class Librsb(AutotoolsPackage):
     version('1.2.0.8',   '8bebd19a1866d80ade13eabfdd0f07ae7e8a485c0b975b5d15f531ac204d80cb')
 
     depends_on('zlib')
+    conflicts('%apple-clang')
     conflicts('%clang')
 
     def configure_args(self):
diff --git a/var/spack/repos/builtin/packages/likwid/package.py b/var/spack/repos/builtin/packages/likwid/package.py
index 8e89693ce1..a104b67003 100644
--- a/var/spack/repos/builtin/packages/likwid/package.py
+++ b/var/spack/repos/builtin/packages/likwid/package.py
@@ -60,9 +60,15 @@ class Likwid(Package):
                     *files)
 
     def install(self, spec, prefix):
-        supported_compilers = {'clang': 'CLANG', 'gcc': 'GCC', 'intel': 'ICC'}
+        supported_compilers = {
+            'apple-clang': 'CLANG',
+            'clang': 'CLANG',
+            'gcc': 'GCC',
+            'intel': 'ICC'
+        }
         if spec.target.family == 'aarch64':
-            supported_compilers = {'gcc': 'GCCARMv8', 'clang': 'ARMCLANG'}
+            supported_compilers = {
+                'gcc': 'GCCARMv8', 'clang': 'ARMCLANG', 'arm': 'ARMCLANG'}
         elif spec.target.family == 'ppc64' or spec.target.family == 'ppc64le':
             supported_compilers = {'gcc': 'GCCPOWER'}
         if self.compiler.name not in supported_compilers:
diff --git a/var/spack/repos/builtin/packages/maverick/package.py b/var/spack/repos/builtin/packages/maverick/package.py
index 6c560bb202..60909c551b 100644
--- a/var/spack/repos/builtin/packages/maverick/package.py
+++ b/var/spack/repos/builtin/packages/maverick/package.py
@@ -17,6 +17,7 @@ class Maverick(MakefilePackage):
 
     conflicts('%gcc@:6.0')
     conflicts('%cce')
+    conflicts('%apple-clang')
     conflicts('%clang')
     conflicts('%intel')
     conflicts('%nag')
diff --git a/var/spack/repos/builtin/packages/mpich/package.py b/var/spack/repos/builtin/packages/mpich/package.py
index c9c7886a08..d862f12522 100644
--- a/var/spack/repos/builtin/packages/mpich/package.py
+++ b/var/spack/repos/builtin/packages/mpich/package.py
@@ -79,6 +79,7 @@ spack package at this time.''',
     # see https://lists.mpich.org/pipermail/discuss/2016-May/004764.html
     # and https://lists.mpich.org/pipermail/discuss/2016-June/004768.html
     patch('mpich32_clang.patch', when='@3.2:3.2.0%clang')
+    patch('mpich32_clang.patch', when='@3.2:3.2.0%apple-clang')
 
     # Fix SLURM node list parsing
     # See https://github.com/pmodels/mpich/issues/3572
diff --git a/var/spack/repos/builtin/packages/mpilander/package.py b/var/spack/repos/builtin/packages/mpilander/package.py
index 276f380a86..9b72915180 100644
--- a/var/spack/repos/builtin/packages/mpilander/package.py
+++ b/var/spack/repos/builtin/packages/mpilander/package.py
@@ -31,6 +31,7 @@ class Mpilander(CMakePackage):
     # compiler support
     conflicts('%gcc@:4.7')
     conflicts('%clang@:3.8')
+    conflicts('%apple-clang@:7.4')
     conflicts('%intel@:16')
 
     def cmake_args(self):
diff --git a/var/spack/repos/builtin/packages/musl/package.py b/var/spack/repos/builtin/packages/musl/package.py
index 7c68735c20..05ec4eb5ff 100644
--- a/var/spack/repos/builtin/packages/musl/package.py
+++ b/var/spack/repos/builtin/packages/musl/package.py
@@ -38,7 +38,7 @@ class Musl(Package):
         if self.compiler.name == 'gcc':
             config.filter("WRAPCC_GCC = .*'", "WRAPCC_GCC = {0}'".
                           format(self.compiler.cc))
-        elif self.compiler.name == 'clang':
+        elif self.compiler.name in ('clang', 'apple-clang'):
             config.filter("WRAPCC_CLANG = .*'", "WRAPCC_CLANG = {0}'".
                           format(self.compiler.cc))
 
@@ -46,7 +46,7 @@ class Musl(Package):
         args = ['--prefix={0}'.format(self.prefix)]
         if self.compiler.name == 'gcc':
             args.append('--enable-wrapper=gcc')
-        elif self.compiler.name == 'clang':
+        elif self.compiler.name in ('clang', 'apple-clang'):
             args.append('--enable-wrapper=clang')
         else:
             args.append('--enable-wrapper=no')
diff --git a/var/spack/repos/builtin/packages/octopus/package.py b/var/spack/repos/builtin/packages/octopus/package.py
index 1073965e81..bed1c1a256 100644
--- a/var/spack/repos/builtin/packages/octopus/package.py
+++ b/var/spack/repos/builtin/packages/octopus/package.py
@@ -97,11 +97,13 @@ class Octopus(Package):
         # When preprocessor expands macros (i.e. CFLAGS) defined as quoted
         # strings the result may be > 132 chars and is terminated.
         # This will look to a compiler as an Unterminated character constant
-        # and produce Line truncated errors. To vercome this, add flags to
+        # and produce Line truncated errors. To overcome this, add flags to
         # let compiler know that the entire line is meaningful.
         # TODO: For the lack of better approach, assume that clang is mixed
         # with GNU fortran.
-        if spec.satisfies('%clang') or spec.satisfies('%gcc'):
+        if (spec.satisfies('%apple-clang') or
+                spec.satisfies('%clang') or
+                spec.satisfies('%gcc')):
             args.extend([
                 'FCFLAGS=-O2 -ffree-line-length-none'
             ])
diff --git a/var/spack/repos/builtin/packages/onednn/package.py b/var/spack/repos/builtin/packages/onednn/package.py
index b723fabd9c..82d22296f9 100644
--- a/var/spack/repos/builtin/packages/onednn/package.py
+++ b/var/spack/repos/builtin/packages/onednn/package.py
@@ -64,7 +64,7 @@ class Onednn(CMakePackage):
     # https://github.com/oneapi-src/oneDNN#requirements-for-building-from-source
     depends_on('cmake@2.8.11:', type='build')
     depends_on('tbb@2017:', when='cpu_runtime=tbb')
-    depends_on('llvm-openmp', when='%clang platform=darwin cpu_runtime=omp')
+    depends_on('llvm-openmp', when='%apple-clang cpu_runtime=omp')
     depends_on('opencl@1.2:', when='gpu_runtime=ocl')
 
     def cmake_args(self):
@@ -81,7 +81,7 @@ class Onednn(CMakePackage):
             args.append('-DDNNL_BUILD_TESTS=OFF')
 
         # https://github.com/oneapi-src/oneDNN/issues/591
-        if self.spec.satisfies('%clang platform=darwin cpu_runtime=omp'):
+        if self.spec.satisfies('%apple-clang cpu_runtime=omp'):
             args.extend([
                 '-DOpenMP_CXX_FLAGS={0}'.format(self.compiler.openmp_flag),
                 '-DOpenMP_C_FLAGS={0}'.format(self.compiler.openmp_flag),
diff --git a/var/spack/repos/builtin/packages/openblas/package.py b/var/spack/repos/builtin/packages/openblas/package.py
index c1a0c1e01b..61a530b37f 100644
--- a/var/spack/repos/builtin/packages/openblas/package.py
+++ b/var/spack/repos/builtin/packages/openblas/package.py
@@ -95,7 +95,7 @@ class Openblas(MakefilePackage):
     patch('openblas-0.3.8-darwin.patch', when='@0.3.8 platform=darwin')
     # Fix ICE in LLVM 9.0.0 https://github.com/xianyi/OpenBLAS/pull/2329
     # Patch as in https://github.com/xianyi/OpenBLAS/pull/2597
-    patch('openblas_appleclang11.patch', when='@0.3.8:0.3.9 %clang@11.0.3-apple')
+    patch('openblas_appleclang11.patch', when='@0.3.8:0.3.9 %apple-clang@11.0.3')
 
     # Add conditions to f_check to determine the Fujitsu compiler
     patch('openblas_fujitsu.patch', when='%fj')
@@ -105,6 +105,9 @@ class Openblas(MakefilePackage):
     conflicts('+consistent_fpcsr', when='threads=none',
               msg='FPCSR consistency only applies to multithreading')
 
+    conflicts('threads=openmp', when='%apple-clang', msg="Apple's clang does not support OpenMP")
+    conflicts('threads=openmp @:0.2.19', when='%clang', msg='OpenBLAS @:0.2.19 does not support OpenMP with clang!')
+
     @property
     def parallel(self):
         # unclear whether setting `-j N` externally was supported before 0.3
@@ -120,18 +123,6 @@ class Openblas(MakefilePackage):
                 'OpenBLAS requires both C and Fortran compilers!'
             )
 
-        # Add support for OpenMP
-        if (self.spec.satisfies('threads=openmp') and
-            self.spec.satisfies('%clang')):
-            if str(self.spec.compiler.version).endswith('-apple'):
-                raise InstallError("Apple's clang does not support OpenMP")
-            if '@:0.2.19' in self.spec:
-                # Openblas (as of 0.2.19) hardcoded that OpenMP cannot
-                # be used with any (!) compiler named clang, bummer.
-                raise InstallError(
-                    'OpenBLAS @:0.2.19 does not support OpenMP with clang!'
-                )
-
     @staticmethod
     def _read_targets(target_file):
         """Parse a list of available targets from the OpenBLAS/TargetList.txt
diff --git a/var/spack/repos/builtin/packages/pdt/package.py b/var/spack/repos/builtin/packages/pdt/package.py
index 35164db1c5..bf3f954982 100644
--- a/var/spack/repos/builtin/packages/pdt/package.py
+++ b/var/spack/repos/builtin/packages/pdt/package.py
@@ -33,7 +33,8 @@ class Pdt(AutotoolsPackage):
     variant('pic', default=False, description="Builds with pic")
 
     def patch(self):
-        if self.spec.satisfies('%clang'):
+        spec = self.spec
+        if spec.satisfies('%clang') or spec.satisfies('%apple-clang'):
             filter_file(r'PDT_GXX=g\+\+ ',
                         r'PDT_GXX=clang++ ', 'ductape/Makefile')
 
diff --git a/var/spack/repos/builtin/packages/petsc/package.py b/var/spack/repos/builtin/packages/petsc/package.py
index f328171072..9da9740674 100644
--- a/var/spack/repos/builtin/packages/petsc/package.py
+++ b/var/spack/repos/builtin/packages/petsc/package.py
@@ -2,11 +2,7 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
-
 import os
-import sys
-from spack import *
 
 
 class Petsc(Package):
@@ -116,9 +112,7 @@ class Petsc(Package):
     # temporary workaround Clang 8.1.0 with XCode 8.3 on macOS, see
     # https://bitbucket.org/petsc/petsc/commits/4f290403fdd060d09d5cb07345cbfd52670e3cbc
     # the patch is an adaptation of the original commit to 3.7.5
-    if sys.platform == "darwin":
-        patch('macos-clang-8.1.0.diff',
-              when='@3.7.5%clang@8.1.0:')
+    patch('macos-clang-8.1.0.diff', when='@3.7.5%apple-clang@8.1.0:')
     patch('pkg-config-3.7.6-3.8.4.diff', when='@3.7.6:3.8.4')
 
     patch('xcode_stub_out_of_sync.patch', when='@:3.10.4')
diff --git a/var/spack/repos/builtin/packages/pixman/package.py b/var/spack/repos/builtin/packages/pixman/package.py
index 1087dabbbf..a145edc638 100644
--- a/var/spack/repos/builtin/packages/pixman/package.py
+++ b/var/spack/repos/builtin/packages/pixman/package.py
@@ -29,7 +29,8 @@ class Pixman(AutotoolsPackage):
     # __builtin_shuffle was removed in clang 5.0.
     # From version 9.1 apple-clang is based on clang 5.0.
     # Patch is obtained from above link.
-    patch('clang.patch', when='@0.34%clang@9.1.0-apple:')
+    patch('clang.patch', when='@0.34%apple-clang@9.1.0:')
+    patch('clang.patch', when='@0.34%clang@5.0.0:')
 
     @run_before('build')
     def patch_config_h_for_intel(self):
diff --git a/var/spack/repos/builtin/packages/plasma/package.py b/var/spack/repos/builtin/packages/plasma/package.py
index bf4d8c68ee..efeb980bd8 100644
--- a/var/spack/repos/builtin/packages/plasma/package.py
+++ b/var/spack/repos/builtin/packages/plasma/package.py
@@ -58,6 +58,7 @@ class Plasma(CMakePackage):
     conflicts("%gcc@:5.99", when='@17.2:')
 
     conflicts("%cce")
+    conflicts('%apple-clang')
     conflicts("%clang")
     conflicts("%intel")
     conflicts("%nag")
diff --git a/var/spack/repos/builtin/packages/precice/package.py b/var/spack/repos/builtin/packages/precice/package.py
index 11250f60bd..45548275c8 100644
--- a/var/spack/repos/builtin/packages/precice/package.py
+++ b/var/spack/repos/builtin/packages/precice/package.py
@@ -60,6 +60,7 @@ class Precice(CMakePackage):
     # We require C++11 compiler support as well as
     # library support for time manipulators (N2071, N2072)
     conflicts('%gcc@:4')
+    conflicts('%apple-clang@:4')
     conflicts('%clang@:3.7')
     conflicts('%intel@:14')
     conflicts('%pgi@:14')
diff --git a/var/spack/repos/builtin/packages/py-dgl/package.py b/var/spack/repos/builtin/packages/py-dgl/package.py
index 544b527f69..9ef1bd9ed8 100644
--- a/var/spack/repos/builtin/packages/py-dgl/package.py
+++ b/var/spack/repos/builtin/packages/py-dgl/package.py
@@ -29,7 +29,7 @@ class PyDgl(CMakePackage):
 
     depends_on('cmake@3.5:', type='build')
     depends_on('cuda', when='+cuda')
-    depends_on('llvm-openmp', when='%clang platform=darwin +openmp')
+    depends_on('llvm-openmp', when='%apple-clang +openmp')
 
     # Python dependencies
     # See python/setup.py
@@ -66,7 +66,7 @@ class PyDgl(CMakePackage):
         if '+openmp' in self.spec:
             args.append('-DUSE_OPENMP=ON')
 
-            if self.spec.satisfies('%clang platform=darwin'):
+            if self.spec.satisfies('%apple-clang'):
                 args.extend([
                     '-DOpenMP_CXX_FLAGS=' +
                     self.spec['llvm-openmp'].headers.include_flags,
diff --git a/var/spack/repos/builtin/packages/py-exodus-bundler/package.py b/var/spack/repos/builtin/packages/py-exodus-bundler/package.py
index c13f30eb46..eddb3da88f 100644
--- a/var/spack/repos/builtin/packages/py-exodus-bundler/package.py
+++ b/var/spack/repos/builtin/packages/py-exodus-bundler/package.py
@@ -15,6 +15,7 @@ class PyExodusBundler(PythonPackage):
 
     version('2.0.2', sha256='4e896a2034b94cf7b4fb33d86a68e29a7d3b08e57541e444db34dddc6ac1ef68')
 
+    depends_on('musl', type='run', when='%apple-clang')
     depends_on('musl', type='run', when='%clang')
     depends_on('musl', type='run', when='%gcc')
     depends_on('py-setuptools', type=('build', 'run'))
diff --git a/var/spack/repos/builtin/packages/py-flye/package.py b/var/spack/repos/builtin/packages/py-flye/package.py
index db3804e11d..090d36cdb9 100644
--- a/var/spack/repos/builtin/packages/py-flye/package.py
+++ b/var/spack/repos/builtin/packages/py-flye/package.py
@@ -26,7 +26,7 @@ class PyFlye(PythonPackage):
     msg = 'C++ compiler with C++11 support required'
     conflicts('%gcc@:4.7',   msg=msg)
     conflicts('%clang@:3.2', msg=msg)
-    # Requires Apple Clang 5.0+ but no way to specify that right now
+    conflicts('apple-clang@:4.9', msg=msg)
 
     def setup_build_environment(self, env):
         if self.spec.target.family == 'aarch64':
diff --git a/var/spack/repos/builtin/packages/py-scikit-learn/package.py b/var/spack/repos/builtin/packages/py-scikit-learn/package.py
index 52c9beae43..d73d3f7576 100644
--- a/var/spack/repos/builtin/packages/py-scikit-learn/package.py
+++ b/var/spack/repos/builtin/packages/py-scikit-learn/package.py
@@ -69,9 +69,7 @@ class PyScikitLearn(PythonPackage):
     depends_on('py-pytest@3.3.0:', type='test')
     depends_on('py-pandas', type='test')
     depends_on('py-setuptools', type='build')
-    # Technically not correct, but currently no way to check if we
-    # are using Apple Clang or not.
-    depends_on('llvm-openmp', when='@0.21: %clang platform=darwin +openmp')
+    depends_on('llvm-openmp', when='@0.21: %apple-clang +openmp')
 
     # Release tarballs are already cythonized. If you wanted to build a release
     # version without OpenMP support, you would need to delete all .c files
@@ -84,7 +82,7 @@ class PyScikitLearn(PythonPackage):
         if self.spec.satisfies('~openmp'):
             env.set('SKLEARN_NO_OPENMP', 'True')
         # https://scikit-learn.org/stable/developers/advanced_installation.html#mac-osx
-        elif self.spec.satisfies('@0.21: %clang platform=darwin +openmp'):
+        elif self.spec.satisfies('@0.21: %apple-clang +openmp'):
             env.append_flags(
                 'CPPFLAGS', self.compiler.openmp_flag)
             env.append_flags(
diff --git a/var/spack/repos/builtin/packages/py-torch/package.py b/var/spack/repos/builtin/packages/py-torch/package.py
index c240144f11..1ffeb81b23 100644
--- a/var/spack/repos/builtin/packages/py-torch/package.py
+++ b/var/spack/repos/builtin/packages/py-torch/package.py
@@ -155,7 +155,7 @@ class PyTorch(PythonPackage, CudaPackage):
     depends_on('nccl', when='+nccl')
     depends_on('gloo', when='+gloo')
     depends_on('opencv', when='+opencv')
-    depends_on('llvm-openmp', when='%clang platform=darwin +openmp')
+    depends_on('llvm-openmp', when='%apple-clang +openmp')
     depends_on('ffmpeg', when='+ffmpeg')
     depends_on('leveldb', when='+leveldb')
     depends_on('lmdb', when='+lmdb')
diff --git a/var/spack/repos/builtin/packages/qmcpack/package.py b/var/spack/repos/builtin/packages/qmcpack/package.py
index 5307b3d2d5..07724f63fe 100644
--- a/var/spack/repos/builtin/packages/qmcpack/package.py
+++ b/var/spack/repos/builtin/packages/qmcpack/package.py
@@ -102,6 +102,7 @@ class Qmcpack(CMakePackage, CudaPackage):
     conflicts('+afqmc', when='@:3.6.0', msg='AFQMC not recommended before v3.7')
     conflicts('+afqmc', when='~mpi', msg='AFQMC requires building with +mpi')
     conflicts('+afqmc', when='%gcc@:6.0', msg='AFQMC code requires gcc@6.1 or greater')
+    conflicts('+afqmc', when='%apple-clang@:9.2', msg='AFQMC code requires clang 4.1 or greater')
     conflicts('+afqmc', when='%clang@:4.0', msg='AFQMC code requires clang 4.1 or greater')
     conflicts('+afqmc', when='%intel@:18', msg='AFQMC code requires intel19 or greater')
 
diff --git a/var/spack/repos/builtin/packages/qt/package.py b/var/spack/repos/builtin/packages/qt/package.py
index ba28042fa3..7e016846ab 100644
--- a/var/spack/repos/builtin/packages/qt/package.py
+++ b/var/spack/repos/builtin/packages/qt/package.py
@@ -286,7 +286,7 @@ class Qt(Package):
             "qmake/qmake.pri",
             "src/tools/bootstrap/bootstrap.pro"
         ]
-        if '%clang' in self.spec:
+        if '%clang' in self.spec or '%apple-clang' in self.spec:
             files_to_filter += [
                 "mkspecs/unsupported/macx-clang-libc++/qmake.conf",
                 "mkspecs/common/clang.conf"
diff --git a/var/spack/repos/builtin/packages/raxml/package.py b/var/spack/repos/builtin/packages/raxml/package.py
index 08d99e6b16..7eb477dfc6 100644
--- a/var/spack/repos/builtin/packages/raxml/package.py
+++ b/var/spack/repos/builtin/packages/raxml/package.py
@@ -25,6 +25,7 @@ class Raxml(Package):
 
     # Compiles with either GCC or ICC.
     conflicts('%cce')
+    conflicts('%apple-clang')
     conflicts('%clang')
     conflicts('%nag')
     conflicts('%pgi')
diff --git a/var/spack/repos/builtin/packages/sgpp/package.py b/var/spack/repos/builtin/packages/sgpp/package.py
index 7e5b907374..728638500f 100644
--- a/var/spack/repos/builtin/packages/sgpp/package.py
+++ b/var/spack/repos/builtin/packages/sgpp/package.py
@@ -97,6 +97,7 @@ class Sgpp(SConsPackage):
 
     # Compiler with C++11 support is required
     conflicts('%gcc@:4.8.4', msg='Compiler with c++11 support is required!')
+    conflicts('%apple-clang@:3.9', msg='Compiler with c++11 support is required!')
     conflicts('%clang@:3.2', msg='Compiler with c++11 support is required!')
     conflicts('%intel@:14', msg='Compiler with c++11 support is required!')
     # Solver python bindings are actually using the pde module at one point:
diff --git a/var/spack/repos/builtin/packages/silo/package.py b/var/spack/repos/builtin/packages/silo/package.py
index 9192937393..3d9ef4e16c 100644
--- a/var/spack/repos/builtin/packages/silo/package.py
+++ b/var/spack/repos/builtin/packages/silo/package.py
@@ -57,6 +57,13 @@ class Silo(AutotoolsPackage):
 
     @when('%clang@9:')
     def patch(self):
+        self.clang_9_patch()
+
+    @when('%apple-clang@11.0.3:')
+    def patch(self):
+        self.clang_9_patch()
+
+    def clang_9_patch(self):
         # Clang 9 and later include macro definitions in <math.h> that conflict
         # with typedefs DOMAIN and RANGE used in Silo plugins.
         # It looks like the upstream fpzip repo has been fixed, but that change
diff --git a/var/spack/repos/builtin/packages/snap-berkeley/package.py b/var/spack/repos/builtin/packages/snap-berkeley/package.py
index 90d970e277..de9928e0c3 100644
--- a/var/spack/repos/builtin/packages/snap-berkeley/package.py
+++ b/var/spack/repos/builtin/packages/snap-berkeley/package.py
@@ -22,6 +22,7 @@ class SnapBerkeley(MakefilePackage):
 
     conflicts('%gcc@6:')
     conflicts('%cce')
+    conflicts('%apple-clang')
     conflicts('%clang')
     conflicts('%intel')
     conflicts('%nag')
diff --git a/var/spack/repos/builtin/packages/sqlitebrowser/package.py b/var/spack/repos/builtin/packages/sqlitebrowser/package.py
index 6cd8eeb9b8..b92d65348e 100644
--- a/var/spack/repos/builtin/packages/sqlitebrowser/package.py
+++ b/var/spack/repos/builtin/packages/sqlitebrowser/package.py
@@ -18,6 +18,7 @@ class Sqlitebrowser(CMakePackage):
 
     msg = 'sqlitebrowser requires C++11 support'
     conflicts('%gcc@:4.8.0', msg=msg)
+    conflicts('%apple-clang@:3.9', msg=msg)
     conflicts('%clang@:3.2', msg=msg)
     conflicts('%intel@:12',  msg=msg)
     conflicts('%xl@:13.0',   msg=msg)
diff --git a/var/spack/repos/builtin/packages/suite-sparse/package.py b/var/spack/repos/builtin/packages/suite-sparse/package.py
index eef38fefae..703fbcf670 100644
--- a/var/spack/repos/builtin/packages/suite-sparse/package.py
+++ b/var/spack/repos/builtin/packages/suite-sparse/package.py
@@ -115,7 +115,7 @@ class SuiteSparse(Package):
         # CFLAGS, but not all compilers use the same flags for these
         # optimizations
         if any([x in spec
-                for x in ('%clang', '%gcc', '%intel')]):
+                for x in ('%apple-clang', '%clang', '%gcc', '%intel')]):
             make_args += ['CFLAGS+=-fno-common -fexceptions']
         elif '%pgi' in spec:
             make_args += ['CFLAGS+=--exceptions']
diff --git a/var/spack/repos/builtin/packages/sundials/package.py b/var/spack/repos/builtin/packages/sundials/package.py
index e374a4bbca..50832e6e5a 100644
--- a/var/spack/repos/builtin/packages/sundials/package.py
+++ b/var/spack/repos/builtin/packages/sundials/package.py
@@ -214,7 +214,7 @@ class Sundials(CMakePackage):
             return 'ON' if varstr in self.spec else 'OFF'
 
         fortran_flag = self.compiler.f77_pic_flag
-        if (spec.satisfies('%clang platform=darwin')) and ('+fcmix' in spec):
+        if (spec.satisfies('%apple-clang')) and ('+fcmix' in spec):
             f77 = Executable(self.compiler.f77)
             libgfortran = LibraryList(f77('--print-file-name',
                                           'libgfortran.a', output=str))
diff --git a/var/spack/repos/builtin/packages/trilinos/package.py b/var/spack/repos/builtin/packages/trilinos/package.py
index 16f916849c..7896d809f2 100644
--- a/var/spack/repos/builtin/packages/trilinos/package.py
+++ b/var/spack/repos/builtin/packages/trilinos/package.py
@@ -683,7 +683,9 @@ class Trilinos(CMakePackage):
 
         # Fortran lib (assumes clang is built with gfortran!)
         if '+fortran' in spec and (
-                spec.satisfies('%gcc') or spec.satisfies('%clang')):
+                spec.satisfies('%gcc') or spec.satisfies('%clang') or
+                spec.satisfies('%apple-clang')
+        ):
             options.append(define('Trilinos_ENABLE_Fortran', True))
             if '+mpi' in spec:
                 libgfortran = os.path.dirname(os.popen(
diff --git a/var/spack/repos/builtin/packages/unblur/package.py b/var/spack/repos/builtin/packages/unblur/package.py
index 428f44c08e..7366ad836a 100644
--- a/var/spack/repos/builtin/packages/unblur/package.py
+++ b/var/spack/repos/builtin/packages/unblur/package.py
@@ -27,6 +27,7 @@ class Unblur(AutotoolsPackage):
     # Requires Intel Fortran compiler
     conflicts('%gcc')
     conflicts('%pgi')
+    conflicts('%apple-clang')
     conflicts('%clang')
     conflicts('%cce')
     conflicts('%xl')
diff --git a/var/spack/repos/builtin/packages/valgrind/package.py b/var/spack/repos/builtin/packages/valgrind/package.py
index 4555b0b886..20d8791f4e 100644
--- a/var/spack/repos/builtin/packages/valgrind/package.py
+++ b/var/spack/repos/builtin/packages/valgrind/package.py
@@ -40,7 +40,7 @@ class Valgrind(AutotoolsPackage, SourcewarePackage):
     variant('ubsan', default=sys.platform != 'darwin',
             description='Activates ubsan support for valgrind')
 
-    conflicts('+ubsan', when='platform=darwin %clang',
+    conflicts('+ubsan', when='%apple-clang',
               msg="""
 Cannot build libubsan with clang on macOS.
 Otherwise with (Apple's) clang there is a linker error:
diff --git a/var/spack/repos/builtin/packages/vtk/package.py b/var/spack/repos/builtin/packages/vtk/package.py
index 0df4c4a5fc..2a334cb1fb 100644
--- a/var/spack/repos/builtin/packages/vtk/package.py
+++ b/var/spack/repos/builtin/packages/vtk/package.py
@@ -285,9 +285,7 @@ class Vtk(CMakePackage):
             # string. This fix was recommended on the VTK mailing list
             # in March 2014 (see
             # https://public.kitware.com/pipermail/vtkusers/2014-March/083368.html)
-            if (self.spec.satisfies('%clang') and
-                    self.compiler.is_apple and
-                    self.compiler.version >= Version('5.1.0')):
+            if self.spec.satisfies('%apple-clang@5.1.0:'):
                 cmake_args.extend(['-DVTK_REQUIRED_OBJCXX_FLAGS='])
 
             # A bug in tao pegtl causes build failures with intel compilers
diff --git a/var/spack/repos/builtin/packages/warpx/package.py b/var/spack/repos/builtin/packages/warpx/package.py
index dc19f916fe..c564528b40 100644
--- a/var/spack/repos/builtin/packages/warpx/package.py
+++ b/var/spack/repos/builtin/packages/warpx/package.py
@@ -68,7 +68,8 @@ class Warpx(MakefilePackage):
 
     def edit(self, spec, prefix):
         comp = 'gcc'
-        vendors = {'%gcc': 'gcc', '%intel': 'intel', '%clang': 'llvm'}
+        vendors = {'%gcc': 'gcc', '%intel': 'intel',
+                   '%apple-clang': 'llvm', '%clang': 'llvm'}
         for key, value in vendors.items():
             if self.spec.satisfies(key):
                 comp = value
diff --git a/var/spack/repos/builtin/packages/xios/package.py b/var/spack/repos/builtin/packages/xios/package.py
index 54eca9fead..2b5c6f9dae 100644
--- a/var/spack/repos/builtin/packages/xios/package.py
+++ b/var/spack/repos/builtin/packages/xios/package.py
@@ -31,6 +31,7 @@ class Xios(Package):
 
     # Workaround bug #17782 in llvm, where reading a double
     # followed by a character is broken (e.g. duration '1d'):
+    patch('llvm_bug_17782.patch', when='@1.1: %apple-clang')
     patch('llvm_bug_17782.patch', when='@1.1: %clang')
 
     depends_on('netcdf-c+mpi')
@@ -45,6 +46,13 @@ class Xios(Package):
 
     @when('%clang')
     def patch(self):
+        self.patch_llvm()
+
+    @when('%apple-clang')
+    def patch(self):
+        self.patch_llvm()
+
+    def patch_llvm(self):
         """Fix type references that are ambiguous for clang."""
         for dirpath, dirnames, filenames in os.walk('src'):
             for filename in filenames:
@@ -99,12 +107,13 @@ OASIS_LIB=""
         param['BOOST_LIB_DIR'] = spec['boost'].prefix.lib
         param['BLITZ_INC_DIR'] = spec['blitz'].prefix.include
         param['BLITZ_LIB_DIR'] = spec['blitz'].prefix.lib
-        if spec.satisfies('%clang platform=darwin'):
+        if spec.satisfies('%apple-clang'):
             param['LIBCXX'] = '-lc++'
         else:
             param['LIBCXX'] = '-lstdc++'
 
-        if any(map(spec.satisfies, ('%gcc', '%intel', '%clang'))):
+        if any(map(spec.satisfies,
+                   ('%gcc', '%intel', '%apple-clang', '%clang'))):
             text = r"""
 %CCOMPILER      {MPICXX}
 %FCOMPILER      {MPIFC}
diff --git a/var/spack/repos/builtin/packages/xsdktrilinos/package.py b/var/spack/repos/builtin/packages/xsdktrilinos/package.py
index b679ee7277..effb4404b0 100644
--- a/var/spack/repos/builtin/packages/xsdktrilinos/package.py
+++ b/var/spack/repos/builtin/packages/xsdktrilinos/package.py
@@ -65,7 +65,9 @@ class Xsdktrilinos(CMakePackage):
         ])
 
         # Fortran lib
-        if spec.satisfies('%gcc') or spec.satisfies('%clang'):
+        if (spec.satisfies('%gcc') or
+                spec.satisfies('%clang') or
+                spec.satisfies('%apple-clang')):
             libgfortran = os.path.dirname(os.popen(
                 '%s --print-file-name libgfortran.a' %
                 join_path(mpi_bin, 'mpif90')).read())
diff --git a/var/spack/repos/builtin/packages/yaml-cpp/package.py b/var/spack/repos/builtin/packages/yaml-cpp/package.py
index 11f1f9fbef..a83471f23a 100644
--- a/var/spack/repos/builtin/packages/yaml-cpp/package.py
+++ b/var/spack/repos/builtin/packages/yaml-cpp/package.py
@@ -35,9 +35,7 @@ class YamlCpp(CMakePackage):
 
     conflicts('%gcc@:4.7', when='@0.6.0:', msg="versions 0.6.0: require c++11 support")
     conflicts('%clang@:3.3.0', when='@0.6.0:', msg="versions 0.6.0: require c++11 support")
-    # currently we can't check for apple-clang's version
-    # conflicts('%clang@:4.0.0-apple', when='@0.6.0:',
-    # msg="versions 0.6.0: require c++11 support")
+    conflicts('%apple-clang@:4.0.0', when='@0.6.0:', msg="versions 0.6.0: require c++11 support")
     conflicts('%intel@:11.1', when='@0.6.0:', msg="versions 0.6.0: require c++11 support")
     conflicts('%xl@:13.1', when='@0.6.0:', msg="versions 0.6.0: require c++11 support")
     conflicts('%xl_r@:13.1', when='@0.6.0:', msg="versions 0.6.0: require c++11 support")