Add a "requires" directive, extend functionality of package requirements (#36286)

Add a "require" directive to packages, which functions exactly like
requirements specified in packages.yaml (uses the same fact-generation
logic); update both to allow making the requirement conditional.

* Packages may now use "require" to add constraints. This can be useful
  for something like "require(%gcc)" (where before we had to add a
  conflict for every compiler except gcc).
* Requirements (in packages.yaml or in a "require" directive) can be
  conditional on a spec, e.g. "require(%gcc, when=@1.0.0)" (version
  1.0.0 can only build with gcc).
* Requirements may include a message which clarifies why they are needed.
  The concretizer assigns a high priority to errors which generate these
  messages (in particular over errors for unsatisfied requirements that
  do not produce messages, but also over a number of more-generic
  errors).

14 files changed, 628 insertions, 136 deletions

diff --git a/lib/spack/docs/build_settings.rst b/lib/spack/docs/build_settings.rst
index 3adbbb4c76..0bbd27e8c3 100644
--- a/lib/spack/docs/build_settings.rst
+++ b/lib/spack/docs/build_settings.rst
@@ -325,42 +325,99 @@ on the command line, because it can specify constraints on packages
 is not possible to specify constraints on dependencies while also keeping
 those dependencies optional.
 
-The package requirements configuration is specified in ``packages.yaml``
-keyed by package name:
+^^^^^^^^^^^^^^^^^^^
+Requirements syntax
+^^^^^^^^^^^^^^^^^^^
+
+The package requirements configuration is specified in ``packages.yaml``,
+keyed by package name and expressed using the Spec syntax. In the simplest
+case you can specify attributes that you always want the package to have
+by providing a single spec string to ``require``:
 
 .. code-block:: yaml
 
    packages:
      libfabric:
        require: "@1.13.2"
+
+In the above example, ``libfabric`` will always build with version 1.13.2. If you
+need to compose multiple configuration scopes ``require`` accepts a list of
+strings:
+
+.. code-block:: yaml
+
+   packages:
+     libfabric:
+       require:
+       - "@1.13.2"
+       - "%gcc"
+
+In this case ``libfabric`` will always build with version 1.13.2 **and** using GCC
+as a compiler.
+
+For more complex use cases, require accepts also a list of objects. These objects
+must have either a ``any_of`` or a ``one_of`` field, containing a list of spec strings,
+and they can optionally have a ``when`` and a ``message`` attribute:
+
+.. code-block:: yaml
+
+   packages:
+     openmpi:
+       require:
+       - any_of: ["@4.1.5", "%gcc"]
+         message: "in this example only 4.1.5 can build with other compilers"
+
+``any_of`` is a list of specs. One of those specs must be satisfied
+and it is also allowed for the concretized spec to match more than one.
+In the above example, that means you could build ``openmpi@4.1.5%gcc``,
+``openmpi@4.1.5%clang`` or ``openmpi@3.9%gcc``, but
+not ``openmpi@3.9%clang``.
+
+If a custom message is provided, and the requirement is not satisfiable,
+Spack will print the custom error message:
+
+.. code-block:: console
+
+   $ spack spec openmpi@3.9%clang
+   ==> Error: in this example only 4.1.5 can build with other compilers
+
+We could express a similar requirement using the ``when`` attribute:
+
+.. code-block:: yaml
+
+   packages:
+     openmpi:
+       require:
+       - any_of: ["%gcc"]
+         when: "@:4.1.4"
+         message: "in this example only 4.1.5 can build with other compilers"
+
+In the example above, if the version turns out to be 4.1.4 or less, we require the compiler to be GCC.
+For readability, Spack also allows a ``spec`` key accepting a string when there is only a single
+constraint:
+
+.. code-block:: yaml
+
+   packages:
      openmpi:
        require:
-       - any_of: ["~cuda", "%gcc"]
+       - spec: "%gcc"
+         when: "@:4.1.4"
+         message: "in this example only 4.1.5 can build with other compilers"
+
+This code snippet and the one before it are semantically equivalent.
+
+Finally, instead of ``any_of`` you can use ``one_of`` which also takes a list of specs. The final
+concretized spec must match one and only one of them:
+
+.. code-block:: yaml
+
+   packages:
      mpich:
-      require:
-      - one_of: ["+cuda", "+rocm"]
-
-Requirements are expressed using Spec syntax (the same as what is provided
-to ``spack install``). In the simplest case, you can specify attributes
-that you always want the package to have by providing a single spec to
-``require``; in the above example, ``libfabric`` will always build
-with version 1.13.2.
-
-You can provide a more-relaxed constraint and allow the concretizer to
-choose between a set of options using ``any_of`` or ``one_of``:
-
-* ``any_of`` is a list of specs. One of those specs must be satisfied
-  and it is also allowed for the concretized spec to match more than one.
-  In the above example, that means you could build ``openmpi+cuda%gcc``,
-  ``openmpi~cuda%clang`` or ``openmpi~cuda%gcc`` (in the last case,
-  note that both specs in the ``any_of`` for ``openmpi`` are
-  satisfied).
-* ``one_of`` is also a list of specs, and the final concretized spec
-  must match exactly one of them.  In the above example, that means
-  you could build ``mpich+cuda`` or ``mpich+rocm`` but not
-  ``mpich+cuda+rocm`` (note the current package definition for
-  ``mpich`` already includes a conflict, so this is redundant but
-  still demonstrates the concept).
+       require:
+       - one_of: ["+cuda", "+rocm"]
+
+In the example above, that means you could build ``mpich+cuda`` or ``mpich+rocm`` but not ``mpich+cuda+rocm``.
 
 .. note::
 
@@ -368,6 +425,13 @@ choose between a set of options using ``any_of`` or ``one_of``:
    preference: items that appear earlier in the list are preferred
    (note that these preferences can be ignored in favor of others).
 
+.. note::
+
+   When using a conditional requirement, Spack is allowed to actively avoid the triggering
+   condition (the ``when=...`` spec) if that leads to a concrete spec with better scores in
+   the optimization criteria. To check the current optimization criteria and their
+   priorities you can run ``spack solve zlib``.
+
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Setting default requirements
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/lib/spack/docs/packaging_guide.rst b/lib/spack/docs/packaging_guide.rst
index 2497c5b83e..2a4d03bc72 100644
--- a/lib/spack/docs/packaging_guide.rst
+++ b/lib/spack/docs/packaging_guide.rst
@@ -2669,9 +2669,9 @@ to allow dependencies to run correctly:
 
 .. _packaging_conflicts:
 
----------
-Conflicts
----------
+--------------------------
+Conflicts and requirements
+--------------------------
 
 Sometimes packages have known bugs, or limitations, that would prevent them
 to build e.g. against other dependencies or with certain compilers. Spack
@@ -2681,27 +2681,51 @@ Adding the following to a package:
 
 .. code-block:: python
 
-    conflicts("%intel", when="@:1.2",
-              msg="<myNicePackage> <= v1.2 cannot be built with Intel ICC, "
-                  "please use a newer release.")
+    conflicts(
+        "%intel",
+         when="@:1.2",
+         msg="<myNicePackage> <= v1.2 cannot be built with Intel ICC, "
+             "please use a newer release."
+    )
 
 we express the fact that the current package *cannot be built* with the Intel
-compiler when we are trying to install a version "<=1.2". The ``when`` argument
-can be omitted, in which case the conflict will always be active.
-Conflicts are always evaluated after the concretization step has been performed,
-and if any match is found a detailed error message is shown to the user.
-You can add an additional message via the ``msg=`` parameter to a conflict that
-provideds more specific instructions for users.
+compiler when we are trying to install a version "<=1.2".
+
+The ``when`` argument can be omitted, in which case the conflict will always be active.
+
+An optional custom error message can be added via the ``msg=`` parameter, and will be printed
+by Spack in case the conflict cannot be avoided and leads to a concretization error.
 
-Similarly, packages that only build on a subset of platforms can use the
-``conflicts`` directive to express that limitation, for example:
+Sometimes, packages allow only very specific choices and they can't use the rest. In those cases
+the ``requires`` directive can be used:
 
 .. code-block:: python
 
-    for platform in ["cray", "darwin", "windows"]:
-        conflicts("platform={0}".format(platform), msg="Only 'linux' is supported")
+    requires(
+        "%apple-clang",
+        when="platform=darwin",
+        msg="<myNicePackage> builds only with Apple-Clang on Darwin"
+    )
+
+In the example above, our package can only be built with Apple-Clang on Darwin.
+The ``requires`` directive is effectively the opposite of the ``conflicts`` directive, and takes
+the same optional ``when`` and ``msg`` arguments.
+
+If a package needs to express more complex requirements, involving more than a single spec,
+that can also be done using the ``requires`` directive. To express that a package can be built
+either with GCC or with Clang we can write:
+
+.. code-block:: python
 
+    requires(
+        "%gcc", "%clang",
+        policy="one_of"
+        msg="<myNicePackage> builds only with GCC or Clang"
+    )
 
+When using multiple specs in a ``requires`` directive, it is advised to set the ``policy=``
+argument explicitly. That argument can take either the value ``any_of`` or the value ``one_of``,
+and the semantic is the same as for :ref:`package-requirements`.
 
 .. _packaging_extensions:
 
diff --git a/lib/spack/spack/directives.py b/lib/spack/spack/directives.py
index 27075d47b4..b997a190c8 100644
--- a/lib/spack/spack/directives.py
+++ b/lib/spack/spack/directives.py
@@ -26,6 +26,7 @@ The available directives are:
   * ``resource``
   * ``variant``
   * ``version``
+  * ``requires``
 
 """
 import collections.abc
@@ -66,6 +67,7 @@ __all__ = [
     "variant",
     "resource",
     "build_system",
+    "requires",
 ]
 
 #: These are variant names used by Spack internally; packages can't use them
@@ -864,6 +866,45 @@ def maintainers(*names: str):
     return _execute_maintainer
 
 
+@directive("requirements")
+def requires(*requirement_specs, policy="one_of", when=None, msg=None):
+    """Allows a package to request a configuration to be present in all valid solutions.
+
+    For instance, a package that is known to compile only with GCC can declare:
+
+        requires("%gcc")
+
+    A package that requires Apple-Clang on Darwin can declare instead:
+
+        requires("%apple-clang", when="platform=darwin", msg="Apple Clang is required on Darwin")
+
+    Args:
+        requirement_specs: spec expressing the requirement
+        when: optional constraint that triggers the requirement. If None the requirement
+            is applied unconditionally.
+
+        msg: optional user defined message
+    """
+
+    def _execute_requires(pkg):
+        if policy not in ("one_of", "any_of"):
+            err_msg = (
+                f"the 'policy' argument of the 'requires' directive in {pkg.name} is set "
+                f"to a wrong value (only 'one_of' or 'any_of' are allowed)"
+            )
+            raise DirectiveError(err_msg)
+
+        when_spec = make_when_spec(when)
+        if not when_spec:
+            return
+
+        # Save in a list the requirements and the associated custom messages
+        when_spec_list = pkg.requirements.setdefault(tuple(requirement_specs), [])
+        when_spec_list.append((when_spec, policy, msg))
+
+    return _execute_requires
+
+
 class DirectiveError(spack.error.SpackError):
     """This is raised when something is wrong with a package directive."""
 
diff --git a/lib/spack/spack/schema/packages.py b/lib/spack/spack/schema/packages.py
index 698787a914..2cc4534d07 100644
--- a/lib/spack/spack/schema/packages.py
+++ b/lib/spack/spack/schema/packages.py
@@ -37,8 +37,17 @@ properties = {
                                             "type": "object",
                                             "additionalProperties": False,
                                             "properties": {
-                                                "one_of": {"type": "array"},
-                                                "any_of": {"type": "array"},
+                                                "one_of": {
+                                                    "type": "array",
+                                                    "items": {"type": "string"},
+                                                },
+                                                "any_of": {
+                                                    "type": "array",
+                                                    "items": {"type": "string"},
+                                                },
+                                                "spec": {"type": "string"},
+                                                "message": {"type": "string"},
+                                                "when": {"type": "string"},
                                             },
                                         },
                                         {"type": "string"},
diff --git a/lib/spack/spack/solver/asp.py b/lib/spack/spack/solver/asp.py
index ae3fc64e33..816ce660be 100644
--- a/lib/spack/spack/solver/asp.py
+++ b/lib/spack/spack/solver/asp.py
@@ -7,12 +7,14 @@ from __future__ import division, print_function
 import collections
 import collections.abc
 import copy
+import enum
 import itertools
 import os
 import pprint
 import re
 import types
 import warnings
+from typing import List
 
 import archspec.cpu
 
@@ -98,24 +100,38 @@ def ast_getter(*names):
 ast_type = ast_getter("ast_type", "type")
 ast_sym = ast_getter("symbol", "term")
 
-#: Order of precedence for version origins. Topmost types are preferred.
-version_origin_fields = [
-    "spec",
-    "dev_spec",
-    "external",
-    "packages_yaml",
-    "package_requirements",
-    "package_py",
-    "installed",
-]
 
-#: Look up version precedence strings by enum id
-version_origin_str = {i: name for i, name in enumerate(version_origin_fields)}
+class Provenance(enum.IntEnum):
+    """Enumeration of the possible provenances of a version."""
+
+    # A spec literal
+    SPEC = enum.auto()
+    # A dev spec literal
+    DEV_SPEC = enum.auto()
+    # An external spec declaration
+    EXTERNAL = enum.auto()
+    # The 'packages' section of the configuration
+    PACKAGES_YAML = enum.auto()
+    # A package requirement
+    PACKAGE_REQUIREMENT = enum.auto()
+    # A 'package.py' file
+    PACKAGE_PY = enum.auto()
+    # An installed spec
+    INSTALLED = enum.auto()
 
-#: Enumeration like object to mark version provenance
-version_provenance = collections.namedtuple(  # type: ignore
-    "VersionProvenance", version_origin_fields
-)(**{name: i for i, name in enumerate(version_origin_fields)})
+    def __str__(self):
+        return f"{self._name_.lower()}"
+
+
+class RequirementKind(enum.Enum):
+    """Purpose / provenance of a requirement"""
+
+    #: Default requirement expressed under the 'all' attribute of packages.yaml
+    DEFAULT = enum.auto()
+    #: Requirement expressed on a virtual package
+    VIRTUAL = enum.auto()
+    #: Requirement expressed on a specific package
+    PACKAGE = enum.auto()
 
 
 DeclaredVersion = collections.namedtuple("DeclaredVersion", ["version", "idx", "origin"])
@@ -635,6 +651,12 @@ class ErrorHandler:
         raise UnsatisfiableSpecError(msg)
 
 
+#: Data class to collect information on a requirement
+RequirementRule = collections.namedtuple(
+    "RequirementRule", ["pkg_name", "policy", "requirements", "condition", "kind", "message"]
+)
+
+
 class PyclingoDriver(object):
     def __init__(self, cores=True):
         """Driver for the Python clingo interface.
@@ -883,8 +905,7 @@ class SpackSolverSetup(object):
         """
 
         def key_fn(version):
-            # Origins are sorted by precedence defined in `version_origin_str`,
-            # then by order added.
+            # Origins are sorted by "provenance" first, see the Provenance enumeration above
             return version.origin, version.idx
 
         pkg = packagize(pkg)
@@ -900,10 +921,7 @@ class SpackSolverSetup(object):
         for weight, declared_version in enumerate(most_to_least_preferred):
             self.gen.fact(
                 fn.version_declared(
-                    pkg.name,
-                    declared_version.version,
-                    weight,
-                    version_origin_str[declared_version.origin],
+                    pkg.name, declared_version.version, weight, str(declared_version.origin)
                 )
             )
 
@@ -1012,35 +1030,83 @@ class SpackSolverSetup(object):
             )
 
     def package_requirement_rules(self, pkg):
+        rules = self.requirement_rules_from_package_py(pkg)
+        rules.extend(self.requirement_rules_from_packages_yaml(pkg))
+        self.emit_facts_from_requirement_rules(rules)
+
+    def requirement_rules_from_package_py(self, pkg):
+        rules = []
+        for requirements, conditions in pkg.requirements.items():
+            for when_spec, policy, message in conditions:
+                rules.append(
+                    RequirementRule(
+                        pkg_name=pkg.name,
+                        policy=policy,
+                        requirements=requirements,
+                        kind=RequirementKind.PACKAGE,
+                        condition=when_spec,
+                        message=message,
+                    )
+                )
+        return rules
+
+    def requirement_rules_from_packages_yaml(self, pkg):
         pkg_name = pkg.name
         config = spack.config.get("packages")
-        requirements, raise_on_failure = config.get(pkg_name, {}).get("require", []), True
+        requirements = config.get(pkg_name, {}).get("require", [])
+        kind = RequirementKind.PACKAGE
         if not requirements:
-            requirements, raise_on_failure = config.get("all", {}).get("require", []), False
-        rules = self._rules_from_requirements(pkg_name, requirements)
-        self.emit_facts_from_requirement_rules(
-            rules, virtual=False, raise_on_failure=raise_on_failure
-        )
+            requirements = config.get("all", {}).get("require", [])
+            kind = RequirementKind.DEFAULT
+        return self._rules_from_requirements(pkg_name, requirements, kind=kind)
 
-    def _rules_from_requirements(self, pkg_name, requirements):
+    def _rules_from_requirements(self, pkg_name: str, requirements, *, kind: RequirementKind):
         """Manipulate requirements from packages.yaml, and return a list of tuples
         with a uniform structure (name, policy, requirements).
         """
         if isinstance(requirements, str):
-            rules = [(pkg_name, "one_of", [requirements])]
+            rules = [self._rule_from_str(pkg_name, requirements, kind)]
         else:
             rules = []
             for requirement in requirements:
                 if isinstance(requirement, str):
                     # A string represents a spec that must be satisfied. It is
                     # equivalent to a one_of group with a single element
-                    rules.append((pkg_name, "one_of", [requirement]))
+                    rules.append(self._rule_from_str(pkg_name, requirement, kind))
                 else:
-                    for policy in ("one_of", "any_of"):
+                    for policy in ("spec", "one_of", "any_of"):
                         if policy in requirement:
-                            rules.append((pkg_name, policy, requirement[policy]))
+                            constraints = requirement[policy]
+
+                            # "spec" is for specifying a single spec
+                            if policy == "spec":
+                                constraints = [constraints]
+                                policy = "one_of"
+
+                            rules.append(
+                                RequirementRule(
+                                    pkg_name=pkg_name,
+                                    policy=policy,
+                                    requirements=constraints,
+                                    kind=kind,
+                                    message=requirement.get("message"),
+                                    condition=requirement.get("when"),
+                                )
+                            )
         return rules
 
+    def _rule_from_str(
+        self, pkg_name: str, requirements: str, kind: RequirementKind
+    ) -> RequirementRule:
+        return RequirementRule(
+            pkg_name=pkg_name,
+            policy="one_of",
+            requirements=[requirements],
+            kind=kind,
+            condition=None,
+            message=None,
+        )
+
     def pkg_rules(self, pkg, tests):
         pkg = packagize(pkg)
 
@@ -1267,27 +1333,55 @@ class SpackSolverSetup(object):
         assert self.possible_virtuals is not None, msg
         for virtual_str in sorted(self.possible_virtuals):
             requirements = packages_yaml.get(virtual_str, {}).get("require", [])
-            rules = self._rules_from_requirements(virtual_str, requirements)
-            self.emit_facts_from_requirement_rules(rules, virtual=True)
+            rules = self._rules_from_requirements(
+                virtual_str, requirements, kind=RequirementKind.VIRTUAL
+            )
+            self.emit_facts_from_requirement_rules(rules)
 
-    def emit_facts_from_requirement_rules(self, rules, *, virtual=False, raise_on_failure=True):
-        """Generate facts to enforce requirements from packages.yaml.
+    def emit_facts_from_requirement_rules(self, rules: List[RequirementRule]):
+        """Generate facts to enforce requirements.
 
         Args:
             rules: rules for which we want facts to be emitted
-            virtual: if True the requirements are on a virtual spec
-            raise_on_failure: if True raise an exception when a requirement condition is invalid
-                for the current spec. If False, just skip that condition
         """
-        for requirement_grp_id, (pkg_name, policy, requirement_grp) in enumerate(rules):
+        for requirement_grp_id, rule in enumerate(rules):
+            virtual = rule.kind == RequirementKind.VIRTUAL
+
+            pkg_name, policy, requirement_grp = rule.pkg_name, rule.policy, rule.requirements
+
+            requirement_weight = 0
+            main_requirement_condition = spack.directives.make_when_spec(rule.condition)
+            if main_requirement_condition is False:
+                continue
+
+            # Write explicitly if a requirement is conditional or not
+            if main_requirement_condition != spack.spec.Spec():
+                msg = f"condition to activate requirement {requirement_grp_id}"
+                try:
+                    main_condition_id = self.condition(
+                        main_requirement_condition, name=pkg_name, msg=msg
+                    )
+                except Exception as e:
+                    if rule.kind != RequirementKind.DEFAULT:
+                        raise RuntimeError("cannot emit requirements for the solver") from e
+                    continue
+
+                self.gen.fact(
+                    fn.requirement_conditional(pkg_name, requirement_grp_id, main_condition_id)
+                )
+
             self.gen.fact(fn.requirement_group(pkg_name, requirement_grp_id))
             self.gen.fact(fn.requirement_policy(pkg_name, requirement_grp_id, policy))
-            requirement_weight = 0
+            if rule.message:
+                self.gen.fact(fn.requirement_message(pkg_name, requirement_grp_id, rule.message))
+            self.gen.newline()
+
             for spec_str in requirement_grp:
                 spec = spack.spec.Spec(spec_str)
                 if not spec.name:
                     spec.name = pkg_name
                 spec.attach_git_version_lookup()
+
                 when_spec = spec
                 if virtual:
                     when_spec = spack.spec.Spec(pkg_name)
@@ -1297,12 +1391,16 @@ class SpackSolverSetup(object):
                         required_spec=when_spec, imposed_spec=spec, name=pkg_name, node=virtual
                     )
                 except Exception as e:
-                    if raise_on_failure:
+                    # Do not raise if the rule comes from the 'all' subsection, since usability
+                    # would be impaired. If a rule does not apply for a specific package, just
+                    # discard it.
+                    if rule.kind != RequirementKind.DEFAULT:
                         raise RuntimeError("cannot emit requirements for the solver") from e
                     continue
 
                 self.gen.fact(fn.requirement_group_member(member_id, pkg_name, requirement_grp_id))
                 self.gen.fact(fn.requirement_has_weight(member_id, requirement_weight))
+                self.gen.newline()
                 requirement_weight += 1
 
     def external_packages(self):
@@ -1345,7 +1443,7 @@ class SpackSolverSetup(object):
             ]
             for version, idx, external_id in external_versions:
                 self.declared_versions[pkg_name].append(
-                    DeclaredVersion(version=version, idx=idx, origin=version_provenance.external)
+                    DeclaredVersion(version=version, idx=idx, origin=Provenance.EXTERNAL)
                 )
 
             # Declare external conditions with a local index into packages.yaml
@@ -1627,7 +1725,7 @@ class SpackSolverSetup(object):
                 v, version_info = item
                 self.possible_versions[pkg_name].add(v)
                 self.declared_versions[pkg_name].append(
-                    DeclaredVersion(version=v, idx=idx, origin=version_provenance.package_py)
+                    DeclaredVersion(version=v, idx=idx, origin=Provenance.PACKAGE_PY)
                 )
                 deprecated = version_info.get("deprecated", False)
                 if deprecated:
@@ -1640,7 +1738,7 @@ class SpackSolverSetup(object):
                 # v can be a string so force it into an actual version for comparisons
                 ver = vn.Version(v)
                 self.declared_versions[pkg_name].append(
-                    DeclaredVersion(version=ver, idx=idx, origin=version_provenance.packages_yaml)
+                    DeclaredVersion(version=ver, idx=idx, origin=Provenance.PACKAGES_YAML)
                 )
                 self.possible_versions[pkg_name].add(ver)
 
@@ -1684,6 +1782,7 @@ class SpackSolverSetup(object):
         self.gen.h2("Default platform")
         platform = spack.platforms.host()
         self.gen.fact(fn.node_platform_default(platform))
+        self.gen.fact(fn.allowed_platform(platform))
 
     def os_defaults(self, specs):
         self.gen.h2("Possible operating systems")
@@ -2005,9 +2104,7 @@ class SpackSolverSetup(object):
             for dep in spec.traverse():
                 self.possible_versions[dep.name].add(dep.version)
                 self.declared_versions[dep.name].append(
-                    DeclaredVersion(
-                        version=dep.version, idx=0, origin=version_provenance.installed
-                    )
+                    DeclaredVersion(version=dep.version, idx=0, origin=Provenance.INSTALLED)
                 )
                 self.possible_oses.add(dep.os)
 
@@ -2078,13 +2175,11 @@ class SpackSolverSetup(object):
 
         # traverse all specs and packages to build dict of possible versions
         self.build_version_dict(possible)
-        self.add_concrete_versions_from_specs(specs, version_provenance.spec)
-        self.add_concrete_versions_from_specs(dev_specs, version_provenance.dev_spec)
+        self.add_concrete_versions_from_specs(specs, Provenance.SPEC)
+        self.add_concrete_versions_from_specs(dev_specs, Provenance.DEV_SPEC)
 
         req_version_specs = _get_versioned_specs_from_pkg_requirements()
-        self.add_concrete_versions_from_specs(
-            req_version_specs, version_provenance.package_requirements
-        )
+        self.add_concrete_versions_from_specs(req_version_specs, Provenance.PACKAGE_REQUIREMENT)
 
         self.gen.h1("Concrete input spec definitions")
         self.define_concrete_input_specs(specs, possible)
@@ -2187,9 +2282,15 @@ def _specs_from_requires(pkg_name, section):
             if isinstance(spec_group, str):
                 spec_strs.append(spec_group)
             else:
-                # Otherwise it is a one_of or any_of: get the values
-                (x,) = spec_group.values()
-                spec_strs.extend(x)
+                # Otherwise it is an object. The object can contain a single
+                # "spec" constraint, or a list of them with "any_of" or
+                # "one_of" policy.
+                if "spec" in spec_group:
+                    new_constraints = [spec_group["spec"]]
+                else:
+                    key = "one_of" if "one_of" in spec_group else "any_of"
+                    new_constraints = spec_group[key]
+                spec_strs.extend(new_constraints)
 
         extracted_specs = []
         for spec_str in spec_strs:
diff --git a/lib/spack/spack/solver/concretize.lp b/lib/spack/spack/solver/concretize.lp
index 632e8926e6..e96e108e0e 100644
--- a/lib/spack/spack/solver/concretize.lp
+++ b/lib/spack/spack/solver/concretize.lp
@@ -461,13 +461,24 @@ error(100, "Attempted to use external for '{0}' which does not satisfy any confi
 % Config required semantics
 %-----------------------------------------------------------------------------
 
-activate_requirement_rules(Package) :- attr("node", Package).
-activate_requirement_rules(Package) :- attr("virtual_node", Package).
+package_in_dag(Package) :-  attr("node", Package).
+package_in_dag(Package) :-  attr("virtual_node", Package).
+
+activate_requirement(Package, X) :-
+  package_in_dag(Package),
+  requirement_group(Package, X),
+  not requirement_conditional(Package, X, _).
+
+activate_requirement(Package, X) :-
+  package_in_dag(Package),
+  requirement_group(Package, X),
+  condition_holds(Y),
+  requirement_conditional(Package, X, Y).
 
 requirement_group_satisfied(Package, X) :-
   1 { condition_holds(Y) : requirement_group_member(Y, Package, X) } 1,
-  activate_requirement_rules(Package),
   requirement_policy(Package, X, "one_of"),
+  activate_requirement(Package, X),
   requirement_group(Package, X).
 
 requirement_weight(Package, Group, W) :-
@@ -479,8 +490,8 @@ requirement_weight(Package, Group, W) :-
 
 requirement_group_satisfied(Package, X) :-
   1 { condition_holds(Y) : requirement_group_member(Y, Package, X) } ,
-  activate_requirement_rules(Package),
   requirement_policy(Package, X, "any_of"),
+  activate_requirement(Package, X),
   requirement_group(Package, X).
 
 requirement_weight(Package, Group, W) :-
@@ -494,12 +505,23 @@ requirement_weight(Package, Group, W) :-
   requirement_policy(Package, Group, "any_of"),
   requirement_group_satisfied(Package, Group).
 
-error(100, "Cannot satisfy the requirements in packages.yaml for package '{0}'. You may want to delete them to proceed with concretization. To check where the requirements are defined run 'spack config blame packages'", Package) :-
-  activate_requirement_rules(Package),
+error(100, "cannot satisfy a requirement for package '{0}'.", Package) :-
+  activate_requirement(Package, X),
   requirement_group(Package, X),
+  not requirement_message(Package, X, _),
   not requirement_group_satisfied(Package, X).
 
+
+error(10, Message) :-
+  activate_requirement(Package, X),
+  requirement_group(Package, X),
+  requirement_message(Package, X, Message),
+  not requirement_group_satisfied(Package, X).
+
+
 #defined requirement_group/2.
+#defined requirement_conditional/3.
+#defined requirement_message/3.
 #defined requirement_group_member/3.
 #defined requirement_has_weight/2.
 #defined requirement_policy/3.
@@ -691,6 +713,8 @@ variant_single_value(Package, "dev_path")
 %-----------------------------------------------------------------------------
 
 % if no platform is set, fall back to the default
+:- attr("node_platform", _, Platform), not allowed_platform(Platform).
+
 attr("node_platform", Package, Platform)
  :- attr("node", Package),
     not attr("node_platform_set", Package),
diff --git a/lib/spack/spack/test/concretize_requirements.py b/lib/spack/spack/test/concretize_requirements.py
index ee022b5f77..255447c909 100644
--- a/lib/spack/spack/test/concretize_requirements.py
+++ b/lib/spack/spack/test/concretize_requirements.py
@@ -2,13 +2,14 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
+import pathlib
 import sys
 
 import pytest
 
 import spack.build_systems.generic
 import spack.config
+import spack.error
 import spack.package_base
 import spack.repo
 import spack.util.spack_yaml as syaml
@@ -595,3 +596,216 @@ def test_non_existing_variants_under_all(concretize_scope, mock_packages):
 
     spec = Spec("callpath ^zmpi").concretized()
     assert "~foo" not in spec
+
+
+@pytest.mark.parametrize(
+    "packages_yaml,spec_str,expected_satisfies",
+    [
+        # In the tests below we set the compiler preference to "gcc" to be explicit on the
+        # fact that "clang" is not the preferred compiler. That helps making more robust the
+        # tests that verify enforcing "%clang" as a requirement.
+        (
+            """\
+    packages:
+      all:
+        compiler: ["gcc", "clang"]
+
+      libelf:
+        require:
+        - one_of: ["%clang"]
+          when: "@0.8.13"
+""",
+            "libelf",
+            [("@0.8.13%clang", True), ("%gcc", False)],
+        ),
+        (
+            """\
+    packages:
+      all:
+        compiler: ["gcc", "clang"]
+
+      libelf:
+        require:
+        - one_of: ["%clang"]
+          when: "@0.8.13"
+""",
+            "libelf@0.8.12",
+            [("%clang", False), ("%gcc", True)],
+        ),
+        (
+            """\
+    packages:
+      all:
+        compiler: ["gcc", "clang"]
+
+      libelf:
+        require:
+        - spec: "%clang"
+          when: "@0.8.13"
+""",
+            "libelf@0.8.12",
+            [("%clang", False), ("%gcc", True)],
+        ),
+        (
+            """\
+    packages:
+      all:
+        compiler: ["gcc", "clang"]
+
+      libelf:
+        require:
+        - spec: "@0.8.13"
+          when: "%clang"
+""",
+            "libelf@0.8.13%gcc",
+            [("%clang", False), ("%gcc", True), ("@0.8.13", True)],
+        ),
+    ],
+)
+def test_conditional_requirements_from_packages_yaml(
+    packages_yaml, spec_str, expected_satisfies, concretize_scope, mock_packages
+):
+    """Test that conditional requirements are required when the condition is met,
+    and optional when the condition is not met.
+    """
+    if spack.config.get("config:concretizer") == "original":
+        pytest.skip("Original concretizer does not support configuration requirements")
+
+    update_packages_config(packages_yaml)
+    spec = Spec(spec_str).concretized()
+    for match_str, expected in expected_satisfies:
+        assert spec.satisfies(match_str) is expected
+
+
+@pytest.mark.parametrize(
+    "packages_yaml,spec_str,expected_message",
+    [
+        (
+            """\
+    packages:
+      mpileaks:
+        require:
+        - one_of: ["~debug"]
+          message: "debug is not allowed"
+""",
+            "mpileaks+debug",
+            "debug is not allowed",
+        ),
+        (
+            """\
+    packages:
+      libelf:
+        require:
+        - one_of: ["%clang"]
+          message: "can only be compiled with clang"
+""",
+            "libelf%gcc",
+            "can only be compiled with clang",
+        ),
+        (
+            """\
+        packages:
+          libelf:
+            require:
+            - one_of: ["%clang"]
+              when: platform=test
+              message: "can only be compiled with clang on the test platform"
+    """,
+            "libelf%gcc",
+            "can only be compiled with clang on ",
+        ),
+        (
+            """\
+            packages:
+              libelf:
+                require:
+                - spec: "%clang"
+                  when: platform=test
+                  message: "can only be compiled with clang on the test platform"
+        """,
+            "libelf%gcc",
+            "can only be compiled with clang on ",
+        ),
+        (
+            """\
+        packages:
+          libelf:
+            require:
+            - one_of: ["%clang", "%intel"]
+              when: platform=test
+              message: "can only be compiled with clang or intel on the test platform"
+    """,
+            "libelf%gcc",
+            "can only be compiled with clang or intel",
+        ),
+    ],
+)
+def test_requirements_fail_with_custom_message(
+    packages_yaml, spec_str, expected_message, concretize_scope, mock_packages
+):
+    """Test that specs failing due to requirements not being satisfiable fail with a
+    custom error message.
+    """
+    if spack.config.get("config:concretizer") == "original":
+        pytest.skip("Original concretizer does not support configuration requirements")
+
+    update_packages_config(packages_yaml)
+    with pytest.raises(spack.error.SpackError, match=expected_message):
+        Spec(spec_str).concretized()
+
+
+def test_skip_requirement_when_default_requirement_condition_cannot_be_met(
+    concretize_scope, mock_packages
+):
+    """Tests that we can express a requirement condition under 'all' also in cases where
+    the corresponding condition spec mentions variants or versions that don't exist in the
+    package. For those packages the requirement rule is not emitted, since it can be
+    determined to be always false.
+    """
+    if spack.config.get("config:concretizer") == "original":
+        pytest.skip("Original concretizer does not support configuration requirements")
+
+    packages_yaml = """
+        packages:
+          all:
+            require:
+            - one_of: ["%clang"]
+              when: "+shared"
+    """
+    update_packages_config(packages_yaml)
+    s = Spec("mpileaks").concretized()
+
+    assert s.satisfies("%clang +shared")
+    # Sanity checks that 'callpath' doesn't have the shared variant, but that didn't
+    # cause failures during concretization.
+    assert "shared" not in s["callpath"].variants
+
+
+def test_requires_directive(concretize_scope, mock_packages):
+    if spack.config.get("config:concretizer") == "original":
+        pytest.skip("Original concretizer does not support configuration requirements")
+    compilers_yaml = pathlib.Path(concretize_scope) / "compilers.yaml"
+    compilers_yaml.write_text(
+        """
+compilers::
+- compiler:
+    spec: gcc@12.0.0
+    paths:
+      cc: /usr/bin/clang-12
+      cxx: /usr/bin/clang++-12
+      f77: null
+      fc: null
+    operating_system: debian6
+    target: x86_64
+    modules: []
+"""
+    )
+    spack.config.config.clear_caches()
+
+    # This package requires either clang or gcc
+    s = Spec("requires_clang_or_gcc").concretized()
+    assert s.satisfies("%gcc@12.0.0")
+
+    # This package can only be compiled with clang
+    with pytest.raises(spack.error.SpackError, match="can only be compiled with Clang"):
+        Spec("requires_clang").concretized()
diff --git a/lib/spack/spack/test/conftest.py b/lib/spack/spack/test/conftest.py
index cf67786136..3a6b36e85b 100644
--- a/lib/spack/spack/test/conftest.py
+++ b/lib/spack/spack/test/conftest.py
@@ -770,7 +770,7 @@ def concretize_scope(mutable_config, tmpdir):
         spack.config.ConfigScope("concretize", str(tmpdir.join("concretize")))
     )
 
-    yield
+    yield str(tmpdir.join("concretize"))
 
     mutable_config.pop_scope()
     spack.repo.path._provider_index = None
diff --git a/var/spack/repos/builtin.mock/packages/requires_clang/package.py b/var/spack/repos/builtin.mock/packages/requires_clang/package.py
new file mode 100644
index 0000000000..9f1c2d0ba4
--- /dev/null
+++ b/var/spack/repos/builtin.mock/packages/requires_clang/package.py
@@ -0,0 +1,18 @@
+# Copyright 2013-2023 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)
+
+from spack.package import *
+
+
+class RequiresClang(Package):
+    """Simple package with no dependencies"""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/b-1.0.tar.gz"
+
+    version("1.0", md5="0123456789abcdef0123456789abcdef")
+    version("0.9", md5="abcd456789abcdef0123456789abcdef")
+
+    requires("%clang", msg="can only be compiled with Clang")
diff --git a/var/spack/repos/builtin.mock/packages/requires_clang_or_gcc/package.py b/var/spack/repos/builtin.mock/packages/requires_clang_or_gcc/package.py
new file mode 100644
index 0000000000..18f924e92f
--- /dev/null
+++ b/var/spack/repos/builtin.mock/packages/requires_clang_or_gcc/package.py
@@ -0,0 +1,18 @@
+# Copyright 2013-2023 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)
+
+from spack.package import *
+
+
+class RequiresClangOrGcc(Package):
+    """Simple package with no dependencies"""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/b-1.0.tar.gz"
+
+    version("1.0", md5="0123456789abcdef0123456789abcdef")
+    version("0.9", md5="abcd456789abcdef0123456789abcdef")
+
+    requires("%gcc", "%clang", policy="one_of")
diff --git a/var/spack/repos/builtin/packages/bucky/package.py b/var/spack/repos/builtin/packages/bucky/package.py
index 5bd0c9fdf6..601729e82e 100644
--- a/var/spack/repos/builtin/packages/bucky/package.py
+++ b/var/spack/repos/builtin/packages/bucky/package.py
@@ -18,13 +18,7 @@ class Bucky(MakefilePackage):
 
     version("1.4.4", sha256="1621fee0d42314d9aa45d0082b358d4531e7d1d1a0089c807c1b21fbdc4e4592")
 
-    # Compilation requires gcc
-    conflicts("%cce")
-    conflicts("%apple-clang")
-    conflicts("%nag")
-    conflicts("%pgi")
-    conflicts("%xl")
-    conflicts("%xl_r")
+    requires("%gcc", msg="bucky can only be compiled with GCC")
 
     build_directory = "src"
 
diff --git a/var/spack/repos/builtin/packages/dyninst/package.py b/var/spack/repos/builtin/packages/dyninst/package.py
index c977fcc408..4079212ecf 100644
--- a/var/spack/repos/builtin/packages/dyninst/package.py
+++ b/var/spack/repos/builtin/packages/dyninst/package.py
@@ -87,19 +87,11 @@ class Dyninst(CMakePackage):
     patch("v9.3.2-auto.patch", when="@9.3.2 %gcc@:4.7")
     patch("tribool.patch", when="@9.3.0:10.0.0 ^boost@1.69:")
 
+    requires("%gcc", msg="dyninst builds only with GCC")
+
     # No Mac support (including apple-clang)
     conflicts("platform=darwin", msg="macOS is not supported")
 
-    # We currently only build with gcc
-    conflicts("%clang")
-    conflicts("%arm")
-    conflicts("%cce")
-    conflicts("%fj")
-    conflicts("%intel")
-    conflicts("%pgi")
-    conflicts("%xl")
-    conflicts("%xl_r")
-
     # Version 11.0 requires a C++11-compliant ABI
     conflicts("%gcc@:5", when="@11.0.0:")
 
diff --git a/var/spack/repos/builtin/packages/ffte/package.py b/var/spack/repos/builtin/packages/ffte/package.py
index 8b5b36c788..ed6de99aa4 100644
--- a/var/spack/repos/builtin/packages/ffte/package.py
+++ b/var/spack/repos/builtin/packages/ffte/package.py
@@ -32,12 +32,7 @@ class Ffte(Package):
 
     depends_on("mpi", when="+mpi")
 
-    conflicts("%cce", when="+cuda", msg="Must use NVHPC compiler")
-    conflicts("%clang", when="+cuda", msg="Must use NVHPC compiler")
-    conflicts("%gcc", when="+cuda", msg="Must use NVHPC compiler")
-    conflicts("%llvm", when="+cuda", msg="Must use NVHPC compiler")
-    conflicts("%nag", when="+cuda", msg="Must use NVHPC compiler")
-    conflicts("%intel", when="+cuda", msg="Must use NVHPC compiler")
+    requires("%nvhpc", when="+cuda", msg="ffte+cuda must use NVHPC compiler")
 
     def edit(self, spec, prefix):
         "No make-file, must create one from scratch."
diff --git a/var/spack/repos/builtin/packages/gcc/package.py b/var/spack/repos/builtin/packages/gcc/package.py
index 15bede82c0..259629d31e 100644
--- a/var/spack/repos/builtin/packages/gcc/package.py
+++ b/var/spack/repos/builtin/packages/gcc/package.py
@@ -273,9 +273,7 @@ class Gcc(AutotoolsPackage, GNUMirrorPackage):
         # See https://gcc.gnu.org/install/prerequisites.html#GDC-prerequisite
         with when("@12:"):
             # All versions starting 12 have to be built GCC:
-            for c in spack.compilers.supported_compilers():
-                if c != "gcc":
-                    conflicts("%{0}".format(c))
+            requires("%gcc")
 
             # And it has to be GCC older than the version we build:
             vv = ["11", "12.1.0", "12.2.0"]