Allow conditional possible values in variants (#29530)

Allow declaring possible values for variants with an associated condition. If the variant takes one of those values, the condition is imposed as a further constraint.

The idea of this PR is to implement part of the mechanisms needed for modeling [packages with multiple build-systems]( https://github.com/spack/seps/pull/3). After this PR the build-system directive can be implemented as:
```python
variant(
    'build-system',
    default='cmake',
    values=(
        'autotools',
        conditional('cmake', when='@X.Y:')
    ), 
    description='...',
)
```

Modifications:
- [x] Allow conditional possible values in variants
- [x] Add a unit-test for the feature
- [x] Add documentation

1 files changed, 34 insertions, 0 deletions

diff --git a/var/spack/repos/builtin.mock/packages/conditional-values-in-variant/package.py b/var/spack/repos/builtin.mock/packages/conditional-values-in-variant/package.py
new file mode 100644
index 0000000000..74895cb7c0
--- /dev/null
+++ b/var/spack/repos/builtin.mock/packages/conditional-values-in-variant/package.py
@@ -0,0 +1,34 @@
+# Copyright 2013-2022 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)
+class ConditionalValuesInVariant(Package):
+    """Package with conditional possible values in a variant"""
+    homepage = "https://dev.null"
+
+    version('1.73.0')
+    version('1.72.0')
+    version('1.62.0')
+    version('1.60.0')
+    version('1.50.0')
+
+    variant(
+        'cxxstd', default='98',
+        values=(
+            '98', '11', '14',
+            # C++17 is not supported by Boost < 1.63.0.
+            conditional('17', when='@1.63.0:'),
+            # C++20/2a is not support by Boost < 1.73.0
+            conditional('2a', when='@1.73.0:')
+        ),
+        multi=False,
+        description='Use the specified C++ standard when building.',
+        when='@1.60.0:'
+    )
+
+    variant(
+        'staging', values=any_combination_of(
+            conditional('flexpath', 'dataspaces', when='@1.73.0:')
+        ),
+        description='Enable dataspaces and/or flexpath staging transports'
+    )