Add a "sticky" property to variants (#28630)

* Add sticky variants

* Add unit tests for sticky variants

* Add documentation for sticky variants

* Revert "Revert 19736 because conflicts are avoided by clingo by default (#26721)"

This reverts commit 33ef7d57c1.

* Add stickiness to "allow-unsupported-compiler"

lib/spack/docs/packaging_guide.rst

@@ -1441,6 +1441,32 @@ The ``when`` clause follows the same syntax and accepts the same
 values as the ``when`` argument of
 :py:func:`spack.directives.depends_on`
 
+^^^^^^^^^^^^^^^
+Sticky Variants
+^^^^^^^^^^^^^^^
+
+The variant directive can be marked as ``sticky`` by setting to ``True`` the
+corresponding argument:
+
+.. code-block:: python
+
+   variant('bar', default=False, sticky=True)
+
+A ``sticky`` variant differs from a regular one in that it is always set
+to either:
+
+#. An explicit value appearing in a spec literal or
+#. Its default value
+
+The concretizer thus is not free to pick an alternate value to work
+around conflicts, but will error out instead.
+Setting this property on a variant is useful in cases where the
+variant allows some dangerous or controversial options (e.g. using unsupported versions
+of a compiler for a library) and the packager wants to ensure that
+allowing these options is done on purpose by the user, rather than
+automatically by the solver.
+
+
 ^^^^^^^^^^^^^^^^^^^
 Overriding Variants
 ^^^^^^^^^^^^^^^^^^^