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"

var/spack/repos/builtin.mock/packages/sticky-variant/package.py

@@ -0,0 +1,18 @@
+# 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)
+from spack import *
+
+
+class StickyVariant(AutotoolsPackage):
+    """Package with a sticky variant and a conflict"""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/a-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    variant('allow-gcc', description='', default=False, sticky=True)
+
+    conflicts('%gcc', when='~allow-gcc')

var/spack/repos/builtin/packages/cuda/package.py

@@ -144,6 +144,8 @@ class Cuda(Package):
     conflicts('arch=darwin-mojave-x86_64')
 
     variant('dev', default=False, description='Enable development dependencies, i.e to use cuda-gdb')
+    variant('allow-unsupported-compilers', default=False, sticky=True,
+            description='Allow unsupported host compiler and CUDA version combinations')
 
     depends_on('libxml2', when='@10.1.243:')
     # cuda-gdb needed libncurses.so.5 before 11.4.0