concretize.lp: fix issue with reuse of conditional variants (#43676)

Currently if you request pkg +example where example is a conditional
variant, and you have a pkg in the database for which the condition
did not hold (so no +example nor ~example), the solver would reuse it
regardless, not imposing +example.

The change rules out exactly one thing: variant_set without variant_value,
which in practice could only happen when not node_has_variant (i.e. when
under the current package.py rules the variant's when condition did not
trigger).

lib/spack/spack/solver/concretize.lp

@@ -891,12 +891,8 @@ error(100, "{0} variant '{1}' cannot have values '{2}' and '{3}' as they come fr
      Set1 < Set2, % see[1]
      build(node(ID, Package)).
 
-% variant_set is an explicitly set variant value. If it's not 'set',
-% we revert to the default value. If it is set, we force the set value
-attr("variant_value", PackageNode, Variant, Value)
- :- attr("node", PackageNode),
-    node_has_variant(PackageNode, Variant),
-    attr("variant_set", PackageNode, Variant, Value).
+:- attr("variant_set", node(ID, Package), Variant, Value),
+   not attr("variant_value", node(ID, Package), Variant, Value).
 
 % The rules below allow us to prefer default values for variants
 % whenever possible. If a variant is set in a spec, or if it is

lib/spack/spack/test/concretize.py

@@ -1350,6 +1350,22 @@ def test_reuse_installed_packages_when_package_def_changes(
         # Structure and package hash will be different without reuse
         assert root.dag_hash() != new_root_without_reuse.dag_hash()
 
+    @pytest.mark.only_clingo("Use case not supported by the original concretizer")
+    @pytest.mark.regression("43663")
+    def test_no_reuse_when_variant_condition_does_not_hold(self, mutable_database, mock_packages):
+        spack.config.set("concretizer:reuse", True)
+
+        # Install a spec for which the `version_based` variant condition does not hold
+        old = Spec("conditional-variant-pkg @1").concretized()
+        old.package.do_install(fake=True, explicit=True)
+
+        # Then explicitly require a spec with `+version_based`, which shouldn't reuse previous spec
+        new1 = Spec("conditional-variant-pkg +version_based").concretized()
+        assert new1.satisfies("@2 +version_based")
+
+        new2 = Spec("conditional-variant-pkg +two_whens").concretized()
+        assert new2.satisfies("@2 +two_whens +version_based")
+
     @pytest.mark.only_clingo("Use case not supported by the original concretizer")
     def test_reuse_with_flags(self, mutable_database, mutable_config):
         spack.config.set("concretizer:reuse", True)