ASP-based solver: add a rule for version uniqueness in virtual packages (#26740)

fixes #26718

A virtual package may or may not have a version, but it
never has more than one. Previously we were missing a rule
for that.

lib/spack/spack/solver/concretize.lp

@@ -28,6 +28,9 @@ version_declared(Package, Version) :- version_declared(Package, Version, _).
 1 { version(Package, Version) : version_declared(Package, Version) } 1
  :- node(Package).
 
+% A virtual package may have or not a version, but never has more than one
+:- virtual_node(Package), 2 { version(Package, _) }.
+
 % If we select a deprecated version, mark the package as deprecated
 deprecated(Package, Version) :- version(Package, Version), deprecated_version(Package, Version).
 

lib/spack/spack/test/concretize.py

@@ -1267,3 +1267,10 @@ def test_newer_dependency_adds_a_transitive_virtual(self):
         s = spack.spec.Spec('root-adds-virtual').concretized()
         assert s['leaf-adds-virtual'].satisfies('@2.0')
         assert 'blas' in s
+
+    @pytest.mark.regression('26718')
+    def test_versions_in_virtual_dependencies(self):
+        # Ensure that a package that needs a given version of a virtual
+        # package doesn't end up using a later implementation
+        s = spack.spec.Spec('hpcviewer@2019.02').concretized()
+        assert s['java'].satisfies('virtual-with-versions@1.8.0')