concretizer: require at least a dependency type to say the dependency holds

fixes #20784

Similarly to the previous bug, here we were deducing
conditions to be imposed on nodes that were not part
of the DAG.

lib/spack/spack/solver/concretize.lp

@@ -93,6 +93,9 @@ dependency_conditions_hold(ID, Parent, Dependency) :-
   attr(Name, Arg1, Arg2)       : required_dependency_condition(ID, Name, Arg1, Arg2);
   attr(Name, Arg1, Arg2, Arg3) : required_dependency_condition(ID, Name, Arg1, Arg2, Arg3);
   dependency_condition(ID, Parent, Dependency);
+  % There must be at least a dependency type declared,
+  % otherwise the dependency doesn't hold
+  dependency_type(ID, _);
   node(Parent);
   not external(Parent).
 

lib/spack/spack/test/concretize.py

@@ -1090,3 +1090,11 @@ def test_reuse_installed_packages(
         ).concretized()
 
         assert root.dag_hash() == new_root.dag_hash()
+
+    @pytest.mark.regression('20784')
+    def test_concretization_of_test_dependencies(self):
+        # With clingo we emit dependency_conditions regardless of the type
+        # of the dependency. We need to ensure that there's at least one
+        # dependency type declared to infer that the dependency holds.
+        s = Spec('test-dep-with-imposed-conditions').concretized()
+        assert 'c' not in s