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

@@ -88,6 +88,9 @@ 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

@@ -1089,3 +1089,11 @@ 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

var/spack/repos/builtin.mock/packages/test-dep-with-imposed-conditions/package.py

@@ -0,0 +1,17 @@
+# Copyright 2013-2021 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 TestDepWithImposedConditions(Package):
+    """Simple package with no dependencies"""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/e-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    depends_on('c@1.0', type='test')