concretize.lp: impose a lower bound on the number of version facts if a solution exists (#31142)

* concretize.lp: impose a lower bound on the number of version facts if a valid version exists

fixes #30864

* Add a unit test

lib/spack/spack/solver/concretize.lp

@@ -120,6 +120,11 @@ version_weight(Package, Weight)
 { version(Package, Version) : version_satisfies(Package, Constraint, Version) }
   :- node_version_satisfies(Package, Constraint).
 
+% If there is at least a version that satisfy the constraint, impose a lower
+% bound on the choice rule to avoid false positives with the error below
+1 { version(Package, Version) : version_satisfies(Package, Constraint, Version) }
+  :- node_version_satisfies(Package, Constraint), version_satisfies(Package, Constraint, _).
+
 % More specific error message if the version cannot satisfy some constraint
 % Otherwise covered by `no_version_error` and `versions_conflict_error`.
 error(1, "No valid version for '{0}' satisfies '@{1}'", Package, Constraint)

lib/spack/spack/test/concretize.py

@@ -1732,3 +1732,24 @@ def test_best_effort_coconcretize_preferences(
             if expected_spec in spec:
                 counter += 1
         assert counter == occurances, concrete_specs
+
+    @pytest.mark.regression('30864')
+    def test_misleading_error_message_on_version(self, mutable_database):
+        # For this bug to be triggered we need a reusable dependency
+        # that is not optimal in terms of optimization scores.
+        # We pick an old version of "b"
+        import spack.solver.asp
+        if spack.config.get('config:concretizer') == 'original':
+            pytest.skip('Original concretizer cannot reuse')
+
+        reusable_specs = [
+            spack.spec.Spec('non-existing-conditional-dep@1.0').concretized()
+        ]
+        root_spec = spack.spec.Spec('non-existing-conditional-dep@2.0')
+
+        with spack.config.override("concretizer:reuse", True):
+            solver = spack.solver.asp.Solver()
+            setup = spack.solver.asp.SpackSolverSetup()
+            with pytest.raises(spack.solver.asp.UnsatisfiableSpecError,
+                               match="'dep-with-variants' satisfies '@999'"):
+                solver.driver.solve(setup, [root_spec], reuse=reusable_specs)