ASP-based solver: fix for unsplittable providers (#40859)

Some providers must provide virtuals "together", i.e.
if they provide one virtual of a set, they must be the
providers also of the others.

There was a bug though, where we were not checking if
the other virtuals in the set were needed at all in
the DAG.

This commit fixes the bug.

lib/spack/spack/solver/concretize.lp

@@ -492,7 +492,9 @@ error(100, "Package '{0}' needs to provide both '{1}' and '{2}' together, but pr
    pkg_fact(Package, provided_together(ID, SetID, Virtual2)),
    Virtual1 != Virtual2,
    attr("virtual_on_incoming_edges", node(X, Package), Virtual1),
-   not attr("virtual_on_incoming_edges", node(X, Package), Virtual2).
+   not attr("virtual_on_incoming_edges", node(X, Package), Virtual2),
+   attr("virtual_node", node(_, Virtual1)),
+   attr("virtual_node", node(_, Virtual2)).
 
 % if a package depends on a virtual, it's not external and we have a
 % provider for that virtual then it depends on the provider

lib/spack/spack/test/concretize.py

@@ -2360,3 +2360,16 @@ def test_condition_triggered_by_edge_property(
 
         for not_expected in expected_not_satisfies:
             assert not s.satisfies(not_expected), str(not_expected)
+
+    def test_virtuals_provided_together_but_only_one_required_in_dag(self):
+        """Tests that we can use a provider that provides more than one virtual together,
+        and is providing only one, iff the others are not needed in the DAG.
+
+        o blas-only-client
+        | [virtual=blas]
+        o openblas (provides blas and lapack together)
+
+        """
+        s = Spec("blas-only-client ^openblas").concretized()
+        assert s.satisfies("^[virtuals=blas] openblas")
+        assert not s.satisfies("^[virtuals=blas,lapack] openblas")