Add a unit test to prevent regression

var/spack/repos/builtin.mock/packages/low-priority-provider/package.py

@@ -0,0 +1,18 @@
+# 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 LowPriorityProvider(Package):
+    """Provides multiple virtuals but is low in the priority of clingo"""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/a-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    provides('lapack')
+    provides('mpi')

var/spack/repos/builtin.mock/packages/many-virtual-consumer/package.py

@@ -0,0 +1,18 @@
+# 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)
+class ManyVirtualConsumer(Package):
+    """PAckage that depends on many virtual packages"""
+    url = "http://www.example.com/"
+    url = "http://www.example.com/2.0.tar.gz"
+
+    version('1.0', 'abcdef1234567890abcdef1234567890')
+
+    depends_on('mpi')
+    depends_on('lapack')
+
+    # This directive is an example of imposing a constraint on a
+    # dependency is that dependency is in the DAG. This pattern
+    # is mainly used with virtual providers.
+    depends_on('low-priority-provider@1.0', when='^low-priority-provider')