Extends: support spec, not just package name (#27754)

var/spack/repos/builtin.mock/packages/extends-spec/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 ExtendsSpec(Package):
+    """Package that tests if the extends directive supports a spec."""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/example-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    extends('extendee@1:')