fix MRO for multimethod.__call__ using iterative algorithm.

Add tests MRO for inherited multimethods with multiple inheritance
Add tests for inherited and overridden multimethods

var/spack/repos/builtin.mock/packages/multimethod-base/package.py

@@ -19,3 +19,6 @@ class when subclass multi-methods do not match.
 
     def base_method(self):
         return "base_method"
+
+    def diamond_inheritance(self):
+        return "base_package"

var/spack/repos/builtin.mock/packages/multimethod-diamond-parent/package.py

@@ -0,0 +1,21 @@
+# Copyright 2013-2018 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.pkg.builtin.mock.multimethod_base import MultimethodBase
+
+
+class MultimethodDiamondParent(MultimethodBase):
+    """This package is designed for use with Spack's multimethod test.
+       It has a bunch of test cases for the @when decorator that the
+       test uses.
+    """
+
+    @when('@3.0')
+    def diamond_inheritance(self):
+        return "second_parent"
+
+    @when('@4.0, 2.0')
+    def diamond_inheritance(self):
+        return "should never be reached"

var/spack/repos/builtin.mock/packages/multimethod-diamond/package.py

@@ -0,0 +1,18 @@
+# Copyright 2013-2018 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)
+
+import spack.pkg.builtin.mock.multimethod_inheritor as mi
+import spack.pkg.builtin.mock.multimethod_diamond_parent as mp
+
+
+class MultimethodDiamond(mi.MultimethodInheritor, mp.MultimethodDiamondParent):
+    """This package is designed for use with Spack's multimethod test.
+       It has a bunch of test cases for the @when decorator that the
+       test uses.
+    """
+
+    @when('@4.0')
+    def diamond_inheritance(self):
+        return 'subclass'

var/spack/repos/builtin.mock/packages/multimethod-inheritor/package.py

@@ -11,3 +11,14 @@ class MultimethodInheritor(Multimethod):
        It has a bunch of test cases for the @when decorator that the
        test uses.
     """
+
+    @when('@1.0')
+    def inherited_and_overridden(self):
+        return "inheritor@1.0"
+
+    #
+    # Test multi-level inheritance
+    #
+    @when('@2:')
+    def base_method(self):
+        return 'multimethod-inheritor'

var/spack/repos/builtin.mock/packages/multimethod/package.py

@@ -124,4 +124,28 @@ def different_by_virtual_dep(self):
     #
     @when("@2:")
     def base_method(self):
-        return "subclass_method"
+        return 'multimethod'
+
+    #
+    # Make sure methods with non-default implementations in a superclass
+    # will invoke those methods when none in the subclass match but one in
+    # the superclass does.
+    #
+    @when("@1.0")
+    def inherited_and_overridden(self):
+        return "base@1.0"
+
+    @when("@2.0")
+    def inherited_and_overridden(self):
+        return "base@2.0"
+
+    #
+    # Make sure that multimethods follow MRO properly with diamond inheritance
+    #
+    @when('@2.0')
+    def diamond_inheritance(self):
+        return 'first_parent'
+
+    @when('@4.0')
+    def diamond_inheritance(self):
+        return "should_not_be_reached"