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/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"