Make multimethods work with inheritance. (#3411)

Previously, this would fail with a NoSuchMethodError:

    class Package(object):
        # this is the default implementation
        def some_method(self):
            ...

    class Foo(Package):
        @when('platform=cray')
        def some_method(self):
            ...

        @when('platform=linux')
        def some_method(self):
            ...

This fixes the implementation of `@when` so that the superclass method
will be invoked when no subclass method matches.

Adds tests to ensure this works, as well.

lib/spack/spack/multimethod.py

@@ -128,10 +128,16 @@ def __call__(self, package_self, *args, **kwargs):
 
         if self.default:
             return self.default(package_self, *args, **kwargs)
+
         else:
-            raise NoSuchMethodError(
-                type(package_self), self.__name__, spec,
-                [m[0] for m in self.method_list])
+            superclass = super(package_self.__class__, package_self)
+            superclass_fn = getattr(superclass, self.__name__, None)
+            if callable(superclass_fn):
+                return superclass_fn(*args, **kwargs)
+            else:
+                raise NoSuchMethodError(
+                    type(package_self), self.__name__, spec,
+                    [m[0] for m in self.method_list])
 
     def __str__(self):
         return "SpecMultiMethod {\n\tdefault: %s,\n\tspecs: %s\n}" % (

lib/spack/spack/test/multimethod.py

@@ -118,3 +118,11 @@ def test_virtual_dep_match(builtin_mock):
 
     pkg = spack.repo.get('multimethod^mpich@1.0')
     assert pkg.different_by_virtual_dep() == 1
+
+
+def test_multimethod_with_base_class(builtin_mock):
+    pkg = spack.repo.get('multimethod@3')
+    assert pkg.base_method() == "subclass_method"
+
+    pkg = spack.repo.get('multimethod@1')
+    assert pkg.base_method() == "base_method"