qapi: Hoist tag collision check to Variants.check()

Checking that a given QAPISchemaObjectTypeVariant.name is a
member of the corresponding QAPISchemaEnumType of the owning
QAPISchemaObjectTypeVariants.tag_member ensures that there are
no collisions in the generated C union for those tag values
(since the enum itself should have no collisions).

However, ever since its introduction in f51d8c3d, this was the
only additional action of of Variant.check(), beyond calling
the superclass Member.check().  This forces a difference in
.check() signatures, just to pass the enum type down.

Simplify things by instead doing the tag name check as part of
Variants.check(), at which point we can rely on inheritance
instead of overriding Variant.check().

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1447836791-369-14-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi.py

@@ -1059,7 +1059,8 @@ class QAPISchemaObjectTypeVariants(object):
             self.tag_member = seen[self.tag_name]
         assert isinstance(self.tag_member.type, QAPISchemaEnumType)
         for v in self.variants:
-            v.check(schema, self.tag_member.type)
+            v.check(schema)
+            assert v.name in self.tag_member.type.values
             if isinstance(v.type, QAPISchemaObjectType):
                 v.type.check(schema)
 
@@ -1075,10 +1076,6 @@ class QAPISchemaObjectTypeVariant(QAPISchemaObjectTypeMember):
     def __init__(self, name, typ):
         QAPISchemaObjectTypeMember.__init__(self, name, typ, False)
 
-    def check(self, schema, tag_type):
-        QAPISchemaObjectTypeMember.check(self, schema)
-        assert self.name in tag_type.values
-
     # This function exists to support ugly simple union special cases
     # TODO get rid of them, and drop the function
     def simple_union_type(self):