qapi/visit.py: assert tag_member contains a QAPISchemaEnumType

This is true by design, but not presently able to be expressed in the
type system. An assertion helps mypy understand our constraints.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20201009161558.107041-35-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/visit.py

@@ -22,7 +22,7 @@
     mcgen,
 )
 from .gen import QAPISchemaModularCVisitor, ifcontext
-from .schema import QAPISchemaObjectType
+from .schema import QAPISchemaEnumType, QAPISchemaObjectType
 
 
 def gen_visit_decl(name, scalar=False):
@@ -84,15 +84,17 @@ def gen_visit_object_members(name, base, members, variants):
         ret += gen_endif(memb.ifcond)
 
     if variants:
+        tag_member = variants.tag_member
+        assert isinstance(tag_member.type, QAPISchemaEnumType)
+
         ret += mcgen('''
     switch (obj->%(c_name)s) {
 ''',
-                     c_name=c_name(variants.tag_member.name))
+                     c_name=c_name(tag_member.name))
 
         for var in variants.variants:
-            case_str = c_enum_const(variants.tag_member.type.name,
-                                    var.name,
-                                    variants.tag_member.type.prefix)
+            case_str = c_enum_const(tag_member.type.name, var.name,
+                                    tag_member.type.prefix)
             ret += gen_if(var.ifcond)
             if var.type.name == 'q_empty':
                 # valid variant and nothing to do