qapi/schema: make c_type() and json_type() abstract methods

These methods should always return a str, it's only the default abstract
implementation that doesn't. They can be marked "abstract", which
requires subclasses to override the method with the proper return type.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240315152301.3621858-9-armbru@redhat.com>

scripts/qapi/schema.py

@@ -16,6 +16,7 @@
 
 # TODO catching name collisions in generated code would be nice
 
+from abc import ABC, abstractmethod
 from collections import OrderedDict
 import os
 import re
@@ -245,9 +246,10 @@ def visit(self, visitor):
         visitor.visit_include(self._sub_module.name, self.info)
 
 
-class QAPISchemaType(QAPISchemaDefinition):
+class QAPISchemaType(QAPISchemaDefinition, ABC):
     # Return the C type for common use.
     # For the types we commonly box, this is a pointer type.
+    @abstractmethod
     def c_type(self):
         pass
 
@@ -259,6 +261,7 @@ def c_param_type(self):
     def c_unboxed_type(self):
         return self.c_type()
 
+    @abstractmethod
     def json_type(self):
         pass