Tweak generator function return type error wording

Author: ddfisher

mypy/messages.py

@@ -30,7 +30,7 @@
 INVALID_EXCEPTION = 'Exception must be derived from BaseException'
 INVALID_EXCEPTION_TYPE = 'Exception type must be derived from BaseException'
 INVALID_RETURN_TYPE_FOR_YIELD = \
-    'The return type of a generator function should be "Generator" or a supertype thereof'
+    'The return type of a generator function should be "Generator" or one of its supertypes'
 INVALID_RETURN_TYPE_FOR_YIELD_FROM = \
     'Iterable function return type expected for "yield from"'
 INCOMPATIBLE_TYPES = 'Incompatible types'

mypy/test/data/check-statements.test

@@ -676,7 +676,7 @@ def f() -> Any:
 [case testYieldInFunctionReturningFunction]
 from typing import Callable
 def f() -> Callable[[], None]:
-    yield object() # E: The return type of a generator function should be "Generator" or a supertype thereof
+    yield object() # E: The return type of a generator function should be "Generator" or one of its supertypes
 [out]
 main: note: In function "f":
 
@@ -688,7 +688,7 @@ def f():
 [case testWithInvalidInstanceReturnType]
 import typing
 def f() -> int:
-    yield 1 # E: The return type of a generator function should be "Generator" or a supertype thereof
+    yield 1 # E: The return type of a generator function should be "Generator" or one of its supertypes
 [builtins fixtures/for.py]
 [out]
 main: note: In function "f":