stubtest: do not error if a stub is async, but runtime is not (#12234)

Author: AlexWaygood

mypy/stubtest.py

@@ -700,18 +700,6 @@ def _verify_signature(
         yield 'runtime does not have **kwargs argument "{}"'.format(stub.varkw.variable.name)
 
 
-def _verify_coroutine(
-    stub: nodes.FuncItem, runtime: Any, *, runtime_is_coroutine: bool
-) -> Optional[str]:
-    if stub.is_coroutine:
-        if not runtime_is_coroutine:
-            return 'is an "async def" function in the stub, but not at runtime'
-    else:
-        if runtime_is_coroutine:
-            return 'is an "async def" function at runtime, but not in the stub'
-    return None
-
-
 @verify.register(nodes.FuncItem)
 def verify_funcitem(
     stub: nodes.FuncItem, runtime: MaybeMissing[Any], object_path: List[str]
@@ -735,21 +723,20 @@ def verify_funcitem(
         stub_sig = Signature.from_funcitem(stub)
         runtime_sig = Signature.from_inspect_signature(signature)
         runtime_sig_desc = f'{"async " if runtime_is_coroutine else ""}def {signature}'
+        stub_desc = f'def {stub_sig!r}'
     else:
-        runtime_sig_desc = None
-
-    coroutine_mismatch_error = _verify_coroutine(
-        stub,
-        runtime,
-        runtime_is_coroutine=runtime_is_coroutine
-    )
+        runtime_sig_desc, stub_desc = None, None
 
-    if coroutine_mismatch_error is not None:
+    # Don't raise an error if the stub is a coroutine, but the runtime isn't.
+    # That results in false positives.
+    # See https://github.com/python/typeshed/issues/7344
+    if runtime_is_coroutine and not stub.is_coroutine:
         yield Error(
             object_path,
-            coroutine_mismatch_error,
+            'is an "async def" function at runtime, but not in the stub',
             stub,
             runtime,
+            stub_desc=stub_desc,
             runtime_desc=runtime_sig_desc
         )
 

mypy/test/teststubtest.py

@@ -209,16 +209,17 @@ class X:
 
     @collect_cases
     def test_coroutines(self) -> Iterator[Case]:
-        yield Case(
-            stub="async def foo() -> int: ...",
-            runtime="def foo(): return 5",
-            error="foo",
-        )
         yield Case(
             stub="def bar() -> int: ...",
             runtime="async def bar(): return 5",
             error="bar",
         )
+        # Don't error for this one -- we get false positives otherwise
+        yield Case(
+            stub="async def foo() -> int: ...",
+            runtime="def foo(): return 5",
+            error=None,
+        )
         yield Case(
             stub="def baz() -> int: ...",
             runtime="def baz(): return 5",