Allow stubtest to raise errors on abstract state mismatch

mypy/stubtest.py

@@ -825,6 +825,13 @@ def verify_funcitem(
         if not callable(runtime):
             return
 
+    if isinstance(stub, nodes.FuncDef):
+        stub_abstract = stub.abstract_status == nodes.IS_ABSTRACT
+        runtime_abstract = getattr(runtime, "__isabstractmethod__", False)
+        # The opposite can exist: some implementations omit `@abstractmethod` decorators
+        if runtime_abstract and not stub_abstract:
+            yield Error(object_path, "runtime method is abstract, but stub is not", stub, runtime)
+
     for message in _verify_static_class_methods(stub, runtime, object_path):
         yield Error(object_path, "is inconsistent, " + message, stub, runtime)
 

mypy/test/teststubtest.py

@@ -1264,6 +1264,40 @@ def test_type_var(self) -> Iterator[Case]:
             )
             yield Case(stub="C = ParamSpec('C')", runtime="C = ParamSpec('C')", error=None)
 
+    @collect_cases
+    def test_abstract_methods(self) -> Iterator[Case]:
+        yield Case(
+            stub="""
+            from abc import abstractmethod
+
+            class A1:
+                def some(self) -> None: ...
+
+            class A2:
+                @abstractmethod
+                def some(self) -> None: ...
+
+            class A3:
+                @abstractmethod
+                def some(self) -> None: ...
+            """,
+            runtime="""
+            from abc import abstractmethod
+
+            class A1:
+                @abstractmethod
+                def some(self) -> None: ...
+
+            class A2:
+                @abstractmethod
+                def some(self) -> None: ...
+
+            class A3:
+                def some(self) -> None: ...
+            """,
+            error="A1.some",
+        )
+
 
 def remove_color_code(s: str) -> str:
     return re.sub("\\x1b.*?m", "", s)  # this works!