Stubtest: handle name-mangling edge cases better

mypy/stubtest.py

@@ -493,7 +493,7 @@ def verify_typeinfo(
     for entry in sorted(to_check):
         mangled_entry = entry
         if entry.startswith("__") and not entry.endswith("__"):
-            mangled_entry = f"_{stub.name}{entry}"
+            mangled_entry = f"_{stub.name.lstrip('_')}{entry}"
         stub_to_verify = next((t.names[entry].node for t in stub.mro if entry in t.names), MISSING)
         assert stub_to_verify is not None
         try:

mypy/test/teststubtest.py

@@ -1154,6 +1154,49 @@ def __mangle_bad(self, text): pass
             """,
             error="X.__mangle_bad",
         )
+        yield Case(
+            stub="""
+            class Klass:
+                class __Mangled1:
+                    class __Mangled2:
+                        def __mangle_good(self, text: str) -> None: ...
+                        def __mangle_bad(self, number: int) -> None: ...
+            """,
+            runtime="""
+            class Klass:
+                class __Mangled1:
+                    class __Mangled2:
+                        def __mangle_good(self, text): pass
+                        def __mangle_bad(self, text): pass
+            """,
+            error="Klass.__Mangled1.__Mangled2.__mangle_bad",
+        )
+        yield Case(
+            stub="""
+            class __Dunder__:
+                def __mangle_good(self, text: str) -> None: ...
+                def __mangle_bad(self, number: int) -> None: ...
+            """,
+            runtime="""
+            class __Dunder__:
+                def __mangle_good(self, text): pass
+                def __mangle_bad(self, text): pass
+            """,
+            error="__Dunder__.__mangle_bad",
+        )
+        yield Case(
+            stub="""
+            class _Private:
+                def __mangle_good(self, text: str) -> None: ...
+                def __mangle_bad(self, number: int) -> None: ...
+            """,
+            runtime="""
+            class _Private:
+                def __mangle_good(self, text): pass
+                def __mangle_bad(self, text): pass
+            """,
+            error="_Private.__mangle_bad",
+        )
 
     @collect_cases
     def test_mro(self) -> Iterator[Case]: