Fix some name-mangling edge cases

stubdefaulter.py

@@ -215,7 +215,10 @@ def gather_funcs(
             return
         for child_name, child_node in node.child_nodes.items():
             if child_name.startswith("__") and not child_name.endswith("__"):
-                maybe_mangled_child_name = f"_{name}{child_name}"
+                unmangled_parent_name = fullname.split(".")[-1]
+                maybe_mangled_child_name = (
+                    f"_{unmangled_parent_name.lstrip('_')}{child_name}"
+                )
             else:
                 maybe_mangled_child_name = child_name
             yield from gather_funcs(

test_stubdefaulter.py

@@ -42,6 +42,10 @@ def method(self, a=False):
                 pass
             async def async_method(self, b=3.14):
                 pass
+    class __Mangled1:
+        class __Mangled2:
+            def __mangled(self, x=True):
+                pass
 
 def overloaded(x=False):
     return 1 if x else "1"
@@ -84,6 +88,9 @@ class NestedKlass1:
         class NestedKlass2:
             def method(self, a: bool = ...) -> None: ...
             async def async_method(self, b: float = ...) -> None: ...
+    class __Mangled1:
+        class __Mangled2:
+            def __mangled(self, x: bool = ...) -> None: ...
 
 @overload
 def overloaded(x: Literal[False] = ...) -> str: ...
@@ -124,6 +131,9 @@ class NestedKlass1:
         class NestedKlass2:
             def method(self, a: bool = False) -> None: ...
             async def async_method(self, b: float = 3.14) -> None: ...
+    class __Mangled1:
+        class __Mangled2:
+            def __mangled(self, x: bool = True) -> None: ...
 
 @overload
 def overloaded(x: Literal[False] = False) -> str: ...