[stubgen] Add known return types to magic methods (#13909)

This is based on the idea of https://github.com/JelleZijlstra/autotyping
Other magic methods can be added later.

CC @JelleZijlstra

Author: sobolevn

mypy/stubgen.py

@@ -189,6 +189,22 @@
     "__iter__",
 }
 
+# These magic methods always return the same type.
+KNOWN_MAGIC_METHODS_RETURN_TYPES: Final = {
+    "__len__": "int",
+    "__length_hint__": "int",
+    "__init__": "None",
+    "__del__": "None",
+    "__bool__": "bool",
+    "__bytes__": "bytes",
+    "__format__": "str",
+    "__contains__": "bool",
+    "__complex__": "complex",
+    "__int__": "int",
+    "__float__": "float",
+    "__index__": "int",
+}
+
 
 class Options:
     """Represents stubgen options.
@@ -735,6 +751,8 @@ def visit_func_def(
             # Always assume abstract methods return Any unless explicitly annotated. Also
             # some dunder methods should not have a None return type.
             retname = None  # implicit Any
+        elif o.name in KNOWN_MAGIC_METHODS_RETURN_TYPES:
+            retname = KNOWN_MAGIC_METHODS_RETURN_TYPES[o.name]
         elif has_yield_expression(o):
             self.add_abc_import("Generator")
             yield_name = "None"

test-data/unit/stubgen.test

@@ -2705,3 +2705,32 @@ def f():
     return 0
 [out]
 def f(): ...
+
+[case testKnownMagicMethodsReturnTypes]
+class Some:
+    def __len__(self): ...
+    def __length_hint__(self): ...
+    def __init__(self): ...
+    def __del__(self): ...
+    def __bool__(self): ...
+    def __bytes__(self): ...
+    def __format__(self, spec): ...
+    def __contains__(self, obj): ...
+    def __complex__(self): ...
+    def __int__(self): ...
+    def __float__(self): ...
+    def __index__(self): ...
+[out]
+class Some:
+    def __len__(self) -> int: ...
+    def __length_hint__(self) -> int: ...
+    def __init__(self) -> None: ...
+    def __del__(self) -> None: ...
+    def __bool__(self) -> bool: ...
+    def __bytes__(self) -> bytes: ...
+    def __format__(self, spec) -> str: ...
+    def __contains__(self, obj) -> bool: ...
+    def __complex__(self) -> complex: ...
+    def __int__(self) -> int: ...
+    def __float__(self) -> float: ...
+    def __index__(self) -> int: ...