Make return type implicitly None for type checked __init__ and __init…

…_subclass__ (python#5677)

Implements python#604 (comment).

mypy/semanal.py

@@ -68,7 +68,7 @@
 from mypy.types import (
  FunctionLike, UnboundType, TypeVarDef, TupleType, UnionType, StarType, function_type,
  CallableType, Overloaded, Instance, Type, AnyType,
- TypeTranslator, TypeOfAny, TypeType,
+ TypeTranslator, TypeOfAny, TypeType, NoneTyp,
 )
 from mypy.nodes import implicit_module_attrs
 from mypy.typeanal import (
@@ -407,6 +407,10 @@ def _visit_func_def(self, defn: FuncDef) -> None:
  add_symbol = False
  if add_symbol:
  self.type.names[defn.name()] = SymbolTableNode(MDEF, defn)
+ if defn.type is not None and defn.name() in ('__init__', '__init_subclass__'):
+ assert isinstance(defn.type, CallableType)
+ if isinstance(defn.type.ret_type, AnyType):
+ defn.type = defn.type.copy_modified(ret_type=NoneTyp())
  self.prepare_method_signature(defn, self.type)
  elif self.is_func_scope():
  # Nested function

test-data/unit/check-classes.test

@@ -577,7 +577,61 @@ import typing
 class A:
  def __init__(self, x: int): pass
 [out]
-main:3: error: The return type of "__init__" must be None
+
+[case testDecoratedConstructorWithImplicitReturnValueType]
+import typing
+from typing import Callable
+
+def deco(fn: Callable) -> Callable:
+ return fn
+
+class A:
+ @deco
+ def __init__(self, x: int): pass
+[out]
+
+[case testOverloadedConstructorWithImplicitReturnValueType]
+from foo import *
+[file foo.pyi]
+from typing import overload
+class Foo:
+ @overload
+ def __init__(self, a: int):
+ pass
+
+ @overload
+ def __init__(self, a: str):
+ pass
+
+[case testConstructorWithAnyReturnValueType]
+import typing
+from typing import Any
+class A:
+ def __init__(self) -> Any: pass # E: The return type of "__init__" must be None
+
+[case testDecoratedConstructorWithAnyReturnValueType]
+import typing
+from typing import Callable, Any
+
+def deco(fn: Callable) -> Callable:
+ return fn
+
+class A:
+ @deco
+ def __init__(self) -> Any: pass # E: The return type of "__init__" must be None
+
+[case testOverloadedConstructorWithAnyReturnValueType]
+from foo import *
+[file foo.pyi]
+from typing import overload, Any
+class Foo:
+ @overload
+ def __init__(self, a: int) -> Any: # E: The return type of "__init__" must be None
+ pass
+
+ @overload
+ def __init__(self, a: str) -> Any: # E: The return type of "__init__" must be None
+ pass
 
 [case testInitSubclassWithReturnValueType]
 import typing
@@ -591,7 +645,62 @@ import typing
 class A:
  def __init_subclass__(cls, x: int=1): pass
 [out]
-main:3: error: The return type of "__init_subclass__" must be None
+
+[case testDecoratedInitSubclassWithImplicitReturnValueType]
+import typing
+from typing import Callable
+
+def deco(fn: Callable) -> Callable:
+ return fn
+
+class A:
+ @deco
+ def __init_subclass__(cls, x: int=1): pass
+[out]
+
+[case testOverloadedInitSubclassWithImplicitReturnValueType]
+from foo import *
+[file foo.pyi]
+from typing import overload
+class Foo:
+ @overload
+ def __init_subclass__(cls, a: int):
+ pass
+
+ @overload
+ def __init_subclass__(cls, a: str):
+ pass
+
+[case testInitSubclassWithAnyReturnValueType]
+import typing
+from typing import Any
+class A:
+ def __init_subclass__(cls) -> Any: pass # E: The return type of "__init_subclass__" must be None
+
+[case testDecoratedInitSubclassWithAnyReturnValueType]
+import typing
+from typing import Callable, Any
+
+def deco(fn: Callable) -> Callable:
+ return fn
+
+class A:
+ @deco
+ def __init_subclass__(cls) -> Any: pass # E: The return type of "__init_subclass__" must be None
+[out]
+
+[case testOverloadedInitSubclassWithAnyReturnValueType]
+from foo import *
+[file foo.pyi]
+from typing import overload, Any
+class Foo:
+ @overload
+ def __init_subclass__(cls, a: int) -> Any: # E: The return type of "__init_subclass__" must be None
+ pass
+
+ @overload
+ def __init_subclass__(cls, a: str) -> Any: # E: The return type of "__init_subclass__" must be None
+ pass
 
 [case testGlobalFunctionInitWithReturnType]
 import typing