Believe arbitrary __new__ return types

Fixes #1020 (comment)
Surprisingly popular comment on a closed issue.

We still issue the warning, but we do trust the return type instead of
overruling it.

Maybe fixes #16012

Author: hauntsaninja

mypy/typeops.py

@@ -197,9 +197,9 @@ def class_callable(
         # by accident. Like `Hashable` is a subtype of `object`. See #11799
         and isinstance(default_ret_type, Instance)
         and not default_ret_type.type.is_protocol
-        # Only use the declared return type from __new__ or declared self in __init__
+        # Only use the declared return type from declared self in __init__
         # if it is actually returning a subtype of what we would return otherwise.
-        and is_subtype(explicit_type, default_ret_type, ignore_type_params=True)
+        and (is_new or is_subtype(explicit_type, default_ret_type, ignore_type_params=True))
     ):
         ret_type: Type = explicit_type
     else:

test-data/unit/check-classes.test

@@ -6883,7 +6883,7 @@ class A:
     def __new__(cls) -> int:  # E: Incompatible return type for "__new__" (returns "int", but must return a subtype of "A")
         pass
 
-reveal_type(A())  # N: Revealed type is "__main__.A"
+reveal_type(A())  # N: Revealed type is "builtins.int"
 
 [case testNewReturnType4]
 from typing import TypeVar, Type
@@ -6960,7 +6960,7 @@ class A:
 class B(A):
     pass
 
-reveal_type(B())  # N: Revealed type is "__main__.B"
+reveal_type(B())  # N: Revealed type is "__main__.A"
 
 [case testNewReturnType10]
 # https://github.com/python/mypy/issues/11398

test-data/unit/check-enum.test

@@ -1360,7 +1360,7 @@ class Bar(Foo):
     A = 1
     B = 2
 
-a = Bar.A
+a = Bar.A  # E: "Type[Foo]" has no attribute "A"
 reveal_type(a.value)    # N: Revealed type is "Any"
 reveal_type(a._value_)  # N: Revealed type is "Any"
 [builtins fixtures/primitives.pyi]