Only return a declared type from __new__ if it is a subtype of the class (#7656)

msullivan · web-flow · commit c95ecc1952b4 · 2019-10-09T13:18:42.000-07:00
Fixes #7597.
diff --git a/mypy/typeops.py b/mypy/typeops.py
@@ -77,11 +77,20 @@ def class_callable(init_type: CallableType, info: TypeInfo, type_type: Instance,
     variables.extend(info.defn.type_vars)
     variables.extend(init_type.variables)
 
+    from mypy.subtypes import is_subtype
+
     init_ret_type = get_proper_type(init_type.ret_type)
-    if is_new and isinstance(init_ret_type, (Instance, TupleType)):
-        ret_type = init_type.ret_type  # type: Type
+    default_ret_type = fill_typevars(info)
+    if (
+        is_new
+        and isinstance(init_ret_type, (Instance, TupleType))
+        # Only use the return type from __new__ if it is actually returning
+        # a subtype of what we would return otherwise.
+        and is_subtype(init_ret_type, default_ret_type, ignore_type_params=True)
+    ):
+        ret_type = init_ret_type  # type: Type
     else:
-        ret_type = fill_typevars(info)
+        ret_type = default_ret_type
 
     callable_type = init_type.copy_modified(
         ret_type=ret_type, fallback=type_type, name=None, variables=variables,
diff --git a/test-data/unit/check-classes.test b/test-data/unit/check-classes.test
@@ -6031,7 +6031,7 @@ 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 'builtins.int'
+reveal_type(A())  # N: Revealed type is '__main__.A'
 
 [case testNewReturnType4]
 from typing import TypeVar, Type
@@ -6099,6 +6099,16 @@ class X:
     def __new__(cls, x: TX) -> TX:  # E: "__new__" must return a class instance (got "TX")
         pass
 
+[case testNewReturnType9]
+class A:
+    def __new__(cls) -> A:
+        pass
+
+class B(A):
+    pass
+
+reveal_type(B())  # N: Revealed type is '__main__.B'
+
 [case testGenericOverride]
 from typing import Generic, TypeVar, Any
 
