Support in assignments too

Author: sterliakov

mypy/checker.py

@@ -2241,7 +2241,11 @@ def check_method_override_for_base_with_name(
 
         if name == "__hash__" and isinstance(original_type, NoneType):
             # Allow defining `__hash__` even if parent class was explicitly made unhashable
-            return False
+            if base.fullname == "builtins.object":
+                # This is only for test stubs to avoid adding object.__hash__
+                # to all of them.
+                return True
+            return self.check_method_override_for_base_with_name(defn, name, base.mro[-1])
 
         if (
             is_property(defn)
@@ -3493,7 +3497,7 @@ def check_compatibility_all_supers(self, lvalue: RefExpr, rvalue: Expression) ->
                                 lvalue, lvalue_type, base_type, base
                             )
                             return
-                    if base is last_immediate_base:
+                    if base is last_immediate_base and base_node.name != "__hash__":
                         # At this point, the attribute was found to be compatible with all
                         # immediate parents.
                         break
@@ -3507,6 +3511,9 @@ def check_compatibility_super(
         base_node: Node,
         always_allow_covariant: bool,
     ) -> bool:
+        if base_node.name == "__hash__" and isinstance(base_type, NoneType):
+            # Allow defining `__hash__` even if parent class was explicitly made unhashable
+            return True
         # TODO: check __set__() type override for custom descriptors.
         # TODO: for descriptors check also class object access override.
         ok = self.check_subtype(

test-data/unit/check-classes.test

@@ -155,7 +155,30 @@ class A:
     __hash__ = None
 
 class B(A):
-    def __hash__(self) -> int: return 0
+    def __hash__(self) -> int:
+        return 0
+
+class C(A):
+    __hash__ = object.__hash__
+
+class D(A):
+    def __hash__(self, x: int) -> str:  # E: Signature of "__hash__" incompatible with supertype "object" \
+                                        # N:      Superclass: \
+                                        # N:          def __hash__(self) -> int \
+                                        # N:      Subclass: \
+                                        # N:          def __hash__(self, x: int) -> str \
+                                        # N:      Superclass: \
+                                        # N:          def __hash__(self) -> int \
+                                        # N:      Subclass: \
+                                        # N:          def __hash__(self, x: int) -> str
+        return ''
+
+def bad_hash(x: E, y: str) -> str:
+    return y
+
+class E(A):
+    __hash__ = bad_hash  # E: Incompatible types in assignment (expression has type "Callable[[str], str]", base class "object" defined the type as "Callable[[], int]")
+
 [builtins fixtures/primitives.pyi]
 
 [case testHashNoneBadOverride]