Fix multiple inheritance check for generic classes

mypy/checker.py

@@ -1387,16 +1387,12 @@ def check_method_override_for_base_with_name(
             if isinstance(original_type, AnyType) or isinstance(typ, AnyType):
                 pass
             elif isinstance(original_type, FunctionLike) and isinstance(typ, FunctionLike):
-                if (isinstance(base_attr.node, (FuncDef, OverloadedFuncDef, Decorator))
-                        and not is_static(base_attr.node)):
-                    bound = bind_self(original_type, self.scope.active_self_type())
-                else:
-                    bound = original_type
-                original = map_type_from_supertype(bound, defn.info, base)
+                original = self.bind_and_map_method(base_attr, original_type,
+                                                    defn.info, base)
                 # Check that the types are compatible.
                 # TODO overloaded signatures
                 self.check_override(typ,
-                                    cast(FunctionLike, original),
+                                    original,
                                     defn.name(),
                                     name,
                                     base.name(),
@@ -1415,6 +1411,23 @@ def check_method_override_for_base_with_name(
                     defn.name(), name, base.name(), context)
         return False
 
+    def bind_and_map_method(self, sym: SymbolTableNode, typ: FunctionLike,
+                            sub_info: TypeInfo, super_info: TypeInfo) -> FunctionLike:
+        """Bind self-type and map type variables for a method.
+
+        Arguments:
+            sym: a symbol that points to method definition
+            typ: method type on the definition
+            sub_info: class where the method is used
+            super_info: class where the method was defined
+        """
+        if (isinstance(sym.node, (FuncDef, OverloadedFuncDef, Decorator))
+                and not is_static(sym.node)):
+            bound = bind_self(typ, self.scope.active_self_type())
+        else:
+            bound = typ
+        return cast(FunctionLike, map_type_from_supertype(bound, sub_info, super_info))
+
     def get_op_other_domain(self, tp: FunctionLike) -> Optional[Type]:
         if isinstance(tp, CallableType):
             if tp.arg_kinds and tp.arg_kinds[0] == ARG_POS:
@@ -1628,22 +1641,35 @@ def determine_type_of_class_member(self, sym: SymbolTableNode) -> Optional[Type]
         return None
 
     def check_compatibility(self, name: str, base1: TypeInfo,
-                            base2: TypeInfo, ctx: Context) -> None:
+                            base2: TypeInfo, ctx: TypeInfo) -> None:
         """Check if attribute name in base1 is compatible with base2 in multiple inheritance.
 
         Assume base1 comes before base2 in the MRO, and that base1 and base2 don't have
         a direct subclass relationship (i.e., the compatibility requirement only derives from
         multiple inheritance).
+
+        This check verifies that a definition taken from base1 (and mapped to the current
+        class ctx), is type compatible with the definition taken from base2 (also mapped), so
+        that unsafe subclassing like this can be detected:
+            class A(Generic[T]):
+                def foo(self, x: T) -> None: ...
+
+            class B:
+                def foo(self, x: str) -> None: ...
+
+            class C(B, A[int]): ...  # this is unsafe because...
+
+            x: A[int] = C()
+            x.foo  # ...runtime type is (str) -> None, while static type is (int) -> None
         """
         if name in ('__init__', '__new__', '__init_subclass__'):
             # __init__ and friends can be incompatible -- it's a special case.
             return
-        first = base1[name]
-        second = base2[name]
+        first = base1.names[name]
+        second = base2.names[name]
         first_type = self.determine_type_of_class_member(first)
         second_type = self.determine_type_of_class_member(second)
 
-        # TODO: What if some classes are generic?
         if (isinstance(first_type, FunctionLike) and
                 isinstance(second_type, FunctionLike)):
             if first_type.is_type_obj() and second_type.is_type_obj():
@@ -1652,8 +1678,9 @@ def check_compatibility(self, name: str, base1: TypeInfo,
                 ok = is_subtype(left=fill_typevars_with_any(first_type.type_object()),
                                 right=fill_typevars_with_any(second_type.type_object()))
             else:
-                first_sig = bind_self(first_type)
-                second_sig = bind_self(second_type)
+                # First bind/map method types when necessary.
+                first_sig = self.bind_and_map_method(first, first_type, ctx, base1)
+                second_sig = self.bind_and_map_method(second, second_type, ctx, base2)
                 ok = is_subtype(first_sig, second_sig, ignore_pos_arg_names=True)
         elif first_type and second_type:
             ok = is_equivalent(first_type, second_type)

test-data/unit/check-multiple-inheritance.test

@@ -624,3 +624,44 @@ class Mixin2:
 class A(Mixin1, Mixin2):
     pass
 [out]
+
+[case testGenericMultipleOverrideRemap]
+from typing import TypeVar, Generic, Tuple
+
+K = TypeVar('K')
+V = TypeVar('V')
+T = TypeVar('T')
+
+class ItemsView(Generic[K, V]):
+    def __iter__(self) -> Tuple[K, V]: ...
+
+class Sequence(Generic[T]):
+    def __iter__(self) -> T: ...
+
+# Override compatible between bases.
+class OrderedItemsView(ItemsView[K, V], Sequence[Tuple[K, V]]):
+    def __iter__(self) -> Tuple[K, V]: ...
+
+class OrderedItemsViewDirect(ItemsView[K, V], Sequence[Tuple[K, V]]):
+    pass
+
+[case testGenericMultipleOverrideReplace]
+from typing import TypeVar, Generic, Union
+
+T = TypeVar('T')
+
+class A(Generic[T]):
+    def foo(self, x: T) -> None: ...
+
+class B(A[T]): ...
+
+class C1:
+    def foo(self, x: str) -> None: ...
+
+class C2:
+    def foo(self, x: Union[str, int]) -> None: ...
+
+class D1(B[str], C1): ...
+class D2(B[Union[int, str]], C2): ...
+class D3(C2, B[str]): ...
+class D4(B[str], C2): ...  # E: Definition of "foo" in base class "A" is incompatible with definition in base class "C2"