Do not ignore generic type args when checking multiple inheritance compatibility

mypy/checker.py

@@ -2744,21 +2744,44 @@ def check_multiple_inheritance(self, typ: TypeInfo) -> None:
         if len(typ.bases) <= 1:
             # No multiple inheritance.
             return
+
         # Verify that inherited attributes are compatible.
-        mro = typ.mro[1:]
-        for i, base in enumerate(mro):
+        # Construct a "typed" MRO that follows regular MRO order, but includes instances
+        # parametrized with their generic args.
+        # This detects e.g. `class A(Mapping[int, str], Iterable[str])` correctly.
+        # For each MRO entry, include it parametrized according to each base inheriting
+        # from it.
+        typed_mro = [
+            map_instance_to_supertype(base, parent)
+            for parent in typ.mro[1:]
+            for base in typ.bases
+            if parent in base.type.mro
+        ]
+        # If the first MRO entry is compatible with everything following, we don't need
+        # (and shouldn't) compare further pairs
+        # (see testMultipleInheritanceExplcitDiamondResolution)
+        seen_names = set()
+        for i, base in enumerate(typed_mro):
             # Attributes defined in both the type and base are skipped.
             # Normal checks for attribute compatibility should catch any problems elsewhere.
-            non_overridden_attrs = base.names.keys() - typ.names.keys()
+            # Sort for consistent messages order.
+            non_overridden_attrs = sorted(typed_mro[i].type.names - typ.names.keys())
             for name in non_overridden_attrs:
                 if is_private(name):
                     continue
-                for base2 in mro[i + 1 :]:
+                if name in seen_names:
+                    continue
+                for base2 in typed_mro[i + 1 :]:
                     # We only need to check compatibility of attributes from classes not
                     # in a subclass relationship. For subclasses, normal (single inheritance)
                     # checks suffice (these are implemented elsewhere).
-                    if name in base2.names and base2 not in base.mro:
+                    if name in base2.type.names and not is_subtype(
+                        base, base2, ignore_promotions=True
+                    ):
+                        # If base1 already inherits from base2 with correct type args,
+                        # we have reported errors if any. Avoid reporting them again.
                         self.check_compatibility(name, base, base2, typ)
+                seen_names.add(name)
 
     def determine_type_of_member(self, sym: SymbolTableNode) -> Type | None:
         if sym.type is not None:
@@ -2783,8 +2806,23 @@ def determine_type_of_member(self, sym: SymbolTableNode) -> Type | None:
         # TODO: handle more node kinds here.
         return None
 
+    def attribute_type_from_base(
+        self, name: str, base: Instance
+    ) -> tuple[ProperType | None, SymbolTableNode]:
+        """For a NameExpr that is part of a class, walk all base classes and try
+        to find the first class that defines a Type for the same name."""
+        base_var = base.type[name]
+        base_type = self.determine_type_of_member(base_var)
+        if base_type is None:
+            return None, base_var
+
+        if not has_no_typevars(base_type):
+            base_type = expand_type_by_instance(base_type, base)
+
+        return get_proper_type(base_type), base_var
+
     def check_compatibility(
-        self, name: str, base1: TypeInfo, base2: TypeInfo, ctx: TypeInfo
+        self, name: str, base1: Instance, base2: Instance, ctx: TypeInfo
     ) -> None:
         """Check if attribute name in base1 is compatible with base2 in multiple inheritance.
 
@@ -2809,10 +2847,9 @@ class C(B, A[int]): ...  # this is unsafe because...
         if name in ("__init__", "__new__", "__init_subclass__"):
             # __init__ and friends can be incompatible -- it's a special case.
             return
-        first = base1.names[name]
-        second = base2.names[name]
-        first_type = get_proper_type(self.determine_type_of_member(first))
-        second_type = get_proper_type(self.determine_type_of_member(second))
+
+        first_type, first = self.attribute_type_from_base(name, base1)
+        second_type, second = self.attribute_type_from_base(name, base2)
 
         # TODO: use more principled logic to decide is_subtype() vs is_equivalent().
         # We should rely on mutability of superclass node, not on types being Callable.
@@ -2822,7 +2859,7 @@ class C(B, A[int]): ...  # this is unsafe because...
         if isinstance(first_type, Instance):
             call = find_member("__call__", first_type, first_type, is_operator=True)
         if call and isinstance(second_type, FunctionLike):
-            second_sig = self.bind_and_map_method(second, second_type, ctx, base2)
+            second_sig = self.bind_and_map_method(second, second_type, ctx, base2.type)
             ok = is_subtype(call, second_sig, ignore_pos_arg_names=True)
         elif isinstance(first_type, FunctionLike) and isinstance(second_type, FunctionLike):
             if first_type.is_type_obj() and second_type.is_type_obj():
@@ -2834,8 +2871,8 @@ class C(B, A[int]): ...  # this is unsafe because...
                 )
             else:
                 # 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)
+                first_sig = self.bind_and_map_method(first, first_type, ctx, base1.type)
+                second_sig = self.bind_and_map_method(second, second_type, ctx, base2.type)
                 ok = is_subtype(first_sig, second_sig, ignore_pos_arg_names=True)
         elif first_type and second_type:
             if isinstance(first.node, Var):
@@ -2844,7 +2881,7 @@ class C(B, A[int]): ...  # this is unsafe because...
                 second_type = expand_self_type(second.node, second_type, fill_typevars(ctx))
             ok = is_equivalent(first_type, second_type)
             if not ok:
-                second_node = base2[name].node
+                second_node = base2.type[name].node
                 if (
                     isinstance(second_type, FunctionLike)
                     and second_node is not None
@@ -2854,22 +2891,22 @@ class C(B, A[int]): ...  # this is unsafe because...
                     ok = is_subtype(first_type, second_type)
         else:
             if first_type is None:
-                self.msg.cannot_determine_type_in_base(name, base1.name, ctx)
+                self.msg.cannot_determine_type_in_base(name, base1.type.name, ctx)
             if second_type is None:
-                self.msg.cannot_determine_type_in_base(name, base2.name, ctx)
+                self.msg.cannot_determine_type_in_base(name, base2.type.name, ctx)
             ok = True
         # Final attributes can never be overridden, but can override
         # non-final read-only attributes.
         if is_final_node(second.node) and not is_private(name):
-            self.msg.cant_override_final(name, base2.name, ctx)
+            self.msg.cant_override_final(name, base2.type.name, ctx)
         if is_final_node(first.node):
             self.check_if_final_var_override_writable(name, second.node, ctx)
         # Some attributes like __slots__ and __deletable__ are special, and the type can
         # vary across class hierarchy.
         if isinstance(second.node, Var) and second.node.allow_incompatible_override:
             ok = True
         if not ok:
-            self.msg.base_class_definitions_incompatible(name, base1, base2, ctx)
+            self.msg.base_class_definitions_incompatible(name, base1.type, base2.type, ctx)
 
     def check_metaclass_compatibility(self, typ: TypeInfo) -> None:
         """Ensures that metaclasses of all parent types are compatible."""

test-data/unit/check-generic-subtyping.test

@@ -946,7 +946,7 @@ x1: X1[str, int]
 reveal_type(list(x1))  # N: Revealed type is "builtins.list[builtins.int]"
 reveal_type([*x1])  # N: Revealed type is "builtins.list[builtins.int]"
 
-class X2(Generic[T, U], Iterator[U], Mapping[T, U]):
+class X2(Generic[T, U], Iterator[U], Mapping[T, U]):  # E: Definition of "__iter__" in base class "Iterable" is incompatible with definition in base class "Iterable"
     pass
 
 x2: X2[str, int]
@@ -1017,10 +1017,7 @@ x1: X1[str, int]
 reveal_type(iter(x1))  # N: Revealed type is "typing.Iterator[builtins.int]"
 reveal_type({**x1})  # N: Revealed type is "builtins.dict[builtins.int, builtins.str]"
 
-# Some people would expect this to raise an error, but this currently does not:
-# `Mapping` has `Iterable[U]` base class, `X2` has direct `Iterable[T]` base class.
-# It would be impossible to define correct `__iter__` method for incompatible `T` and `U`.
-class X2(Generic[T, U],  Mapping[U, T], Iterable[T]):
+class X2(Generic[T, U],  Mapping[U, T], Iterable[T]):  # E: Definition of "__iter__" in base class "Iterable" is incompatible with definition in base class "Iterable"
     pass
 
 x2: X2[str, int]
@@ -1065,3 +1062,31 @@ class F(E[T_co], Generic[T_co]): ...  # E: Variance of TypeVar "T_co" incompatib
 
 class G(Generic[T]): ...
 class H(G[T_contra], Generic[T_contra]): ...  # E: Variance of TypeVar "T_contra" incompatible with variance in parent type
+
+[case testMultipleInheritanceCompatibleTypeVar]
+from typing import Generic, TypeVar
+
+T = TypeVar("T")
+U = TypeVar("U")
+
+class A(Generic[T]):
+    x: T
+    def fn(self, t: T) -> None: ...
+
+class A2(A[T]):
+    y: str
+    z: str
+
+class B(Generic[T]):
+    x: T
+    def fn(self, t: T) -> None: ...
+
+class C1(A2[str], B[str]): pass
+class C2(A2[str], B[int]): pass  # E: Definition of "fn" in base class "A" is incompatible with definition in base class "B" \
+                                 # E: Definition of "x" in base class "A" is incompatible with definition in base class "B"
+class C3(A2[T], B[T]): pass
+class C4(A2[U], B[U]): pass
+class C5(A2[U], B[T]): pass  # E: Definition of "fn" in base class "A" is incompatible with definition in base class "B" \
+                             # E: Definition of "x" in base class "A" is incompatible with definition in base class "B"
+
+[builtins fixtures/tuple.pyi]

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

@@ -669,7 +669,6 @@ 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"
 
-
 [case testMultipleInheritanceOverridingOfFunctionsWithCallableInstances]
 from typing import Any, Callable
 
@@ -706,3 +705,47 @@ class C34(B3, B4): ...
 class C41(B4, B1): ...
 class C42(B4, B2): ...
 class C43(B4, B3): ...
+
+[case testMultipleInheritanceTransitive]
+class A:
+    def fn(self, x: int) -> None: ...
+class B(A): ...
+class C(A):
+    def fn(self, x: "int | str") -> None: ...
+class D(B, C): ...
+
+[case testMultipleInheritanceCompatErrorPropagation]
+class A:
+    foo: bytes
+class B(A):
+    foo: str  # type: ignore[assignment]
+
+class Ok(B, A): pass
+
+class C(A): pass
+class Ok2(B, C): pass
+
+[case testMultipleInheritanceExplcitDiamondResolution]
+class A:
+    class M:
+        pass
+
+class B0(A):
+    class M(A.M):
+        pass
+
+class B1(A):
+    class M(A.M):
+        pass
+
+class C(B0,B1):
+    class M(B0.M, B1.M):
+        pass
+
+class D0(B0):
+    pass
+class D1(B1):
+    pass
+
+class D(D0,D1,C):
+    pass