Fix attribute type resolution with multiple inheritance

mypy/checker.py

@@ -2733,19 +2733,20 @@ def check_multiple_inheritance(self, typ: TypeInfo) -> None:
             return
         # Verify that inherited attributes are compatible.
         mro = typ.mro[1:]
-        for i, base in enumerate(mro):
+        all_names = {name for base in mro for name in base.names}
+        for name in sorted(all_names - typ.names.keys()):
+            # Sort for reproducible message order.
             # 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()
-            for name in non_overridden_attrs:
-                if is_private(name):
-                    continue
-                for base2 in 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:
-                        self.check_compatibility(name, base, base2, typ)
+            if is_private(name):
+                continue
+            # Compare the first base defining a name with the rest.
+            # Remaining bases may not be pairwise compatible as the first base provides
+            # the used definition.
+            i, base = next((i, base) for i, base in enumerate(mro) if name in base.names)
+            for base2 in mro[i + 1 :]:
+                if name in base2.names and base2 not in base.mro:
+                    self.check_compatibility(name, base, base2, typ)
 
     def determine_type_of_member(self, sym: SymbolTableNode) -> Type | None:
         if sym.type is not None:
@@ -2826,8 +2827,10 @@ class C(B, A[int]): ...  # this is unsafe because...
                 ok = is_subtype(first_sig, second_sig, ignore_pos_arg_names=True)
         elif first_type and second_type:
             if isinstance(first.node, Var):
+                first_type = get_proper_type(map_type_from_supertype(first_type, ctx, base1))
                 first_type = expand_self_type(first.node, first_type, fill_typevars(ctx))
             if isinstance(second.node, Var):
+                second_type = get_proper_type(map_type_from_supertype(second_type, ctx, base2))
                 second_type = expand_self_type(second.node, second_type, fill_typevars(ctx))
             ok = is_equivalent(first_type, second_type)
             if not ok:

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

@@ -1065,3 +1065,38 @@ 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"
+
+class D1(A[str], B[str]): pass
+class D2(A[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 D3(A[T], B[T]): pass
+class D4(A[U], B[U]): pass
+class D5(A[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

@@ -706,3 +706,29 @@ class C34(B3, B4): ...
 class C41(B4, B1): ...
 class C42(B4, B2): ...
 class C43(B4, B3): ...
+
+[case testMultipleInheritanceExplicitDiamondResolution]
+# Adapted from #14279
+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