Map TypeVar-valued attributes to the checked base

mypy/checker.py

@@ -2826,8 +2826,10 @@ 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 = 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 = 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 +1065,39 @@ 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 "x" in base class "A" is incompatible with definition in base class "B" \
+                                 # E: Definition of "fn" 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 "x" in base class "A" is incompatible with definition in base class "B" \
+                             # E: Definition of "fn" 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 "x" in base class "A" is incompatible with definition in base class "B" \
+                                 # E: Definition of "fn" 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 "x" in base class "A" is incompatible with definition in base class "B" \
+                             # E: Definition of "fn" in base class "A" is incompatible with definition in base class "B"
+
+[builtins fixtures/tuple.pyi]