Ignore transitive errors in multiple inheritance

Author: sterliakov

mypy/checker.py

@@ -2841,9 +2841,19 @@ class C(B, A[int]): ...  # this is unsafe because...
                 and ctx.mro.index(b1type) > ctx.mro.index(b2type)
             ):
                 b1type, b2type = b2type, b1type
+                base1, base2 = base2, base1
                 first_type, second_type = second_type, first_type
                 first_node, second_node = second_node, first_node
 
+            if (
+                base1 is not None
+                and base2 is not None
+                and is_subtype(base1, base2, ignore_promotions=True)
+            ):
+                # If base1 already inherits from base2 with correct type args,
+                # we have reported errors if any. Avoid reporting them again.
+                return
+
         # 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.
 

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

@@ -1090,3 +1090,14 @@ class C5(A2[U], B[T]): pass  # E: Definition of "x" in base class "A" is incompa
                              # E: Definition of "fn" in base class "A" is incompatible with definition in base class "B"
 
 [builtins fixtures/tuple.pyi]
+
+[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