types overlap only if all type args match (#9452)

Fixes #9451

Author: JelleZijlstra

mypy/meet.py

@@ -339,9 +339,9 @@ def _type_object_overlap(left: Type, right: Type) -> bool:
             # Or, to use a more concrete example, List[Union[A, B]] and List[Union[B, C]]
             # would be considered partially overlapping since it's possible for both lists
             # to contain only instances of B at runtime.
-            for left_arg, right_arg in zip(left.args, right.args):
-                if _is_overlapping_types(left_arg, right_arg):
-                    return True
+            if all(_is_overlapping_types(left_arg, right_arg)
+                   for left_arg, right_arg in zip(left.args, right.args)):
+                return True
 
         return False
 

test-data/unit/check-overloading.test

@@ -5151,3 +5151,25 @@ compose(ID, fakeint)("test")
 reveal_type(compose(ID, fakeint))  # N: Revealed type is 'def (Union[builtins.str, builtins.bytes]) -> __main__.ID*'
 
 [builtins fixtures/tuple.pyi]
+
+[case testOverloadTwoTypeArgs]
+from typing import Generic, overload, TypeVar, Any
+
+T1 = TypeVar("T1")
+T2 = TypeVar("T2")
+
+class A: ...
+class B: ...
+class G(Generic[T1, T2]): ...
+
+@overload
+def f1(g: G[A, A]) -> A: ...
+@overload
+def f1(g: G[A, B]) -> B: ...
+def f1(g: Any) -> Any: ...
+
+@overload
+def f2(g: G[A, Any]) -> A: ...  # E: Overloaded function signatures 1 and 2 overlap with incompatible return types
+@overload
+def f2(g: G[A, B], x: int = ...) -> B: ...
+def f2(g: Any, x: int = ...) -> Any: ...