Fix overload diagnostic when vararg and varkwarg can match (#19614)

Fixes #19612

Author: hauntsaninja

mypy/typeops.py

@@ -521,15 +521,18 @@ def callable_corresponding_argument(
 
         # def right(a: int = ...) -> None: ...
         # def left(__a: int = ..., *, a: int = ...) -> None: ...
-        from mypy.subtypes import is_equivalent
+        from mypy.subtypes import is_subtype
 
         if (
             not (by_name.required or by_pos.required)
             and by_pos.name is None
             and by_name.pos is None
-            and is_equivalent(by_name.typ, by_pos.typ)
         ):
-            return FormalArgument(by_name.name, by_pos.pos, by_name.typ, False)
+            # We actually want the intersection of by_name.typ and by_pos.typ
+            if is_subtype(by_name.typ, by_pos.typ):
+                return FormalArgument(by_name.name, by_pos.pos, by_name.typ, False)
+            if is_subtype(by_pos.typ, by_name.typ):
+                return FormalArgument(by_name.name, by_pos.pos, by_pos.typ, False)
     return by_name if by_name is not None else by_pos
 
 

test-data/unit/check-overloading.test

@@ -231,9 +231,21 @@ def f(x: 'A') -> Any: # E: Overloaded function implementation does not accept al
 
 reveal_type(f(A())) # N: Revealed type is "__main__.B"
 reveal_type(f(B())) # N: Revealed type is "__main__.A"
-
 [builtins fixtures/isinstance.pyi]
 
+[case testTypeCheckOverloadImplOverlapVarArgsAndKwargs]
+from __future__ import annotations
+from typing import overload
+
+@overload
+def foo(x: int) -> None: ...
+@overload
+def foo(a: str, /) -> None: ...
+
+def foo(*args: int | str, **kw: int) -> None:
+    pass
+[builtins fixtures/tuple.pyi]
+
 [case testTypeCheckOverloadWithImplTooSpecificRetType]
 from typing import overload, Any