From 2700f4f80abd327e67938ea507ed3828655a9be8 Mon Sep 17 00:00:00 2001
From: atty57 <99388680+atty57@users.noreply.github.com>
Date: Fri, 9 Oct 2026 17:18:57 -0400
Subject: [PATCH] Don't cache partial protocol implementation checks

Partial protocol checks can skip incompatible members and therefore must
not populate the full subtype cache. Add regression tests for inference
against a union of callback protocols in both call orders.

Fixes #22151.
---
 mypy/subtypes.py                    |  4 ++-
 test-data/unit/check-protocols.test | 40 +++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/mypy/subtypes.py b/mypy/subtypes.py
index d9c1b47c317f..cbab19f6247f 100644
--- a/mypy/subtypes.py
+++ b/mypy/subtypes.py
@@ -1390,7 +1390,9 @@ def f(self) -> A: ...
         subtype_context=SubtypeContext(ignore_pos_arg_names=ignore_names),
         proper_subtype=proper_subtype,
     )
-    type_state.record_subtype_cache_entry(subtype_kind, left, right)
+    # A partial protocol check does not prove full subtype compatibility.
+    if not skip:
+        type_state.record_subtype_cache_entry(subtype_kind, left, right)
     return True
 
 
diff --git a/test-data/unit/check-protocols.test b/test-data/unit/check-protocols.test
index 75a0dfe2f5f4..9be565136723 100644
--- a/test-data/unit/check-protocols.test
+++ b/test-data/unit/check-protocols.test
@@ -2730,6 +2730,46 @@ def test(func: A[T, S]) -> Tuple[T, S]: ...
 reveal_type(test(f))  # N: Revealed type is "tuple[builtins.str, builtins.int]"
 [builtins fixtures/tuple.pyi]
 
+[case testUnionOfCallbackProtocolsInference]
+from typing import Protocol, TypeVar
+
+T = TypeVar("T")
+PT = TypeVar("PT", contravariant=True)
+
+class OneArg(Protocol[PT]):
+    def __call__(self, x: PT, /) -> None: ...
+
+class TwoArgs(Protocol[PT]):
+    def __call__(self, x: PT, y: int, /) -> None: ...
+
+def first_arg(fn: OneArg[T] | TwoArgs[T]) -> T: ...
+
+def one(x: int) -> None: ...
+def two(x: str, y: int) -> None: ...
+
+reveal_type(first_arg(one))  # N: Revealed type is "builtins.int"
+reveal_type(first_arg(two))  # N: Revealed type is "builtins.str"
+
+[case testUnionOfCallbackProtocolsInferenceReversed]
+from typing import Protocol, TypeVar
+
+T = TypeVar("T")
+PT = TypeVar("PT", contravariant=True)
+
+class OneArg(Protocol[PT]):
+    def __call__(self, x: PT, /) -> None: ...
+
+class TwoArgs(Protocol[PT]):
+    def __call__(self, x: PT, y: int, /) -> None: ...
+
+def first_arg(fn: OneArg[T] | TwoArgs[T]) -> T: ...
+
+def one(x: int) -> None: ...
+def two(x: str, y: int) -> None: ...
+
+reveal_type(first_arg(two))  # N: Revealed type is "builtins.str"
+reveal_type(first_arg(one))  # N: Revealed type is "builtins.int"
+
 [case testProtocolsAlwaysABCs]
 from typing import Protocol