From 60342acc9965d78ffe517733b944e9013bcf969f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Edgar=20Ram=C3=ADrez=20Mondrag=C3=B3n?=
 <edgarrm358@gmail.com>
Date: Mon, 21 Sep 2026 10:03:01 -0600
Subject: [PATCH 1/2] Fix stubtest false positive for sentinel default values
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Signed-off-by: Edgar Ramírez Mondragón <edgarrm358@gmail.com>
---
 mypy/stubtest.py          | 15 ++++++++++
 mypy/test/teststubtest.py | 58 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 73 insertions(+)

diff --git a/mypy/stubtest.py b/mypy/stubtest.py
index 4a38939a03907..d4c530ff92354 100644
--- a/mypy/stubtest.py
+++ b/mypy/stubtest.py
@@ -863,6 +863,8 @@ def _verify_arg_default_value(
                 and type(runtime_arg.default) is not object
                 # And ellipsis
                 and runtime_arg.default is not ...
+                # And sentinels
+                and not is_sentinel(runtime_type)
                 and not is_subtype_helper(runtime_type, stub_type)
             ):
                 yield (
@@ -2090,6 +2092,19 @@ def is_subtype_helper(left: mypy.types.Type, right: mypy.types.Type) -> bool:
         return mypy.subtypes.is_subtype(left, right)
 
 
+def is_sentinel(runtime: mypy.types.Type) -> bool:
+    """Checks whether ``runtime`` is the type of a ``sentinel()`` marker object."""
+    runtime = mypy.types.get_proper_type(runtime)
+    return isinstance(runtime, mypy.types.Instance) and runtime.type.fullname in (
+        "builtins.sentinel",
+        "typing_extensions.sentinel",
+        # typing_extensions 4.14.0-4.15.x defined the class as `Sentinel`, with
+        # no lowercase alias; the rename (and `Sentinel = sentinel` alias)
+        # landed in 4.16.0.
+        "typing_extensions.Sentinel",
+    )
+
+
 def get_mypy_node_for_name(module: str, type_name: str) -> mypy.nodes.SymbolNode | None:
     stub = get_stub(module)
     if stub is None:
diff --git a/mypy/test/teststubtest.py b/mypy/test/teststubtest.py
index fb83ff0ef528c..8af55a93e58c1 100644
--- a/mypy/test/teststubtest.py
+++ b/mypy/test/teststubtest.py
@@ -123,6 +123,11 @@ class frozenset(Generic[T]): ...
 
 class function: pass
 
+class sentinel:
+    __name__: str
+    __module__: str
+    def __new__(cls, name: str, /, *, repr: str | None = None) -> sentinel: ...
+
 class int: ...
 class float: ...
 class bool(int): ...
@@ -3075,6 +3080,59 @@ def __init__(self, formatter_class=HelpFormatter): ...
         )
 
 
+    @collect_cases
+    def test_object_marker(self) -> Iterator[Case]:
+        yield Case(
+            stub="""
+            def f(x: int = ...) -> None: ...
+            """,
+            runtime="""
+            _MISSING = object()
+
+
+            def f(x=_MISSING):
+                pass
+            """,
+            error=None,
+        )
+
+    @collect_cases
+    def test_sentinel_object_like_marker(self) -> Iterator[Case]:
+        yield Case(
+            stub="""
+            def f(x: int = ...) -> None: ...
+            """,
+            runtime="""
+            from typing_extensions import sentinel
+
+            _MISSING = sentinel("_MISSING")
+
+
+            def f(x=_MISSING):
+                pass
+            """,
+            error=None,
+        )
+
+    @collect_cases
+    def test_legacy_sentinel_object_like_marker(self) -> Iterator[Case]:
+        yield Case(
+            stub="""
+            def f(x: int = ...) -> None: ...
+            """,
+            runtime="""
+            from typing_extensions import Sentinel
+
+            _MISSING = Sentinel("_MISSING")
+
+
+            def f(x=_MISSING):
+                pass
+            """,
+            error=None,
+        )
+
+
 def remove_color_code(s: str) -> str:
     return re.sub("\\x1b.*?m", "", s)  # this works!
 

From a788363098130b640f5cc2806b2d7b669c124a38 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Mon, 21 Sep 2026 16:54:27 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypy/test/teststubtest.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/mypy/test/teststubtest.py b/mypy/test/teststubtest.py
index 8af55a93e58c1..788f1ed545115 100644
--- a/mypy/test/teststubtest.py
+++ b/mypy/test/teststubtest.py
@@ -3079,7 +3079,6 @@ def __init__(self, formatter_class=HelpFormatter): ...
             error=None,
         )
 
-
     @collect_cases
     def test_object_marker(self) -> Iterator[Case]:
         yield Case(