Fix stubtest false positive for sentinel default values

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:

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): ...
@@ -3074,6 +3079,58 @@ def __init__(self, formatter_class=HelpFormatter): ...
             error=None,
         )
 
+    @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!