Forbid Sentinel conversion to bool

Going with a strict implementation as early as possible.
This restriction can always be relaxed at a later time.

A truth test on a sentinel implies that the sentinel is mixed in
with other types during the test.
In nearly all cases this is incorrect and could result in
unexpected behavior.

Author: HexDecimal

doc/index.rst

@@ -1044,9 +1044,12 @@ Sentinel objects
    of the sentinel object. If not provided, *name* will be used.
 
    All sentinels with the same *name* and *module_name* have the same identity.
-   Sentinel objects are tested using :py:ref:`is`.
    Sentinel identity is preserved across :py:mod:`copy` and :py:mod:`pickle`.
 
+   Sentinel objects are tested using :py:ref:`is`.
+   Sentinels have no truthiness and attempting to convert a sentinel to
+   :py:class:`bool` will raise :py:exc:`TypeError`.
+
    Example::
 
       >>> from typing_extensions import Sentinel, assert_type

src/test_typing_extensions.py

@@ -9337,6 +9337,11 @@ def test_sentinel_picklable_anonymous(self):
             ):
                 self.assertIs(anonymous_sentinel, pickle.loads(pickle.dumps(anonymous_sentinel, protocol=proto)))
 
+    def test_sentinel_bool(self):
+        with self.assertRaisesRegex(
+            TypeError, rf"{self.SENTINEL!r} is not convertable to bool",
+        ):
+            bool(self.SENTINEL)
 
 
 if __name__ == '__main__':

src/typing_extensions.py

@@ -4247,6 +4247,10 @@ def __reduce__(self) -> str:
         """Reduce this sentinel to a singleton."""
         return self._name  # Module is set from __module__ attribute
 
+    def __bool__(self) -> Never:
+        """Raise TypeError."""
+        raise TypeError(f"Sentinel {self!r} is not convertable to bool.")
+
 
 # Aliases for items that are in typing in all supported versions.
 # We use hasattr() checks so this library will continue to import on