Drop runtime error in PEP 705 implementation (#333)

See https://discuss.python.org/t/pep-705-read-only-typeddict-items/37867/10

Author: JelleZijlstra

CHANGELOG.md

@@ -1,5 +1,8 @@
 # Unreleased
 
+- Drop runtime error when a mutable `TypedDict` key overrides a read-only
+  one. Type checkers should still flag this as an error. Patch by Jelle
+  Zijlstra.
 - Speedup `issubclass()` checks against simple runtime-checkable protocols by
   around 6% (backporting https://github.com/python/cpython/pull/112717, by Alex
   Waygood).

src/test_typing_extensions.py

@@ -4143,13 +4143,12 @@ class Child2(Base2):
         self.assertEqual(Child1.__readonly_keys__, frozenset({'a'}))
         self.assertEqual(Child1.__mutable_keys__, frozenset({'b'}))
 
-    def test_cannot_make_mutable_key_readonly(self):
+    def test_make_mutable_key_readonly(self):
         class Base(TypedDict):
             a: int
 
-        with self.assertRaises(TypeError):
-            class Child(Base):
-                a: ReadOnly[int]
+        class Child(Base):
+            a: ReadOnly[int]  # type checker error, but allowed at runtime
 
     def test_can_make_readonly_key_mutable(self):
         class Base(TypedDict):

src/typing_extensions.py

@@ -942,11 +942,6 @@ def __new__(cls, name, bases, ns, *, total=True):
                 else:
                     optional_keys.add(annotation_key)
                 if ReadOnly in qualifiers:
-                    if annotation_key in mutable_keys:
-                        raise TypeError(
-                            f"Cannot override mutable key {annotation_key!r}"
-                            " with read-only key"
-                        )
                     readonly_keys.add(annotation_key)
                 else:
                     mutable_keys.add(annotation_key)