Backport the ability to define `__init__` methods on Protocol classes (#142)

CHANGELOG.md

diff --git a/CHANGELOG.md b/CHANGELOG.md
index d22b404..7fa1319 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -29,6 +29,10 @@
   `typing_extensions` may no longer be considered instances of that protocol
   using the new release, and vice versa. Most users are unlikely to be affected
   by this change. Patch by Alex Waygood.
+- Backport the ability to define `__init__` methods on Protocol classes, a
+  change made in Python 3.11 (originally implemented in 
+  https://github.com/python/cpython/pull/31628 by Adrian Garcia Badaracco).
+  Patch by Alex Waygood.
 - Speedup `isinstance(3, typing_extensions.SupportsIndex)` by >10x on Python
   <3.12. Patch by Alex Waygood.
 

src/test_typing_extensions.py

diff --git a/src/test_typing_extensions.py b/src/test_typing_extensions.py
index 0e4a1b7..19a116f 100644
--- a/src/test_typing_extensions.py
+++ b/src/test_typing_extensions.py
@@ -1454,6 +1454,32 @@
         class CG(PG[T]): pass
         self.assertIsInstance(CG[int](), CG)
 
+    def test_protocol_defining_init_does_not_get_overridden(self):
+        # check that P.__init__ doesn't get clobbered
+        # see https://bugs.python.org/issue44807
+
+        class P(Protocol):
+            x: int
+            def __init__(self, x: int) -> None:
+                self.x = x
+        class C: pass
+
+        c = C()
+        P.__init__(c, 1)
+        self.assertEqual(c.x, 1)
+
+    def test_concrete_class_inheriting_init_from_protocol(self):
+        class P(Protocol):
+            x: int
+            def __init__(self, x: int) -> None:
+                self.x = x
+
+        class C(P): pass
+
+        c = C(1)
+        self.assertIsInstance(c, C)
+        self.assertEqual(c.x, 1)
+
     def test_cannot_instantiate_abstract(self):
         @runtime_checkable
         class P(Protocol):

src/typing_extensions.py

diff --git a/src/typing_extensions.py b/src/typing_extensions.py
index b7864a9..16a8fdd 100644
--- a/src/typing_extensions.py
+++ b/src/typing_extensions.py
@@ -662,7 +662,8 @@
                         isinstance(base, _ProtocolMeta) and base._is_protocol):
                     raise TypeError('Protocols can only inherit from other'
                                     f' protocols, got {repr(base)}')
-            cls.__init__ = _no_init
+            if cls.__init__ is Protocol.__init__:
+                cls.__init__ = _no_init
 
     def runtime_checkable(cls):
         """Mark a protocol class as a runtime protocol, so that it