From c11956a8bddd75f02ccc7b4da7e4d8123e1f3c5f Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Wed, 8 Sep 2021 13:25:09 +0300
Subject: [PATCH] bpo-45121: Fix RecursionError when calling Protocol.__init__
 from a subclass' __init__ (GH-28206)

---
 Lib/test/test_typing.py                                | 10 ++++++++++
 Lib/typing.py                                          |  5 +++++
 .../2021-09-07-17-10-16.bpo-45121.iG-Hsf.rst           |  2 ++
 3 files changed, 17 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2021-09-07-17-10-16.bpo-45121.iG-Hsf.rst

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
index 847d583cdaf..fa49b90886c 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -1610,6 +1610,16 @@ class ProtocolTests(BaseTestCase):
         with self.assertRaisesRegex(TypeError, "@runtime_checkable"):
             isinstance(1, P)
 
+    def test_super_call_init(self):
+        class P(Protocol):
+            x: int
+
+        class Foo(P):
+            def __init__(self):
+                super().__init__()
+
+        Foo()  # Previously triggered RecursionError
+
 
 class GenericTests(BaseTestCase):
 
diff --git a/Lib/typing.py b/Lib/typing.py
index 892f1b35068..e29d699283d 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1406,6 +1406,11 @@ def _no_init_or_replace_init(self, *args, **kwargs):
     if cls._is_protocol:
         raise TypeError('Protocols cannot be instantiated')
 
+    # Already using a custom `__init__`. No need to calculate correct
+    # `__init__` to call. This can lead to RecursionError. See bpo-45121.
+    if cls.__init__ is not _no_init_or_replace_init:
+        return
+
     # Initially, `__init__` of a protocol subclass is set to `_no_init_or_replace_init`.
     # The first instantiation of the subclass will call `_no_init_or_replace_init` which
     # searches for a proper new `__init__` in the MRO. The new `__init__`
diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-09-07-17-10-16.bpo-45121.iG-Hsf.rst b/Misc/NEWS.d/next/Core and Builtins/2021-09-07-17-10-16.bpo-45121.iG-Hsf.rst
new file mode 100644
index 00000000000..19eb3314125
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2021-09-07-17-10-16.bpo-45121.iG-Hsf.rst	
@@ -0,0 +1,2 @@
+Fix issue where ``Protocol.__init__`` raises ``RecursionError`` when it's
+called directly or via ``super()``. Patch provided by Yurii Karabas.