From 784a4ac8d2dd84e5d0d13a80e63432899bb218fe Mon Sep 17 00:00:00 2001
From: Yurii Karabas <1998uriyyo@gmail.com>
Date: Wed, 8 Sep 2021 13:25:09 +0300
Subject: [PATCH 1/2] bpo-45121: Fix RecursionError when calling
 Protocol.__init__ from a subclass' __init__ (GH-28206)

---
 Lib/test/test_typing.py                        | 18 ++++++++++++++++++
 Lib/typing.py                                  |  5 +++++
 .../2021-09-07-17-10-16.bpo-45121.iG-Hsf.rst   |  2 ++
 3 files changed, 25 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 e05ac4539fb503..db03fae6bc53f3 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -1459,6 +1459,24 @@ class CustomProtocol(TestCase, Protocol):
         class CustomContextManager(typing.ContextManager, Protocol):
             pass
 
+    def test_non_runtime_protocol_isinstance_check(self):
+        class P(Protocol):
+            x: int
+
+        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):
 
     def test_basics(self):
diff --git a/Lib/typing.py b/Lib/typing.py
index 9010775c0de7e1..775f17822afc1a 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1085,6 +1085,11 @@ 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 00000000000000..19eb3314125167
--- /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.

From ab284379fa6bbaa6c8a397c72a5fbfa491c2383e Mon Sep 17 00:00:00 2001
From: Fidget-Spinner <28750310+Fidget-Spinner@users.noreply.github.com>
Date: Wed, 8 Sep 2021 18:32:27 +0800
Subject: [PATCH 2/2] remove unrelated test

---
 Lib/test/test_typing.py | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
index db03fae6bc53f3..930f675c624ec4 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -1459,13 +1459,6 @@ class CustomProtocol(TestCase, Protocol):
         class CustomContextManager(typing.ContextManager, Protocol):
             pass
 
-    def test_non_runtime_protocol_isinstance_check(self):
-        class P(Protocol):
-            x: int
-
-        with self.assertRaisesRegex(TypeError, "@runtime_checkable"):
-            isinstance(1, P)
-
     def test_super_call_init(self):
         class P(Protocol):
             x: int