From 5ae38dd3706ad8906d39bb717bb2616dca1cc701 Mon Sep 17 00:00:00 2001
From: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Date: Mon, 3 Mar 2025 00:20:23 -0800
Subject: [PATCH] Fix parser for TypeVar bounds (#4602)

---
 CHANGES.md                              | 3 +++
 src/blib2to3/Grammar.txt                | 6 +++---
 tests/data/cases/type_param_defaults.py | 6 ++++++
 tests/data/cases/type_params.py         | 6 ++++++
 4 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/CHANGES.md b/CHANGES.md
index d8348c9..a81e407 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -26,6 +26,9 @@
 
 <!-- Changes to the parser or to version autodetection -->
 
+- Fix bug where certain unusual expressions (e.g., lambdas) were not accepted
+  in type parameter bounds and defaults. (#4602)
+
 ### Performance
 
 <!-- Changes that improve Black's performance. -->
diff --git a/src/blib2to3/Grammar.txt b/src/blib2to3/Grammar.txt
index c8800e2..b779b49 100644
--- a/src/blib2to3/Grammar.txt
+++ b/src/blib2to3/Grammar.txt
@@ -12,9 +12,9 @@ file_input: (NEWLINE | stmt)* ENDMARKER
 single_input: NEWLINE | simple_stmt | compound_stmt NEWLINE
 eval_input: testlist NEWLINE* ENDMARKER
 
-typevar: NAME [':' expr] ['=' expr]
-paramspec: '**' NAME ['=' expr]
-typevartuple: '*' NAME ['=' (expr|star_expr)]
+typevar: NAME [':' test] ['=' test]
+paramspec: '**' NAME ['=' test]
+typevartuple: '*' NAME ['=' (test|star_expr)]
 typeparam: typevar | paramspec | typevartuple
 typeparams: '[' typeparam (',' typeparam)* [','] ']'
 
diff --git a/tests/data/cases/type_param_defaults.py b/tests/data/cases/type_param_defaults.py
index feba64a..f8c24eb 100644
--- a/tests/data/cases/type_param_defaults.py
+++ b/tests/data/cases/type_param_defaults.py
@@ -20,6 +20,8 @@ def trailing_comma1[T=int,](a: str):
 def trailing_comma2[T=int](a: str,):
     pass
 
+def weird_syntax[T=lambda: 42, **P=lambda: 43, *Ts=lambda: 44](): pass
+
 # output
 
 type A[T = int] = float
@@ -61,3 +63,7 @@ def trailing_comma2[T = int](
     a: str,
 ):
     pass
+
+
+def weird_syntax[T = lambda: 42, **P = lambda: 43, *Ts = lambda: 44]():
+    pass
diff --git a/tests/data/cases/type_params.py b/tests/data/cases/type_params.py
index 720a775..f8fc385 100644
--- a/tests/data/cases/type_params.py
+++ b/tests/data/cases/type_params.py
@@ -13,6 +13,8 @@ def it_gets_worse[WhatIsTheLongestTypeVarNameYouCanThinkOfEnoughToMakeBlackSplit
 
 def magic[Trailing, Comma,](): pass
 
+def weird_syntax[T: lambda: 42, U: a or b](): pass
+
 # output
 
 
@@ -56,3 +58,7 @@ def magic[
     Comma,
 ]():
     pass
+
+
+def weird_syntax[T: lambda: 42, U: a or b]():
+    pass