gh-148157: Check for _PyPegen_add_type_comment_to_arg fail in _PyPegen_name_default_pair (GH-148158)

(cherry picked from commit 1795fccfbc7ccb89ead5c529b2f55f54622d1314)

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: StanFromIreland

Lib/test/test_type_comments.py

@@ -398,6 +398,9 @@ def test_non_utf8_type_comment_with_ignore_cookie(self):
         with self.assertRaises(UnicodeDecodeError):
             _testcapi.Py_CompileStringExFlags(
                 b"a=1 # type: \x80", "<test>", 256, flags)
+        with self.assertRaises(UnicodeDecodeError):
+            _testcapi.Py_CompileStringExFlags(
+                b"def a(f=8, #type: \x80\n\x80", "<test>", 256, flags)
 
     def test_func_type_input(self):
 

Misc/NEWS.d/next/Core_and_Builtins/2026-04-06-11-15-46.gh-issue-148157.JFnZDn.rst

@@ -0,0 +1,2 @@
+Fix an unlikely crash when parsing an invalid type comments for function
+parameters. Found by OSS Fuzz in :oss-fuzz:`492782951`.

Parser/action_helpers.c

@@ -434,6 +434,9 @@ _PyPegen_name_default_pair(Parser *p, arg_ty arg, expr_ty value, Token *tc)
         return NULL;
     }
     a->arg = _PyPegen_add_type_comment_to_arg(p, arg, tc);
+    if (!a->arg) {
+        return NULL;
+    }
     a->value = value;
     return a;
 }