gh-130618: Fix parser error when using lambdas inside f-strings (#130638)

Lib/test/test_grammar.py

@@ -1972,6 +1972,18 @@ class GrammarTests(unittest.TestCase):
         with self.assertRaises(Done):
             foo().send(None)
 
+    def test_complex_lambda(self):
+        def test1(foo, bar):
+            return ""
+
+        def test2():
+            return f"{test1(
+                foo=lambda: '、、、、、、、、、、、、、、、、、',
+                bar=lambda: 'abcdefghijklmnopqrstuvwxyz 123456789 123456789',
+            )}"
+
+        self.assertEqual(test2(), "")
+
 
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2025-02-27-15-07-06.gh-issue-130618.JTcsRB.rst

@@ -0,0 +1,3 @@
+Fix a bug that was causing ``UnicodeDecodeError`` or ``SystemError`` to be
+raised when using f-strings with ``lambda`` expressions with non-ASCII
+characters. Patch by Pablo Galindo

Parser/lexer/lexer.c

@@ -211,9 +211,13 @@ _PyLexer_update_fstring_expr(struct tok_state *tok, char cur)
             break;
         case '}':
         case '!':
-        case ':':
             tok_mode->last_expr_end = strlen(tok->start);
             break;
+        case ':':
+            if (tok_mode->last_expr_end == -1) {
+               tok_mode->last_expr_end = strlen(tok->start);
+            }
+            break;
         default:
             Py_UNREACHABLE();
     }