gh-122026: Fix identification of mismatched parentheses inside f-strings (#122028)

Author: pablogsal

Lib/test/test_fstring.py

@@ -896,6 +896,7 @@ def test_missing_expression(self):
                              "f'{:2}'",
                              "f'''{\t\f\r\n:a}'''",
                              "f'{:'",
+                             "F'{[F'{:'}[F'{:'}]]]",
                              ])
 
         self.assertAllRaise(SyntaxError,

Misc/NEWS.d/next/Core and Builtins/2024-07-19-15-28-05.gh-issue-122026.sta2Ca.rst

@@ -0,0 +1,2 @@
+Fix a bug that caused the tokenizer to not correctly identify mismatched
+parentheses inside f-strings in some situations. Patch by Pablo Galindo

Parser/lexer/lexer.c

@@ -1238,6 +1238,9 @@ tok_get_normal_mode(struct tok_state *tok, tokenizer_mode* current_tok, struct t
 
         if (INSIDE_FSTRING(tok)) {
             current_tok->curly_bracket_depth--;
+            if (current_tok->curly_bracket_depth < 0) {
+                return MAKE_TOKEN(_PyTokenizer_syntaxerror(tok, "f-string: unmatched '%c'", c));
+            }
             if (c == '}' && current_tok->curly_bracket_depth == current_tok->curly_bracket_expr_start_depth) {
                 current_tok->curly_bracket_expr_start_depth--;
                 current_tok->kind = TOK_FSTRING_MODE;