gh-109114: Relax the check for invalid lambdas inside f-strings to avoid false positives (#109121)

Grammar/python.gram

@@ -1170,7 +1170,7 @@ invalid_expression:
         _PyPegen_check_legacy_stmt(p, a) ? NULL : p->tokens[p->mark-1]->level == 0 ? NULL :
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Perhaps you forgot a comma?") }
    | a=disjunction 'if' b=disjunction !('else'|':') { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected 'else' after 'if' expression") }
-   | a='lambda' [lambda_params] b=':' &(FSTRING_MIDDLE | fstring_replacement_field)  {
+   | a='lambda' [lambda_params] b=':' &FSTRING_MIDDLE  {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "f-string: lambda expressions are not allowed without parentheses") }
 
 invalid_named_expression(memo):

Lib/test/test_fstring.py

@@ -1027,6 +1027,10 @@ x = (
                              "f'{lambda x:}'",
                              "f'{lambda :}'",
                              ])
+        # Ensure the detection of invalid lambdas doesn't trigger detection
+        # for valid lambdas in the second error pass
+        with self.assertRaisesRegex(SyntaxError, "invalid syntax"):
+            compile("lambda name_3=f'{name_4}': {name_3}\n1 $ 1", "<string>", "exec")
 
         # but don't emit the paren warning in general cases
         with self.assertRaisesRegex(SyntaxError, "f-string: expecting a valid expression after '{'"):

Misc/NEWS.d/next/Core and Builtins/2023-09-08-01-50-41.gh-issue-109114.adqgtb.rst

@@ -0,0 +1,3 @@
+Relax the detection of the error message for invalid lambdas inside
+f-strings to not search for arbitrary replacement fields to avoid false
+positives. Patch by Pablo Galindo