Raise specialised syntax error for invalid lambda parameters (GH-20776)

(cherry picked from commit c6483c9)

Co-authored-by: Pablo Galindo <[email protected]>

Author: miss-islington

Grammar/python.gram

@@ -329,7 +329,11 @@ expression[expr_ty] (memo):
     | lambdef
 
 lambdef[expr_ty]:
-    | 'lambda' a=[lambda_parameters] ':' b=expression { _Py_Lambda((a) ? a : CHECK(_PyPegen_empty_arguments(p)), b, EXTRA) }
+    | 'lambda' a=[lambda_params] ':' b=expression { _Py_Lambda((a) ? a : CHECK(_PyPegen_empty_arguments(p)), b, EXTRA) }
+
+lambda_params[arguments_ty]:
+    | invalid_lambda_parameters
+    | lambda_parameters
 
 # lambda_parameters etc. duplicates parameters but without annotations
 # or type comments, and if there's no comma after a parameter, we expect
@@ -669,6 +673,9 @@ invalid_dict_comprehension:
 invalid_parameters:
     | param_no_default* (slash_with_default | param_with_default+) param_no_default {
         RAISE_SYNTAX_ERROR("non-default argument follows default argument") }
+invalid_lambda_parameters:
+    | lambda_param_no_default* (lambda_slash_with_default | lambda_param_with_default+) lambda_param_no_default {
+        RAISE_SYNTAX_ERROR("non-default argument follows default argument") }
 invalid_star_etc:
     | '*' (')' | ',' (')' | '**')) { RAISE_SYNTAX_ERROR("named arguments must follow bare *") }
     | '*' ',' TYPE_COMMENT { RAISE_SYNTAX_ERROR("bare * has associated type comment") }

Lib/test/test_positional_only_arg.py

@@ -4,7 +4,7 @@
 import pickle
 import unittest
 
-from test.support import check_syntax_error, use_old_parser
+from test.support import check_syntax_error
 
 
 def global_pos_only_f(a, b, /):
@@ -23,12 +23,10 @@ def assertRaisesSyntaxError(self, codestr, regex="invalid syntax"):
             compile(codestr + "\n", "<test>", "single")
 
     def test_invalid_syntax_errors(self):
-        if use_old_parser():
-            check_syntax_error(self, "def f(a, b = 5, /, c): pass", "non-default argument follows default argument")
-            check_syntax_error(self, "def f(a = 5, b, /, c): pass", "non-default argument follows default argument")
-            check_syntax_error(self, "def f(a = 5, b=1, /, c, *, d=2): pass", "non-default argument follows default argument")
-            check_syntax_error(self, "def f(a = 5, b, /): pass", "non-default argument follows default argument")
-
+        check_syntax_error(self, "def f(a, b = 5, /, c): pass", "non-default argument follows default argument")
+        check_syntax_error(self, "def f(a = 5, b, /, c): pass", "non-default argument follows default argument")
+        check_syntax_error(self, "def f(a = 5, b=1, /, c, *, d=2): pass", "non-default argument follows default argument")
+        check_syntax_error(self, "def f(a = 5, b, /): pass", "non-default argument follows default argument")
         check_syntax_error(self, "def f(*args, /): pass")
         check_syntax_error(self, "def f(*args, a, /): pass")
         check_syntax_error(self, "def f(**kwargs, /): pass")
@@ -46,12 +44,10 @@ def test_invalid_syntax_errors(self):
         check_syntax_error(self, "def f(a, *, c, /, d, e): pass")
 
     def test_invalid_syntax_errors_async(self):
-        if use_old_parser():
-            check_syntax_error(self, "async def f(a, b = 5, /, c): pass", "non-default argument follows default argument")
-            check_syntax_error(self, "async def f(a = 5, b, /, c): pass", "non-default argument follows default argument")
-            check_syntax_error(self, "async def f(a = 5, b=1, /, c, d=2): pass", "non-default argument follows default argument")
-            check_syntax_error(self, "async def f(a = 5, b, /): pass", "non-default argument follows default argument")
-
+        check_syntax_error(self, "async def f(a, b = 5, /, c): pass", "non-default argument follows default argument")
+        check_syntax_error(self, "async def f(a = 5, b, /, c): pass", "non-default argument follows default argument")
+        check_syntax_error(self, "async def f(a = 5, b=1, /, c, d=2): pass", "non-default argument follows default argument")
+        check_syntax_error(self, "async def f(a = 5, b, /): pass", "non-default argument follows default argument")
         check_syntax_error(self, "async def f(*args, /): pass")
         check_syntax_error(self, "async def f(*args, a, /): pass")
         check_syntax_error(self, "async def f(**kwargs, /): pass")
@@ -235,11 +231,9 @@ def test_lambdas(self):
         self.assertEqual(x(1, 2), 3)
 
     def test_invalid_syntax_lambda(self):
-        if use_old_parser():
-            check_syntax_error(self, "lambda a, b = 5, /, c: None", "non-default argument follows default argument")
-            check_syntax_error(self, "lambda a = 5, b, /, c: None", "non-default argument follows default argument")
-            check_syntax_error(self, "lambda a = 5, b, /: None", "non-default argument follows default argument")
-
+        check_syntax_error(self, "lambda a, b = 5, /, c: None", "non-default argument follows default argument")
+        check_syntax_error(self, "lambda a = 5, b, /, c: None", "non-default argument follows default argument")
+        check_syntax_error(self, "lambda a = 5, b, /: None", "non-default argument follows default argument")
         check_syntax_error(self, "lambda *args, /: None")
         check_syntax_error(self, "lambda *args, a, /: None")
         check_syntax_error(self, "lambda **kwargs, /: None")