bpo-40334: Spacialized error message for invalid args after bare '*' (GH-19865)

When parsing things like `def f(*): pass` the old parser used to output `SyntaxError: named arguments must follow bare *`, which the new parser wasn't able to do.

Grammar/python.gram

@@ -249,6 +249,7 @@ star_etc[StarEtc*]:
     | '*' ',' b=param_maybe_default+ c=[kwds] {
         _PyPegen_star_etc(p, NULL, b, c) }
     | a=kwds { _PyPegen_star_etc(p, NULL, NULL, a) }
+    | invalid_star_etc
 
 kwds[arg_ty]: '**' a=param_no_default { a }
 
@@ -356,6 +357,7 @@ lambda_star_etc[StarEtc*]:
     | '*' ',' b=lambda_param_maybe_default+ c=[lambda_kwds] {
         _PyPegen_star_etc(p, NULL, b, c) }
     | a=lambda_kwds { _PyPegen_star_etc(p, NULL, NULL, a) }
+    | invalid_lambda_star_etc
 
 lambda_kwds[arg_ty]: '**' a=lambda_param_no_default { a }
 
@@ -636,6 +638,10 @@ invalid_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_star_etc:
+    | '*' (')' | ',' (')' | '**')) { RAISE_SYNTAX_ERROR("named arguments must follow bare *") }
+invalid_lambda_star_etc:
+    | '*' (':' | ',' (':' | '**')) { RAISE_SYNTAX_ERROR("named arguments must follow bare *") }
 invalid_double_type_comments:
     | TYPE_COMMENT NEWLINE TYPE_COMMENT NEWLINE INDENT {
         RAISE_SYNTAX_ERROR("Cannot have two type comments on def") }

Lib/test/test_exceptions.py

@@ -242,11 +242,11 @@ class ExceptionTests(unittest.TestCase):
         check('from __future__ import doesnt_exist', 1, 1)
         check('from __future__ import braces', 1, 1)
         check('x=1\nfrom __future__ import division', 2, 1)
+        check('def f(*):\n  pass', 1, 7 if support.use_old_parser() else 8)
 
     @support.skip_if_new_parser("Pegen column offsets might be different")
     def testSyntaxErrorOffsetCustom(self):
         self.check('for 1 in []: pass', 1, 5)
-        self.check('def f(*):\n  pass', 1, 7)
         self.check('[*x for x in xs]', 1, 2)
         self.check('def f():\n  x, y: int', 2, 3)
         self.check('(yield i) = 2', 1, 1)

Lib/test/test_peg_parser.py

@@ -603,6 +603,12 @@ FAIL_SPECIALIZED_MESSAGE_CASES = [
     ("1 += 1", "cannot assign to literal"),
     ("pass\n    pass", "unexpected indent"),
     ("def f():\npass", "expected an indented block"),
+    ("def f(*): pass", "named arguments must follow bare *"),
+    ("def f(*,): pass", "named arguments must follow bare *"),
+    ("def f(*, **a): pass", "named arguments must follow bare *"),
+    ("lambda *: pass", "named arguments must follow bare *"),
+    ("lambda *,: pass", "named arguments must follow bare *"),
+    ("lambda *, **a: pass", "named arguments must follow bare *"),
 ]
 
 GOOD_BUT_FAIL_TEST_CASES = [

Lib/test/test_syntax.py

@@ -627,9 +627,9 @@ Corner-cases that used to fail to raise the correct error:
     Traceback (most recent call last):
     SyntaxError: cannot assign to __debug__
 
-    # >>> with (lambda *:0): pass
-    # Traceback (most recent call last):
-    # SyntaxError: named arguments must follow bare *
+    >>> with (lambda *:0): pass
+    Traceback (most recent call last):
+    SyntaxError: named arguments must follow bare *
 
 Corner-cases that used to crash: