Reject a var-positional or var-keyword parameter in an optional group

They were reported as an error on the following line, which was confusing: the closing ']' was blamed for following a var-keyword parameter.

Author: serhiy-storchaka

Lib/test/test_clinic.py

@@ -2130,9 +2130,8 @@ def test_disallowed_grouping__parameter_after_group(self):
         # Only positional-only parameters can follow an optional group.
         group_err = ("You cannot use optional groups ('[' and ']') unless all "
                      "parameters are positional-only ('/')")
-        kwds_err = ("Function 'bar' uses a var-keyword parameter and other "
-                    "non-positional parameters, which Argument Clinic does "
-                    "not currently support: '**kwds: dict'")
+        kwds_err = ("cannot use a var-keyword parameter with pos-or-keyword "
+                    "or keyword-only parameters")
         dataset = (("""
             module foo
             foo.bar
@@ -2717,7 +2716,7 @@ def test_group_with_var_keyword(self):
                 **kwds: dict
                 ]
         """
-        err = "parameters cannot follow var-keyword parameter: ']'"
+        err = "A var-keyword parameter cannot be in an optional group."
         self.expect_failure(block, err)
 
     def test_group_with_var_positional(self):
@@ -2727,8 +2726,7 @@ def test_group_with_var_positional(self):
                 *args: tuple
                 ]
         """
-        err = ("You cannot use optional groups ('[' and ']') unless all "
-               "parameters are positional-only ('/')")
+        err = "A var-positional parameter cannot be in an optional group."
         self.expect_failure(block, err)
 
     def test_depr_star_must_come_after_slash(self):
@@ -3630,9 +3628,8 @@ def test_var_keyword_with_pos_or_kw(self):
                x: int
                **kwds: dict
         """
-        err = ("Function 'bar' uses a var-keyword parameter and other "
-               "non-positional parameters, which Argument Clinic does "
-               "not currently support: '**kwds: dict'")
+        err = ("Function 'bar' cannot use a var-keyword parameter with "
+               "pos-or-keyword or keyword-only parameters.")
         self.expect_failure(block, err)
 
     def test_var_keyword_with_kw_only(self):
@@ -3645,9 +3642,8 @@ def test_var_keyword_with_kw_only(self):
                y: int
                **kwds: dict
         """
-        err = ("Function 'bar' uses a var-keyword parameter and other "
-               "non-positional parameters, which Argument Clinic does "
-               "not currently support: '**kwds: dict'")
+        err = ("Function 'bar' cannot use a var-keyword parameter with "
+               "pos-or-keyword or keyword-only parameters.")
         self.expect_failure(block, err)
 
     def test_var_keyword_with_pos_or_kw_and_kw_only(self):
@@ -3661,9 +3657,8 @@ def test_var_keyword_with_pos_or_kw_and_kw_only(self):
                z: int
                **kwds: dict
         """
-        err = ("Function 'bar' uses a var-keyword parameter and other "
-               "non-positional parameters, which Argument Clinic does "
-               "not currently support: '**kwds: dict'")
+        err = ("Function 'bar' cannot use a var-keyword parameter with "
+               "pos-or-keyword or keyword-only parameters.")
         self.expect_failure(block, err)
 
     def test_allow_negative_accepted_by_py_ssize_t_converter_only(self):

Tools/clinic/libclinic/dsl_parser.py

@@ -1020,9 +1020,9 @@ def parse_parameter(self, line: str) -> None:
                 for p in self.function.parameters.values()
             )
             if has_non_positional_param:
-                fail(f'Function {self.function.name!r} uses a var-keyword parameter '
-                     f'and other non-positional parameters, which Argument Clinic '
-                     f'does not currently support: {line!r}')
+                fail(f'Function {self.function.name!r} cannot use a var-keyword '
+                     f'parameter with pos-or-keyword or keyword-only '
+                     f'parameters.')
             is_var_keyword = True
             parameter = function_args.kwarg
         else:
@@ -1161,6 +1161,9 @@ def bad_node(self, node: ast.AST) -> None:
 
         kind: inspect._ParameterKind
         if is_vararg:
+            if self.group_stack:
+                fail("A var-positional parameter cannot be in an optional "
+                     "group.")
             if any(p.group for p in self.function.parameters.values()):
                 # With "foo([a, b], *args)" the number of arguments does not
                 # tell whether the group is passed or all arguments belong
@@ -1169,6 +1172,9 @@ def bad_node(self, node: ast.AST) -> None:
                      f"groups with a var-positional parameter.")
             kind = inspect.Parameter.VAR_POSITIONAL
         elif is_var_keyword:
+            if self.group_stack:
+                fail("A var-keyword parameter cannot be in an optional "
+                     "group.")
             kind = inspect.Parameter.VAR_KEYWORD
         elif self.keyword_only:
             kind = inspect.Parameter.KEYWORD_ONLY