Fix assertion failure for fwd params after rest

Author: haldun

include/prism/diagnostic.h

@@ -168,6 +168,7 @@ typedef enum {
     PM_ERR_PARAMETER_ORDER,
     PM_ERR_PARAMETER_SPLAT_MULTI,
     PM_ERR_PARAMETER_STAR,
+    PM_ERR_PARAMETER_UNEXPECTED_FWD,
     PM_ERR_PARAMETER_WILD_LOOSE_COMMA,
     PM_ERR_PATTERN_EXPRESSION_AFTER_BRACKET,
     PM_ERR_PATTERN_EXPRESSION_AFTER_HROCKET,

src/diagnostic.c

@@ -202,6 +202,7 @@ static const char* const diagnostic_messages[PM_DIAGNOSTIC_ID_LEN] = {
     [PM_ERR_PARAMETER_ORDER]                    = "Unexpected parameter order",
     [PM_ERR_PARAMETER_SPLAT_MULTI]              = "Unexpected multiple `*` splat parameters",
     [PM_ERR_PARAMETER_STAR]                     = "Unexpected parameter `*`",
+    [PM_ERR_PARAMETER_UNEXPECTED_FWD]           = "Unexpected `...` in parameters",
     [PM_ERR_PARAMETER_WILD_LOOSE_COMMA]         = "Unexpected `,` in parameters",
     [PM_ERR_PATTERN_EXPRESSION_AFTER_BRACKET]   = "Expected a pattern expression after the `[` operator",
     [PM_ERR_PATTERN_EXPRESSION_AFTER_COMMA]     = "Expected a pattern expression after `,`",

src/prism.c

@@ -9292,6 +9292,14 @@ parse_parameters(
 
                     pm_parser_local_add_token(parser, &parser->previous);
                     pm_forwarding_parameter_node_t *param = pm_forwarding_parameter_node_create(parser, &parser->previous);
+                    if (params->keyword_rest != NULL) {
+                        // If we already have a keyword rest parameter, then we replace it with the
+                        // forwarding parameter and move the keyword rest parameter to the posts list.
+                        pm_node_t *keyword_rest = params->keyword_rest;
+                        pm_parameters_node_posts_append(params, keyword_rest);
+                        pm_diagnostic_list_append(&parser->error_list, parser->previous.start, parser->previous.end, PM_ERR_PARAMETER_UNEXPECTED_FWD);
+                        params->keyword_rest = NULL;
+                    }
                     pm_parameters_node_keyword_rest_set(params, (pm_node_t *)param);
                 } else {
                     update_parameter_state(parser, &parser->current, &order);

test/prism/errors_test.rb

@@ -1348,6 +1348,13 @@ def test_loop_conditional_is_closed
       ]
     end
 
+    def test_forwarding_arg_after_keyword_rest
+      source = "def f(**,...);end"
+      assert_errors expression(source), source, [
+        ["Unexpected `...` in parameters", 9..12],
+      ]
+    end
+
     private
 
     def assert_errors(expected, source, errors, compare_ripper: RUBY_ENGINE == "ruby")