fix: function paramter list can end in comma. Fixes# 613 (#664)

* fix: function paramter list can end in comma. Fixes# 613

* [WIP] add: regression tests

* fix: cpp2 does not syntax-check cpp1

* fix: test name such that it fits the naming scheme

* Tweak error message wording

Signed-off-by: Herb Sutter <herb.sutter@gmail.com>

---------

Signed-off-by: Herb Sutter <herb.sutter@gmail.com>
Co-authored-by: Herb Sutter <herb.sutter@gmail.com>

Author: jobnz

regression-tests/pure2-bugfix-for-parameter-decl-list-error.cpp2

@@ -0,0 +1 @@
+main: (args,) = {}

regression-tests/test-results/pure2-bugfix-for-parameter-decl-list-error.cpp2.output

@@ -0,0 +1,3 @@
+pure2-bugfix-for-parameter-decl-list.cpp2...
+pure2-bugfix-for-parameter-decl-list.cpp2(1,13): error: invalid parameter list: expected expr (at ')')
+

source/parse.h

@@ -6182,8 +6182,11 @@ class parser
         auto param = std::make_unique<parameter_declaration_node>();
 
         auto count = 1;
+        auto expect_another_param_decl = false;
+
         while ((param = parameter_declaration(is_returns, is_named, is_template, is_statement)) != nullptr)
         {
+            expect_another_param_decl = false;
             param->ordinal = count;
             ++count;
 
@@ -6209,9 +6212,15 @@ class parser
                 }
                 return {};
             }
+
+            expect_another_param_decl = true;
             next();
         }
 
+        if (expect_another_param_decl) {
+            error("invalid parameter list: a comma must be followed by another parameter", true, {}, true);
+        }
+
         if (curr().type() != closer) {
             if (is_statement) {
                 pos = start_pos;    // backtrack