Add diagnostic for that a variadic declaration must have a name, closes #708

Author: hsutter

regression-tests/test-results/version

@@ -1,5 +1,5 @@
 
-cppfront compiler v0.2.1   Build 8928:1030
+cppfront compiler v0.2.1   Build 8928:1101
 Copyright(c) Herb Sutter   All rights reserved
 
 SPDX-License-Identifier: CC-BY-NC-ND-4.0

source/build.info

@@ -1 +1 @@
-"8928:1030"
+"8928:1101"

source/parse.h

@@ -8312,6 +8312,14 @@ class parser
                 return {};
             }
 
+            if (id->to_string() == "...") {
+                errors.emplace_back(
+                    curr().position(),
+                    "a variadic declaration must have a name - did you forget to write a name before '...'?"
+                );
+                pos = start_pos;    // backtrack
+            }
+
             auto is_variadic = false;
             if (curr().type() == lexeme::Ellipsis) {
                 is_variadic = true;