Fix unqualified_id handling of empty template arguments, closes #819

A fully empty `<>` list is allowed, but `unqualified_id_node::to_string` didn't handle empty `<>` correctly and included the `>`. Now it doesn't include it.

A nonempty `<>` list shouldn't have an empty argument in the list.

Author: hsutter

regression-tests/test-results/version

@@ -1,5 +1,5 @@
 
-cppfront compiler v0.3.0   Build 8B10:1325
+cppfront compiler v0.3.0   Build 8B13:1628
 Copyright(c) Herb Sutter   All rights reserved
 
 SPDX-License-Identifier: CC-BY-NC-ND-4.0

source/build.info

@@ -1 +1 @@
-"8B10:1325"
+"8B13:1628"

source/parse.h

@@ -1294,7 +1294,9 @@ auto unqualified_id_node::to_string() const
             }
             separator = ",";
         }
-        ret += ">";
+        if (std::ssize(template_args) > 0) {
+            ret += ">";
+        }
     }
     return ret;
 }
@@ -6237,6 +6239,14 @@ class parser
                     term.arg = std::move(i);
                 }
 
+                //  Else if we already got at least one template-argument, this is a
+                //  ',' followed by something that isn't a valid template-arg
+                else if (std::ssize(n->template_args) > 0) {
+                    error( "expected a template argument after ','", false);
+                    return {};
+                }
+
+                //  Else this is an empty '<>' list which is okay
                 else {
                     break;
                 }
@@ -7873,7 +7883,7 @@ class parser
             next();
             auto idx = id_expression();
             if (!idx) {
-                error("'@' must be followed by a a metafunction name", false);
+                error("'@' must be followed by a metafunction name", false);
                 return {};
             }
             n->metafunctions.push_back( std::move(idx) );