fix(parse): make template-argument-list optional (#473)

* fix(parse): make template-argument-list optional

* refactor: use `open_angle` as cue for temp arg list

* test: add `test-results/gcc-13`

Author: JohelEGP

regression-tests/pure2-bugfix-for-optional-template-argument-list.cpp2

@@ -0,0 +1,2 @@
+plus: const std::plus<> = ();
+main: () -> int         = std::plus<>()(0, 0);

regression-tests/test-results/gcc-13/gcc-version.output

@@ -0,0 +1,5 @@
+c++ (GCC) 13.1.1 20230429
+Copyright (C) 2023 Free Software Foundation, Inc.
+This is free software; see the source for copying conditions.  There is NO
+warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+

regression-tests/test-results/gcc-13/pure2-bugfix-for-optional-template-argument-list.cpp.execution

@@ -0,0 +1,23 @@
+
+#define CPP2_USE_MODULES         Yes
+
+//=== Cpp2 type declarations ====================================================
+
+
+#include "cpp2util.h"
+
+
+
+//=== Cpp2 type definitions and function declarations ===========================
+
+#line 1 "pure2-bugfix-for-optional-template-argument-list.cpp2"
+extern std::plus<> const plus;
+[[nodiscard]] auto main() -> int;
+
+
+//=== Cpp2 function definitions =================================================
+
+#line 1 "pure2-bugfix-for-optional-template-argument-list.cpp2"
+std::plus<> const plus {}; 
+[[nodiscard]] auto main() -> int { return std::plus<>()(0, 0); }
+

regression-tests/test-results/gcc-13/pure2-bugfix-for-optional-template-argument-list.cpp.output

@@ -0,0 +1,2 @@
+pure2-bugfix-for-optional-template-argument-list.cpp2... ok (all Cpp2, passes safety checks)
+

regression-tests/test-results/pure2-bugfix-for-optional-template-argument-list.cpp

@@ -1666,7 +1666,7 @@ class cppfront
         assert(n.identifier);
         emit(*n.identifier, is_qualified);  // inform the identifier if we know this is qualified
 
-        if (!n.template_args.empty()) {
+        if (n.open_angle != source_position{}) {
             printer.print_cpp2("<", n.open_angle);
             auto first = true;
             for (auto& a : n.template_args) {

regression-tests/test-results/pure2-bugfix-for-optional-template-argument-list.cpp2.output

@@ -886,7 +886,7 @@ struct unqualified_id_node
     auto get_token() const
         -> token const*
     {
-        if (template_args.empty()) {
+        if (open_angle == source_position{}) {
             assert (identifier);
             return identifier;
         }
@@ -911,12 +911,13 @@ struct unqualified_id_node
         assert (identifier);
         v.start(*identifier, depth+1);
 
-        if (!template_args.empty()) {
+        if (open_angle != source_position{}) {
             //  Inform the visitor that this is a template args list
             v.start(template_args_tag{}, depth);
             assert(open_angle  != source_position{});
             assert(close_angle != source_position{});
-            assert(template_args.front().comma == source_position{});
+            assert(template_args.empty()
+                   || template_args.front().comma == source_position{});
             for (auto& a : template_args) {
                 try_visit<expression>(a.arg, v, depth+1);
                 try_visit<type_id   >(a.arg, v, depth+1);
@@ -1118,7 +1119,7 @@ auto unqualified_id_node::to_string() const
 {
     assert(identifier);
     auto ret = identifier->to_string(true);
-    if (!template_args.empty()) {
+    if (open_angle != source_position{}) {
         auto separator = std::string{"<"};
         for (auto& t : template_args) {
             ret += separator;
@@ -4815,18 +4816,9 @@ class parser
             //  Remember current position, in case this < is isn't a template argument list
             auto start_pos = pos;
 
-            //  And since we'll do this in two places, factor it into a local function
-            auto back_out_template_arg_list = [&]{
-                //  Aha, this wasn't a template argument list after all,
-                //  so back out just that part and return the identifier
-                n->open_angle = source_position{};
-                n->template_args.clear();
-                pos = start_pos;
-            };
-
             n->open_angle = curr().position();
             next();
-
+            
             auto term = unqualified_id_node::term{};
 
             do {
@@ -4838,8 +4830,7 @@ class parser
                     term.arg = std::move(i);
                 }
                 else {
-                    back_out_template_arg_list();
-                    return n;
+                    break;
                 }
                 n->template_args.push_back( std::move(term) );
             }
@@ -4853,7 +4844,11 @@ class parser
                 //      next  term.comma = curr().position();
 
             if (curr().type() != lexeme::Greater) {
-                back_out_template_arg_list();
+                //  Aha, this wasn't a template argument list after all,
+                //  so back out just that part and return the identifier
+                n->open_angle = source_position{};
+                n->template_args.clear();
+                pos = start_pos;
                 return n;
             }
             n->close_angle = curr().position();