Corrected order of _template-argument_ productions, see #50

Examples like `f<a+a>` should be parsed as an _expression_ first (if it can be an expression, it is), and then if that fails parsed as an _id-expression_
The original comment was correct about the production order, so put that back to how it was
Added a regression test case

Author: hsutter

regression-tests/mixed-test-parens.cpp2

@@ -1,10 +1,13 @@
 #include <vector>
 
-template<bool>
+template<auto>
 auto f(auto, auto) -> void { }
 
+constexpr int a = 1;
+
 main: () -> int = {
     v : std::vector<int> = ( 1, 2, 3 );
     std::cout << (1+2)*(3+v[0]);
     f<(1>2)>(3,4);
+    f< a+a >(5,6);
 }

regression-tests/test-results/mixed-test-parens.cpp

@@ -4,17 +4,20 @@
 #line 1 "mixed-test-parens.cpp2"
 #include <vector>
 
-template<bool>
+template<auto>
 auto f(auto, auto) -> void { }
 
+constexpr int a = 1;
+
 [[nodiscard]] auto main() -> int;
 
 //=== Cpp2 definitions ==========================================================
 
-#line 5 "mixed-test-parens.cpp2"
+#line 7 "mixed-test-parens.cpp2"
 
 [[nodiscard]] auto main() -> int{
     std::vector<int> v { 1, 2, 3 }; 
     std::cout << (1 + 2) * (3 + v[0]);
     f<(1 > 2)>(3, 4);
+    f< a + a>(5, 6);
 }

source/parse.h

@@ -1761,8 +1761,8 @@ class parser
     //G     template-argument-list , template-argument
     //G
     //G template-argument:
-    //G     id-expression
     //G     expression
+    //G     id-expression
     //G
     auto unqualified_id() -> std::unique_ptr<unqualified_id_node>
     {
@@ -1804,12 +1804,12 @@ class parser
             unqualified_id_node::term term;
 
             do {
-                if (auto i = id_expression()) {
-                    term.arg = std::move(i);
-                }
-                else if (auto e = expression(false)) {  // disallow relational comparisons in template args
+                if (auto e = expression(false)) {   // disallow unparenthesized relational comparisons in template args
                     term.arg = std::move(e);
                 }
+                else if (auto i = id_expression()) {
+                    term.arg = std::move(i);
+                }
                 else {
                     back_out_template_arg_list();
                     return n;