fix(parse): recognize const and pointer template arguments (hsutter#521)

* fix(parse): recognize const and pointer template arguments

* Minor: Massage the new code's format and comment

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

* Update parse.h

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: 2 people

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

@@ -0,0 +1,3 @@
+main: () = //
+  std::is_void_v<* i32> //
+  && std::is_void_v<const i32>;

regression-tests/test-results/gcc-13/pure2-bugfix-for-template-argument.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-template-argument.cpp2"
+auto main() -> int;
+  
+
+//=== Cpp2 function definitions =================================================
+
+#line 1 "pure2-bugfix-for-template-argument.cpp2"
+auto main() -> int { //
+  std::is_void_v<cpp2::i32*> //
+  && std::is_void_v<cpp2::i32 const>; }
+

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

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

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

@@ -4822,6 +4822,7 @@ class parser
     //G
     //G template-argument:
     //G     # note: < > << >> are not allowed in expressions until new ( is opened
+    //G     'const' type-id
     //G     expression
     //G     type-id
     //G
@@ -4855,16 +4856,32 @@ class parser
             auto term = unqualified_id_node::term{};
 
             do {
-                // disallow unparenthesized relational comparisons in template args
-                if (auto e = expression(false)) {
+                //  If it doesn't start with * or const (which can only be a type id),
+                //  try parsing it as an expression
+                if (auto e = [&]{
+                        if (
+                            curr().type() == lexeme::Multiply // '*'
+                            || curr() == "const"              // 'const'
+                        )
+                        {
+                            return decltype(expression()){};
+                        }
+                        return expression(false);   // false == disallow unparenthesized relational comparisons in template args
+                    }()
+                ) 
+                {
                     term.arg = std::move(e);
                 }
+                    
+                //  Else try parsing it as a type id
                 else if (auto i = type_id()) {
                     term.arg = std::move(i);
                 }
+                    
                 else {
                     break;
                 }
+                
                 n->template_args.push_back( std::move(term) );
             }
             //  Use the lambda trick to jam in a "next" clause