[to_cpp1] Default initialize function argument

Author: sookach

source/parse.h

@@ -661,6 +661,7 @@ struct expression_list_node
     token const* open_paren  = {};
     token const* close_paren = {};
     bool inside_initializer  = false;
+    bool default_initializer = false;
 
     struct term {
         passing_style                    pass = {};
@@ -5393,6 +5394,7 @@ class parser
     };
     mutable std::vector<function_body_extent> function_body_extents;
     mutable bool                              is_function_body_extents_sorted = false;
+    bool                                      is_inside_call_expr = false;
 
 public:
     auto is_within_function_body(source_position p) const
@@ -5747,6 +5749,8 @@ class parser
                 expr_list->inside_initializer = false;
             }
             n->expression_list_is_fold_expression = expr_list->is_fold_expression();
+            expr_list->default_initializer =
+                is_inside_call_expr && std::empty(expr_list->expressions);
             n->expr = std::move(expr_list);
             return n;
         }
@@ -5899,8 +5903,12 @@ class parser
                 //  Next should be an expression-list followed by a ')'
                 //  If not, then this wasn't a call expression so backtrack to
                 //  the '(' which will be part of the next grammar production
+		        is_inside_call_expr = true;
 
                 term.expr_list = expression_list(term.op, lexeme::RightParen);
+				
+		        is_inside_call_expr = false;
+
                 if (
                     term.expr_list
                     && curr().type() == lexeme::RightParen

source/to_cpp1.h

@@ -3959,6 +3959,11 @@ class cppfront
     )
         -> void
     {   STACKINSTR
+        if (n.default_initializer) {
+          printer.print_cpp2("{}", n.position());
+          return;
+        }
+
         auto add_parens =
             should_add_expression_list_parens()
             && !n.inside_initializer