Default initialize function argument, closes #1090 (#1092)

* Default initialize function argument

* Add output to test to show that defaults were applied

---------

Co-authored-by: Herb Sutter <[email protected]>

Author: sookach

regression-tests/mixed-default-arguments.cpp2

@@ -0,0 +1,14 @@
+auto cxx(int x, std::string y) -> void {
+    std::cout << x << " \"" << y << "\"" << "\n";
+}
+
+cxx2 : (x : int, y : std::string) = {
+    std::cout << "(x)$ \"(y)$\"\n";
+}
+
+main : () = {
+	cxx(1, "test");
+	cxx((), ());
+	cxx2(1, "test");
+	cxx2((), ());
+}

regression-tests/test-results/apple-clang-14-c++2b/mixed-default-arguments.cpp.execution

@@ -0,0 +1,4 @@
+1 "test"
+0 ""
+1 "test"
+0 ""

regression-tests/test-results/apple-clang-14-c++2b/mixed-default-arguments.cpp.output

@@ -0,0 +1,4 @@
+1 "test"
+0 ""
+1 "test"
+0 ""

regression-tests/test-results/apple-clang-15-c++2b/mixed-default-arguments.cpp.execution

@@ -1,4 +1,5 @@
-g++-14 (Ubuntu 14-20240412-0ubuntu1) 14.0.1 20240412 (experimental) [master r14-9935-g67e1433a94f]
+g++ (GCC) 14.1.1 20240607 (Red Hat 14.1.1-5)
 Copyright (C) 2024 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/apple-clang-15-c++2b/mixed-default-arguments.cpp.output

@@ -0,0 +1,4 @@
+1 "test"
+0 ""
+1 "test"
+0 ""

regression-tests/test-results/clang-12-c++20/mixed-default-arguments.cpp.execution

@@ -1,5 +1,5 @@
-# This is intended to be run in the /test-results/gcc-13 subdirectory
-# in a Linux shell with gcc 13 installed
+# This is intended to be run in the /test-results/gcc-14 subdirectory
+# in a Linux shell with gcc 14 installed
 #
 cp ../*.cpp .
 rm -f *.output
@@ -9,7 +9,7 @@ g++ --version > gcc-version.output 2>&1
 for f in *.cpp
 do
     let count=count+1
-    printf "[%s] Starting gcc 13 %s\n" "$count" "$f"
+    printf "[%s] Starting gcc 14 %s\n" "$count" "$f"
     g++ -I../../../include -std=c++2b -pthread -Wold-style-cast -Wunused-parameter -o test.exe $f > $f.output 2>&1
     rm -f $f
     if test -f "test.exe"; then

regression-tests/test-results/clang-12-c++20/mixed-default-arguments.cpp.output

@@ -0,0 +1,40 @@
+
+
+//=== Cpp2 type declarations ====================================================
+
+
+#include "cpp2util.h"
+
+#line 1 "mixed-default-arguments.cpp2"
+
+
+//=== Cpp2 type definitions and function declarations ===========================
+
+#line 1 "mixed-default-arguments.cpp2"
+auto cxx(int x, std::string y) -> void {
+    std::cout << x << " \"" << y << "\"" << "\n";
+}
+
+#line 5 "mixed-default-arguments.cpp2"
+auto cxx2(cpp2::impl::in<int> x, cpp2::impl::in<std::string> y) -> void;
+
+#line 9 "mixed-default-arguments.cpp2"
+auto main() -> int;
+
+//=== Cpp2 function definitions =================================================
+
+#line 1 "mixed-default-arguments.cpp2"
+
+#line 5 "mixed-default-arguments.cpp2"
+auto cxx2(cpp2::impl::in<int> x, cpp2::impl::in<std::string> y) -> void{
+    std::cout << (cpp2::to_string(x) + " \"" + cpp2::to_string(y) + "\"\n");
+}
+
+#line 9 "mixed-default-arguments.cpp2"
+auto main() -> int{
+ cxx(1, "test");
+ cxx({}, {});
+ cxx2(1, "test");
+ cxx2({}, {});
+}
+

regression-tests/test-results/clang-15-c++20-libcpp/mixed-default-arguments.cpp.execution

@@ -0,0 +1,2 @@
+mixed-default-arguments.cpp2... ok (mixed Cpp1/Cpp2, Cpp2 code passes safety checks)
+

regression-tests/test-results/clang-15-c++20-libcpp/mixed-default-arguments.cpp.output

@@ -0,0 +1,4 @@
+1 "test"
+0 ""
+1 "test"
+0 ""

regression-tests/test-results/clang-15-c++20/mixed-default-arguments.cpp.execution

@@ -0,0 +1 @@
+mixed-default-arguments.cpp

regression-tests/test-results/clang-15-c++20/mixed-default-arguments.cpp.output

@@ -657,6 +657,7 @@ 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 = {};
@@ -5389,6 +5390,7 @@ 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
@@ -5743,6 +5745,8 @@ 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;
         }
@@ -5897,8 +5901,10 @@ 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

regression-tests/test-results/clang-18-c++20/mixed-default-arguments.cpp.execution

@@ -3966,6 +3966,11 @@ 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