[BUG] Cannot value initialise a function argument with equivalent of C++ `{}`

[Neil Henderson]

Describe the bug
I can't find a Cpp2 syntax to value initialise an argument passed to a function using its default constructor.

To Reproduce
Run cppfront on this code:

log: (message: std::string_view,
      context: std::string,
      event_id: int) =
{
    std::println("{} {} {}", message, context, event_id);
}

main: () = {
    log("event", (), 123);  // (1) Error in C++ compiler
    log("event", _, 123);   // (2) Error in C++ compiler
    log("event", {}, 123);  // (3) Error in cppfront
}

(1) produces this error:

main.cpp2:10:19: error: expected expression
   10 |     log("event", (), 123);
      |                   ^

(2) produces this error:

main.cpp2:10:18: error: use of undeclared identifier '_'
   10 |     log("event", _, 123);
      |                  ^

(3) produces a cppfront syntax error.

Repro on Godbolt

Additional context
I was translating the ranges::find_end example from cppreference and have used explicit arguments like this:

found4:= std::ranges::find_end(
    secret, "SWORD"sv,
    std::ranges::equal_to(),
    std::identity(),
    :(c: char) std::tolower(c));

instead of the original:

const auto found4 = std::ranges::find_end(secret, "SWORD"sv,
        {},
        {},
        [](char c) { return std::tolower(c); });

[Andrew Sukach]

Hi Neil,
The solution I think might be simplest for this is to use '{}' for default initialized arguments just like c++, and adding it would be pretty straight forward. One approach is to allow brace expressions here:

cppfront/source/parse.h

Lines 5715 to 5718 in 130f149

 if (curr().type() == lexeme::LeftParen 
     //  If in the future (not now) we decide to allow braced-expressions 
     //      || curr().type() == lexeme::LeftBrace 
     ) 

However that approach might not be best one given the comment right above it. I can probably add logic to the parser to grab '{}' only in this specific instance, but I'd like to hear from our BDFL, @hsutter, before going down any route.

Thanks.

[Herb Sutter]

Thanks! In Cpp2 so far, { } are used only for scopes.

Hot take: () does already mean default construction (e.g., x : int = ();) so an empty list argument () could be lowered to {}.

[Andrew Sukach]

I like that idea, I'll try it out.

[Derp McDerp]

if () is changed to lower to {} then wouldn't that imply that explicit constructors should be included in the overload set? i.e.

The following doesn't work in C++ (but should)

struct B {
	explicit B(int);
};

void foo(B);

B asdf() {
	B _ = auto{1}; // should mean B _ = B{1};
	foo({1});      // should mean foo(B{1}); (some might disagree)
	foo(auto{1});  // should mean foo(B{1});

	if (rand())
		return {1};     // should mean return B{1}; (some might disagree)
	else
		return auto{1}; // should mean return B{1};
}

The following should work in Cpp2:

B: type = {
	operator=: (out this, _: i32) = {} // explicit ctor
}

foo: (_: B) = {}

B asdf() {
	_: B = (1);
	foo((1)); // some might disagree that this should work
	foo(:_ = (1));
	foo(_(1));

	if (rand())
		return (1); // some might disagree that this should work
	else if (rand())
		return :_ = (1);
	else
		return _(1);
}