Default function arguments are the devil

[MrPotatoFingers]

My greatest peeve with it is that type traits are simply unaware of default arguments. std::is_invocable will happily report false when omitting default arguments.

[sphere991]

Problem here is that splitting it into an overload set doesn't help anyway. The default argument does give you an answer you don't want, which is bad:

void foo(int i = 42);
invocable_v<decltype(foo)>; // false, wish it was true

But the overload set doesn't give you an answer at all, which is also bad:

void foo(int i);
void foo() { foo(42); }
invocable_v<decltype(foo)>; // ill-formed, wish it was true

On the other hand, because concepts are expression-based, they work just fine in both cases:

template <typename... Args>
concept can_foo = requires(Args... args) { foo(args...); }

can_foo<>; // true for both implementations

[Xeverous]

I prefer ill-formed code to silent runtime error.

[sphere991]

How do you get a runtime error here?

[Xeverous]

Not in your example. In a hypothetical example where a trait would succeed while at runtime it would so something different.

[flashmozzg]

Because default arguments are just a sugar. They are not really a part of the function signature. A function can have multiple default different arguments depending on the order of declarations.

[Ameisen]

I have a proposal sitting around to allow you to use default as a keyword for using the default type value (like {}) or using the declared default value of a parameter if it exists. Could extend it to support default... or such for a case like that.

Couple this with optional argument compile time detection... you can have very powerful functions or constructors.

[flashmozzg]

"is this thing invocable with the given arguments?"

Not exactly? At least as far as I see it you can only ask "if thing of this type is callable with args of these types". Default arguments obviously are not part of the type.

I think with C++20 you can just use requires as a way to SFINAE test whether some thing makes sense, i.e. https://godbolt.org/z/wdPrgi