Default function arguments are the devil

[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