literal_ipaddr | C17 constexpr implementation of inet_addr / inet_aton / | File Utils library

ipSourceAddress does not get its value at compile time (as-if rule nonwithstanding).

ip's value is not usable in a constant expression since it is not declared constexpr and doesn't qualify for one of the other exceptions to the rule of lvalue-to-rvalue conversion in a constant expression. Therefore IPAddr::inet_pton(ip) is not a constant expression.

You can see this clearly by making ipSourceAddress constexpr (which auto does not imply).

constexpr on a variable requires initialization at compile-time and because the initializer is not a constant expression it will fail.

See godbolt.

I don't know how you come to a different conclusion.

Note however that the library does seem to require that the char array passed to it is exactly as long as the string it contains (plus null-terminator). It fails if you hand it a longer array and outputs the value you are seeing.

See godbolt.

It seems the author intends the functions to only be called directly with string literals.