gcc 11, C++20: error: 'f' is not a constant expression

[D-r-P-3-p-p-3-r]

Under Linux with gcc 11 the following example builds fine with -std=c++17 but fails to build with -std=c++20:

#include <iostream>
#include "fmt/format.h"

template<typename... Args>
void foo(const char * const f, Args... args){
	const auto bar = fmt::format(f, args...);
	std::cout << bar << std::endl;
}

int main() {
	foo("test {} {}", 1, 2);
}

libfmt 8.0.1 and 8.1.1:

main.cpp: In instantiation of 'void foo(const char*, Args ...) [with Args = {int, int}]':
main.cpp:11:5: required from here
main.cpp:6:30: in 'constexpr' expansion of 'fmt::v8::basic_format_string<char, int&, int&>(f)'
main.cpp:6:37: error: 'f' is not a constant expression
6 | const auto bar = fmt::format(f, args...);
| ~~~~~~~~~~~^~~~~~~~~~~~

Why?

[vitaut]

The format string must be known at compile time or wrapped in fmt::runtime.

[antonysigma]

I also encounter this problem with g++-11. Perhaps this issue is a duplicate of #2455 (comment) ?

The format string must be known at compile time or wrapped in fmt::runtime.

I thought std::string_view is always a constant and known at compile time? This is my pseudo code:

#include <iostream>
#include "fmt/format.h"

template<typename... Args>
void foo(std::string_view f, Args... args){
	const auto bar = fmt::format(f, args...);
	std::cout << bar << std::endl;
}

int main() {
	foo("test {} {}", 1, 2);
}

Update: I am forcing runtime for the time being, until this issue is resolved. #2438

[vitaut]

I thought std::string_view is always a constant and known at compile time?

It is not.

[antonysigma]

Update: Turns out if I am willing to give up const char* or std::string_view as function parameters, I can enable compile-time format string checking. https://stackoverflow.com/a/69647103

@vitaut Sorry for posting new messages to the "closed" issue.

[vitaut]

No worries, glad you've found the solution.