<format> header present in Emscripten libc++ but __cpp_lib_format undefined

[anutosh491]

I saw the header present in sysroot and I tried

// file: check_format.cpp
#include <iostream>
#include <version>   // feature-test macros live here
#include <format>    // attempt to include format if available

int main() {
#if defined(__cpp_lib_format)
    std::cout << "__cpp_lib_format is defined: " << __cpp_lib_format << std::endl;
#else
    std::cout << "__cpp_lib_format is undefined" << std::endl;
#endif
    return 0;
}

And then compiled using

anutosh491@Anutoshs-MacBook-Air clang-wasm-repl % em++ check_format.cpp -std=c++20 -o check_format.js
anutosh491@Anutoshs-MacBook-Air clang-wasm-repl % node check_format.js
__cpp_lib_format is undefined

Am I missing something here ?

[sbc100]

What version of emscripten are you using?

It certainly looks like it being defined in the latest version.

$ cat test.cc 
#include <iostream>
#include <version>
#include <format>

#ifndef __cpp_lib_format
#error "NOT DEFINED"
#endif
$ emcc -std=c++20 -c test.cc 

You can see the check is working because if you drop the -std=c++20 it fails:

$ emcc -c test.cc 
test.cc:6:2: error: "NOT DEFINED"
    6 | #error "NOT DEFINED"
      |  ^
1 error generated.

[anutosh491]

I was on 3.1.73 !

[sbc100]

It looks like this macro was adding in the libc++ upgrade in #22994, which was adding in 4.0.0, just two releases after 3.1.73