Forgot the second link, as in C, as you predicted, it works

https://godbolt.org/z/hdGo6sxTn

On Thu, Apr 04, 2024 at 09:23:10AM +0200, Federico Kircheis via Std-Discussion wrote:
>On Wed, Apr 03, 2024 at 11:26:51AM -0700, Thiago Macieira via Std-Discussion wrote:
>>On Wednesday 3 April 2024 05:52:43 PDT Federico Kircheis via Std-Discussion
>>wrote:
>>>The question is if something similar was considered for functions,
>>>because there are similar advantages
>>
>>constexpr functions are implicitly inline. Therefore, you MUST see the body in
>>order to call them. We don't have extern inline like C does. It seems to me
>>that to have what you're requesting, this is what you need to focus on first.
>>
>>I mean, extern inlines compile, but they does not do what they do in C, which
>>is to emit an out-of-line non-weak/non-mergeable copy of the function. Compare
>>https://c.godbolt.org/z/Y4xebq4PE to https://gcc.godbolt.org/z/cx5qorn5T
>>
>
>Oh, this is a good point
>
>// lib0.hpp
>const char* foo(int);
>
>// lib0.cpp
>inline const char* foo(int){
> return "hello world!";
>}
>
>// main.cpp
>#include "lib0.hpp"
>
>#include <cstdio>
>
>int main(){
> std::puts(foo(42));
>}
>
>fails to compile (and succeeds when inline is removed from lib0.cpp)
>
>https://godbolt.org/z/EPzhdncMe