MSC unreachable code warning because of if constexpr (..)

[torsten48]

It looks like a new fmt version is comming soon (ChangeLog.md), so I testet the current main in my project and got annoying unreachable code warnings.
Example (fmt/base.h::2927)

template <typename... T>
FMT_INLINE void println(FILE* f, format_string<T...> fmt, T&&... args) {
  vargs<T...> va = {{args...}};
  if FMT_CONSTEXPR20 (detail::use_utf8) return vprintln(f, fmt.str, va);
  detail::vprint_mojibake(f, fmt.str, va, true);
}

In my case, the compiler sees detail::vprint_mojibake(f, fmt.str, va, true); as dead (unreachable) code.

Would it be acceptable for you if I supply a pr to add else in such places to avoid the warning?

template <typename... T>
FMT_INLINE void println(FILE* f, format_string<T...> fmt, T&&... args) {
  vargs<T...> va = {{args...}};
  if FMT_CONSTEXPR20 (detail::use_utf8) return vprintln(f, fmt.str, va);
    else detail::vprint_mojibake(f, fmt.str, va, true);
}

[vitaut]

Please provide a godbolt repro demonstrating the warning.

[torsten48]

I can't. When I select an MSC compiler on Godbolt the trunk version of fmt is not available (include file not found), and the latest tagged version is 12.0.0.

Here is my local minimal example (using fmt 12.2.0):

test.cpp

#include<fmt/format.h>

int main(void)
{
  fmt::println("The answer is {}", 42);
  return 0;
}

C:\work\test>cl /std:c++20 /Wall /utf-8 /EHsc /wd5045 /wd4514 /wd4820 /wd4623 /wd4365 /I . /DFMT_HEADER_ONLY test.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.51.36243 for x86
Copyright (C) Microsoft Corporation. All rights reserved.

test.cpp
...
C:\work\test\fmt\base.h(2927) : warning C4702: unreachable code
Microsoft (R) Incremental Linker Version 14.51.36243.0
Copyright (C) Microsoft Corporation. All rights reserved.

/out:test.exe
test.obj

[torsten48]

Hi Victor @vitaut, I don't know how to continue here. Do you want/need other input?
It's a simple change and does not harm other compiler.
println() is a void function so we could also write:

template <typename... T>
FMT_INLINE void println(FILE* f, format_string<T...> fmt, T&&... args) {
  vargs<T...> va = {{args...}};
  if FMT_CONSTEXPR20 (detail::use_utf8) vprintln(f, fmt.str, va);
    else detail::vprint_mojibake(f, fmt.str, va, true);
}

With or without return and braces, whatever style is prefered.

[torsten48]

#4822 completed all checks