Compile failure with MSVC with /W4 /WX - ostream.h(153): warning C4127: conditional expression is constant

[stefan301]

The following Code doesn't compile with 'Treat warnings as errors' and warning level 4:

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

int main()
{
	fmt::print(std::cerr, "Don't {}!", "panic");
}

compiling with cl /EHsc /std:c++20 /W4 /WX /utf-8 x.cpp /MDd /I D:\libs\fmt\include D:\libs\fmt\x64\Debug\fmtd.lib gives:

D:\libs\fmt\include\fmt/ostream.h(153): error C2220: the following warning is treated as an error
D:\libs\fmt\include\fmt/ostream.h(153): warning C4127: conditional expression is constant
D:\libs\fmt\include\fmt/ostream.h(153): note: consider using 'if constexpr' statement instead

The problem is this if in print because detail::use_utf8 is a compile time constant

if (detail::use_utf8) return vprint(os, fmt.str, vargs);

Changing print this way might solve the problem for C++17 and higher but doesn't compile with C++14

FMT_EXPORT template <typename... T>
void print(std::ostream& os, format_string<T...> fmt, T&&... args) {
  fmt::vargs<T...> vargs = {{args...}};
  if constexpr( detail::use_utf8 )
  {
          return vprint( os, fmt.str, vargs );
  }
  else
  {
          auto buffer = memory_buffer();
          detail::vformat_to( buffer, fmt.str, vargs );
          detail::write_buffer( os, buffer );
  }
}

[vitaut]

Fixing all possible warnings (especially level 4 which have tons of false positive) is a non-goal but you can suppress them with FMT_SYSTEM or other means.

[ZehMatt]

I also ran into this while bumping the dependency verison, wouldn't something like following solve the problem?

if FMT_CONSTEXPR (detail::use_utf8) return vprint(os, fmt.str, vargs);

This will probably not work for C++14 so it might be worth to have FMT_CONSTEXPR17 just like FMT_CONSTEXPR20

Another benefit would be that this condition is shifted to compile time and not runtime, I know optimizations are pretty clever but that will not happen for debug builds while with constexpr it will be a certain thing.

[phprus]

@ZehMatt

Use const_check function, like this:

fmt/include/fmt/base.h

Line 2913 in aabe639

if (detail::const_check(!detail::use_utf8))

[ZehMatt]

@ZehMatt

Use const_check function, like this:

fmt/include/fmt/base.h

Line 2913 in aabe639

if (detail::const_check(!detail::use_utf8))

Does that mean I should create a PR? We consume the library directly from this repo so a local patch wouldn't be ideal.

[ZehMatt]

Also it seems that detail::const_check exists to suppress the warning which isn't the same as actually having it constexpr. What is the reason behind this instead of just adding constexpr where the value is known compile time?

[vitaut]

A PR to add const_check would be welcome. I think if constexpr is an overkill for this case and it won't handle other cases with mixed compile-time and runtime conditions without complicating the code.