<chrono>: Spurious warning C4702: unreachable code #15
Description
Basic Info
-
I have read the rules for the Standard Library Modules Bug Bash.
-
I have pulled and rebuilt the
bug-bashbranch in the last 24 hours. -
My current commit, as printed by
git rev-parse HEAD, is: 6c96fe7 -
This bug is: warning-on-valid
Command-Line Test Case
My test case that works with classic includes but fails with named modules:
D:\GitHub\STL>type repro.cpp
#ifdef USE_CLASSIC_INCLUDES
#include <chrono>
#include <format>
#include <iostream>
#else
import std;
#endif
int main() {
std::cout << std::format("{}\n", std::chrono::seconds{1});
}D:\GitHub\STL>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od repro.cpp /DUSE_CLASSIC_INCLUDES && repro.exe
repro.cpp
1s
D:\GitHub\STL>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od repro.cpp std.obj && repro.exe
repro.cpp
D:\GitHub\STL\out\x64\out\inc\chrono(5028) : warning C4702: unreachable code
D:\GitHub\STL\out\x64\out\inc\chrono(5603) : warning C4702: unreachable code
D:\GitHub\STL\out\x64\out\inc\chrono(5704) : warning C4702: unreachable code
D:\GitHub\STL\out\x64\out\inc\chrono(5471) : warning C4702: unreachable code
1s
The STL internally suppresses this warning, but Standard Library Modules isn't handling that suppression.
Additional Context
This test case is from @nico-engels in #14 ; after working around the compiler error reported there, I found this warning.
I've fully reduced this and reported it as internal VSO-1582381 "Standard Library Modules: Bogus warning C4702: unreachable code". Minimal repro:
C:\Temp>type library.ixx
export module library;
#pragma warning(push)
#pragma warning(disable : 4702)
namespace library {
export template <typename T>
int func(T t) {
if constexpr (sizeof(T) == 1) {
return t * 11;
}
return t * 111;
}
} // namespace library
#pragma warning(pop)C:\Temp>type meow.cpp
#include <stdio.h>
import library;
int main() {
printf("%d\n", library::func(static_cast<unsigned char>(3)));
printf("%d\n", library::func(4));
}C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /Femeow.exe library.ixx meow.cpp && meow
library.ixx
meow.cpp
Generating Code...
C:\Temp\library.ixx(12) : warning C4702: unreachable code
33
444
Without modules, suppression works:
C:\Temp>type classic.cpp
#include <stdio.h>
#pragma warning(push)
#pragma warning(disable : 4702)
namespace library {
template <typename T>
int func(T t) {
if constexpr (sizeof(T) == 1) {
return t * 11;
}
return t * 111;
}
} // namespace library
#pragma warning(pop)
int main() {
printf("%d\n", library::func(static_cast<unsigned char>(3)));
printf("%d\n", library::func(4));
}C:\Temp>cl /EHsc /nologo /W4 /std:c++latest classic.cpp && classic
classic.cpp
33
444