win32_helpers.h no longer compiles with WIL_SUPPRESS_EXCEPTIONS and C++20

[sylveon]

Compile the following in C++20 mode:

#define WIL_SUPPRESS_EXCEPTIONS
#include <Windows.h>
#include <wil/win32_helpers.h>

int main()
{

}

Observe error:

Build started at 03:35...
1>------ Build started: Project: ConsoleApplication10, Configuration: Debug x64 ------
1>ConsoleApplication10.cpp
1>Z:\Throwaway\ConsoleApplication10\ConsoleApplication10\vcpkg_installed\x64-windows\x64-windows\include\wil\win32_helpers.h(213,21): error C2039: 'bit_cast': is not a member of 'std'
1>(compiling source file 'ConsoleApplication10.cpp')
1>    C:\Program Files (x86)\Windows Kits\10\Include\10.0.26100.0\ucrt\new.h(24,19):
1>    see declaration of 'std'
1>Z:\Throwaway\ConsoleApplication10\ConsoleApplication10\vcpkg_installed\x64-windows\x64-windows\include\wil\win32_helpers.h(213,30): error C2062: type 'unsigned __int64' unexpected
1>(compiling source file 'ConsoleApplication10.cpp')
1>Z:\Throwaway\ConsoleApplication10\ConsoleApplication10\vcpkg_installed\x64-windows\x64-windows\include\wil\win32_helpers.h(210,34): error C3615: constexpr function 'wil::filetime::to_int64' cannot result in a constant expression
1>(compiling source file 'ConsoleApplication10.cpp')
1>    Z:\Throwaway\ConsoleApplication10\ConsoleApplication10\vcpkg_installed\x64-windows\x64-windows\include\wil\win32_helpers.h(210,34):
1>    failure was caused by control reaching the end of a constexpr function
1>Z:\Throwaway\ConsoleApplication10\ConsoleApplication10\vcpkg_installed\x64-windows\x64-windows\include\wil\win32_helpers.h(224,21): error C2039: 'bit_cast': is not a member of 'std'
1>(compiling source file 'ConsoleApplication10.cpp')
1>    C:\Program Files (x86)\Windows Kits\10\Include\10.0.26100.0\ucrt\new.h(24,19):
1>    see declaration of 'std'
1>Z:\Throwaway\ConsoleApplication10\ConsoleApplication10\vcpkg_installed\x64-windows\x64-windows\include\wil\win32_helpers.h(224,30): error C2275: 'FILETIME': expected an expression instead of a type
1>(compiling source file 'ConsoleApplication10.cpp')
1>Z:\Throwaway\ConsoleApplication10\ConsoleApplication10\vcpkg_installed\x64-windows\x64-windows\include\wil\win32_helpers.h(221,38): error C3615: constexpr function 'wil::filetime::from_int64' cannot result in a constant expression
1>(compiling source file 'ConsoleApplication10.cpp')
1>    Z:\Throwaway\ConsoleApplication10\ConsoleApplication10\vcpkg_installed\x64-windows\x64-windows\include\wil\win32_helpers.h(221,38):
1>    failure was caused by control reaching the end of a constexpr function
1>Done building project "ConsoleApplication10.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
========== Build completed at 03:35 and took 00.784 seconds ==========

This seems to be caused by #490: inclusion of <bit> is now ultimately guarded by WIL_USE_STL, however further down the file, the feature check for __cpp_lib_bit_cast is done without checking WIL_USE_STL first. Should be easy to fix.

WIL_USE_STL seems to be automatically determined based on exception support, but the entire <bit> header is exception-free. Should we exclude exception-free usage of STL facilities from that WIL_USE_STL macro, or add something like WIL_USE_NOEXCEPT_STL to allow inclusion of exception-free STL bits? With the obvious fix (adding WIL_USE_STL to the feature checks), people who suppress exceptions are being pessimized (worse codegen, no constexpr filetime) for no valid reason.

[dunhor]

however further down the file, the feature check for __cpp_lib_bit_cast is done without checking WIL_USE_STL first. Should be easy to fix

Correct, the use should also be guarded. This instance was missed

Should we exclude exception-free usage of STL facilities from that WIL_USE_STL macro, or add something like WIL_USE_NOEXCEPT_STL to allow inclusion of exception-free STL bits

I'd rather we not increase the complexity in this space. The default value of WIL_USE_STL is heuristically chosen, but can be overridden by the consumer. If you want to disable exceptions, but still use parts of the STL, compile with -DWIL_USE_STL=1. Uses of STL features that throw exceptions should still be guarded by WIL_ENABLE_EXCEPTIONS.