<future>: Emits warnings when compiling with /we5204

[bgianfo]

Describe the bug
Recent versions of MSVC provide a disabled by default warning /we5204, which flags classes with virtual functions but no virtual destructors. When enabling this warning across our code base, we had to suppress errors coming for <future>, it would be nice
if these could be fixed / suppressed internally to allow the warning to be used cleanly in user code bases.

Command-line test case

C:\Temp>type repro.cpp

#include <future>

int main()
{
    return 0;
}

C:\Temp>CL.exe repro.cpp /EHsc /we5204
Microsoft (R) C/C++ Optimizing Compiler Version 19.26.28806 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

repo.cpp
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.26.28801\include\pplwin.h(78): 
error C5204: 'Concurrency::details::_DefaultPPLTaskScheduler': 
class has virtual functions, but its trivial destructor is not virtual;
instances of objects derived from this class may not be destructed correctly

Expected behavior
I expect to be able to compile code using <future> and /we5204 and not have <future> or any other STL header emit a warning.

STL version
Microsoft Visual Studio Enterprise 2019
Version 16.6.2

[StephanTLavavej]

Thanks for the bug report! This is related to #186. While /Wall was historically not supported by the STL, we're moving towards doing so, and this is a good example of an off-by-default warning that makes sense to enable.

In this case, pplwin.h is Parallel Patterns Library machinery, used by <future> but not part of this repo, so we'll need to fix this in the Microsoft-internal repo (in addition to fixes and test coverage in GitHub).

[StephanTLavavej]

Fixed by internal MSVC-PR-497556 on 2023-09-14 marking this class as final, in conjunction with #4026, shipped in VS 2022 17.9. I've verified that your repro now compiles cleanly. Sorry for forgetting to resolve this issue at that time! ⛔ 🧠 🐈