Add macro to detect vNext ABI break is in force

[ned14]

I, like quite a few people, have macro preprocessor logic to special case things for the Microsoft STL to work around shortcomings which will be fixed by the vNext ABI break.

Could we get a macro added NOW which would let us detect if the vNext ABI break is in force or not? Then we could turn off those workarounds if on vNext or later. This, down the line, would aid the testing and conformance for the vNext ABI break.

[StephanTLavavej]

This seems potentially counterproductive, encouraging people to begin adding preprocessor checks that may break if vNext doesn't end up containing what they think it'll contain. I am inclined to reject this. If (big if) and when we start work on vNext, we'll introduce some mechanism to detect it, but I don't think we should add one now.

[ned14]

I see and understand the rationale.

To do this properly, every deviation from the standard where vNext is mentioned in source comments ought to gain an internal feature macro, and then one can do #if _MSVC_STL_VNEXT_MOVE_ONLY_OBJECTS_IN_NESTED_CONTAINERS and then all would be good right?

Problem is somebody then needs to go add those internal feature macros. Still, as a documentation exercise around 'where we don't conform to the standard', it would be valuable work.

[StephanTLavavej]

I don't think a bunch of fine-grained detection macros would serve any purpose. Different names are valuable only if the answers can vary.