Fix C4365: '=': signed/unsigned mismatch with /J

[fsb4000]

Fixes #3294

[strega-nil-ms]

I think this is a bug in the compiler; similar code without all of the stuff around it does not fire this warning. I would prefer we fix the compiler rather than work around it in the library, (or at least mark it with a transition).

[strega-nil-ms]

Okay, now that I look at it, it is an off-by-default warning; I personally think it's very silly to warn for:

unsigned char c = ...;
c = c + 1;

and I'd prefer we just turned off that warning or something along those lines.

[fsb4000]

We already have the same static_cast for fixing the warning:

STL/stl/inc/xcharconv_ryu.h

Line 830 in 77ec5c6

*_First++ = static_cast<char>('0' + __digits);

_First is char* and __digits is uint32_t, this is why it is triggered without /J

[strega-nil-ms]

I disagree with that - __digits is not the same type as the type we're assigning into, which means it makes sense (to me) to cast it down - it's a narrowing cast.

The unsigned char thing is just a silly result of C's arithmetic promotion rules. It should not be treated as something to warn on.

[StephanTLavavej]

We talked about this at the weekly maintainer meeting; warning C4365 is indeed one of the few off-by-default warnings we attempt to be clean for:

STL/tests/std/tests/prefix.lst

Line 6 in 8ddf4da

PM_CL="/FIforce_include.hpp /w14365 /D_ENFORCE_FACET_SPECIALIZATIONS=1 /D_STL_CALL_ABORT_INSTEAD_OF_INVALID_PARAMETER"

We additionally (grudgingly) support /J, in the running for the worst compiler option ever, so this is technically a bug.

[strega-nil-ms]

I severely dislike both this warning and this option, but it is a correct change. Thanks!

[StephanTLavavej]

Thanks for the fix and updated test coverage! This is a minimally invasive change for the Ryu-derived code.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

I had to push an additional commit to fix the internal test harness, as /clr:pure is incompatible with /J. I've chosen to change the usual_matrix to the impure_matrix, omitting a bit of unnecessary test coverage. (There are an extremely small number of /clr:pure users, virtually all of which are in C++14 mode, so the combination of /clr:pure and /Zc:alignedNew- isn't worth testing.)

[StephanTLavavej]

Thanks for fixing this warning! 🛠️ ⚠️ ✨