<xiosbase>: Remove non-conforming extension ios_base::hexfloat

[frederick-vs-ja]

Fixes #3296.

All uses of ios_base::hexfloat are replaced with ios_base::scientific | ios_base::fixed. Also, it seems to me that we should no longer test ios_base::hexfloat.

If there's too many breaking for removing, we may fallback to deprecation.

[StephanTLavavej]

I previously thought that we couldn't remove ios_base::hexfloat until vNext, but in #4125 shipping in VS 2022 17.9 Preview 2 I was able to remove other static constexpr data members. (The enums had to be retained due to an internal tool that we didn't have time to fix.)

I think that TR1-era ios_base::hexfloat was sufficiently obscure that we should try removing this outright, given how simple it is to fix any affected code (as you mentioned, (std::ios_base::scientific | std::ios_base::fixed) is a targeted replacement, std::hexfloat is only slightly more work). Worst case, a bunch of Real World Code breaks and we have to reluctantly go back to your deprecation approach.

[frederick-vs-ja]

Worst case, a bunch of Real World Code breaks and we have to reluctantly go back to your deprecation approach.

Hmm, it seems that there're some codes on Github relying on ios::hexfloat.

[StephanTLavavej]

I'm willing to break a little code at compile-time, when that code is non-portable and easily fixed. Many changes (including even deprecation warnings!) can result in compiler errors, it's just a question of their magnitude and how easy they are to deal with.

That said, it would be an option to deprecate in 17.10, and wait until the next update for outright removal. I just don't think we need to move that slowly.

[frederick-vs-ja]

Thanks! I've pushed a commit for removal.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for removing this hex from the Standard Library! 🪄 😸 🎉