<xtree>: Use scope guard for node copy failure

[frederick-vs-ja]

Towards #2307.

Also drops an if-statement and reuses _Newroot since _Scary->_Myhead->_Isnil is always true. Closes #1913.

[AlexGuteniev]

Is there test coverage?

[frederick-vs-ja]

Is there test coverage?

I think the partial test coverage is (roughly) in libcxx test's std/containers/associative/from_range_associative_containers.h. The coverage doesn't seem complete though.

Perhaps we should add a test file to MSVC STL which covers copy/move ctors, corresponding allocator-extended ctors, and copy/move assignment operators of associative containers.

[AlexGuteniev]

I expect that non-throwing path is implicitly covered somehow, I think it is enough if there's a test for throwing path

[AlexGuteniev]

I mean I'm asking for the coverage for the guard, not for the unneeded conditional removal.

[StephanTLavavej]

We didn't have test coverage for the _RERAISE before and the transformation is fairly straightforward so I'm willing to accept this PR as-is, but adding test coverage in a followup that exercises the guard would certainly be reasonable.

[AlexGuteniev]

#2308 has an example of coverage

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for eliminating yet another occurrence of the try-catch-rethrow pattern! 🐈 🐈‍⬛ 😸