[PATCH] D38757: [libc++] Fix PR34898 - vector iterator constructors and assign method perform push_back instead of emplace_back.

[David L. Jones via Phabricator via cfe-commits]

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Hmm, looking more at this change... while it does make the behaviour consistent for Forward and Input iterators, I think it's just making them both do the wrong thing.

Specifically, based on this:

"... i and j denote iterators satisfying input iterator requirements and refer to elements implicitly convertible to value_­type..."

https://timsong-cpp.github.io/cppwp/n4659/container.requirements#sequence.reqmts-3

So, for example, in test_emplacable_concept, the vector constructor should be diagnosed, because there is no way to *implicitly* convert from the dereferenced iterator type to the inserted type. The selected constructor is explicit. Using emplacement just omits a *second* potentially-expensive conversion: the explicit constructor behaviour (invoked through forwarding) may still be undesired.


https://reviews.llvm.org/D38757