This fixes MSVC warning C4127: conditional expression is constant.
Testing `TEST_STD_AT_LEAST_20_OR_RUNTIME_EVALUATED` by itself doesn't
emit this warning, but the condition here is more complicated. I'm
expanding the macro and mechanically simplifying the resulting code.
(Yeah, this warning is often annoying, and I introduced
`TEST_STD_AT_LEAST_20_OR_RUNTIME_EVALUATED` to avoid this warning
elsewhere, so it's disappointing that it doesn't make the compiler happy
here. If this change is undesirable, I can replace it with
`ADDITIONAL_COMPILE_FLAGS(cl-style-warnings)`, but ideally I'd like to
avoid having to suppress it.)
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Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
The Standard is currently unimplementable. We have to pick between:
1. Not implementing constexpr support properly in std::array<T, 0>
2. Making std::array<T, 0> non-trivial even when T is trivial
3. Returning nullptr from std::array<T, 0>::begin()
Libc++ initially picked (1). In 77b9abfc8e89, we started implementing constexpr properly, but lost the guarantee of triviality. Since it seems like both (1) and (2) are really important, it seems like (3) is the only viable option for libc++, after all. This is also what other implementations are doing.
This patch moves libc++ from (1) to (3).
It also:
- Improves the test coverage for the various ways of initializing std::array
- Adds tests for the triviality of std::array
- Adds tests for the aggregate-ness of std::array
Reviewed By: #libc, miscco, EricWF, zoecarver
Differential Revision: https://reviews.llvm.org/D80821
