[all-commits] [llvm/llvm-project] 4e0ea2: [libc++] Use enable_if_t instead of _EnableIf

[Louis Dionne via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 4e0ea2cf2e7c5f04c526f5476eff70280f3c0871
      https://github.com/llvm/llvm-project/commit/4e0ea2cf2e7c5f04c526f5476eff70280f3c0871
  Author: Louis Dionne <ldionne.2 at gmail.com>
  Date:   2021-09-08 (Wed, 08 Sep 2021)

  Changed paths:
    M libcxx/include/__functional/bind_back.h
    M libcxx/include/__functional/bind_front.h
    M libcxx/include/__functional/not_fn.h
    M libcxx/include/__functional/perfect_forward.h
    M libcxx/include/__memory/construct_at.h
    M libcxx/include/array
    M libcxx/include/bit
    M libcxx/include/cmath
    M libcxx/include/deque
    M libcxx/include/forward_list
    M libcxx/include/list
    M libcxx/include/map
    M libcxx/include/optional
    M libcxx/include/queue
    M libcxx/include/set
    M libcxx/include/stack
    M libcxx/include/string
    M libcxx/include/type_traits
    M libcxx/include/unordered_map
    M libcxx/include/unordered_set
    M libcxx/include/vector

  Log Message:
  -----------
  [libc++] Use enable_if_t instead of _EnableIf

I just ran into a compiler error involving __bind_back and some overloads
that were being disabled with _EnableIf. I noticed that the error message
was quite bad and did not mention the reason for the overload being
excluded. Specifically, the error looked like this:

     candidate template ignored: substitution failure [with _Args =
     <ContiguousView>]: no member named '_EnableIfImpl' in 'std::_MetaBase<false>'

Instead, when using enable_if or enable_if_t, the compiler is clever and
can produce better diagnostics, like so:

     candidate template ignored: requirement 'is_invocable_v<
          std::__bind_back_op<1, std::integer_sequence<unsigned long, 0>>,
          std::ranges::views::__transform::__fn &, std::tuple<PlusOne> &,
          ContiguousView>' was not satisfied [with _Args = <ContiguousView>]

Basically, it tries to do a poor man's implementation of concepts, which
is already a lot better than simply complaining about substitution failure.

Hence, this commit uses enable_if_t instead of _EnableIf whenever
possible. That is both more straightforward than using the internal
helper, and also leads to better error messages in those cases.

I understand the motivation for _EnableIf's implementation was to improve
compile-time performance, however I believe striving to improve error
messages is even more important for our QOI, hence this patch. Furthermore,
it is unclear that _EnableIf actually improved compile-time performance
in any noticeable way (see discussion in the review for details).

Differential Revision: https://reviews.llvm.org/D108216