[libcxx-commits] [PATCH] D108216: [libc++] Formulate _EnableIf in terms of std::enable_if

[Louis Dionne via Phabricator via libcxx-commits]

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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 does two things:

1. Use 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.

2. Reformulate _EnableIf in terms of enable_if so that pre-C++14 places get better error messages too.

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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D108216

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

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