[libcxx-commits] [PATCH] D116388: [libc++] Properly handle specializations of std::is_placeholder.

[Arthur O'Dwyer via Phabricator via libcxx-commits]

Quuxplusone updated this revision to Diff 396707.
Quuxplusone marked an inline comment as done.
Quuxplusone added a comment.

...and I forgot to answer @ldionne's question about conformingness, so I'll do it here!

My understanding is that `std::bind` is just massively underspecified. For example, https://eel.is/c++draft/func.bind.isbind says

> The function template `bind` uses `is_­bind_­expression` to detect subexpressions. [...] A program may specialize this template for a program-defined type `T` to have a base characteristic of `true_­type` to indicate that `T` should be treated as a subexpression in a `bind` call.

But, like, //how// does it use `is_­bind_­expression` to detect subexpressions? and what does it mean to "be treated as a subexpression"? and what happens if this template is specialized to have a base characteristic of something-other-than-`true_type`? None of this is written out. But I think clearly the //intent// is what's happening in this PR: a single specialization for `T` should magically apply to all cv-qualifications and value-categories of `T`.

It is also never really specified what should happen if the user //queries// `std::is_placeholder_v<U>` for any particular `U`;  `is_placeholder` is fundamentally a mechanism for //customizing// the library's behavior, not for //querying// it.

  #include <functional>
  struct P {};
  template<> struct std::is_placeholder<P> : std::true_type {};
  static_assert(std::is_placeholder_v<const P>);  // unspecified

Here's the exhaustive rundown: https://godbolt.org/z/jcTnqY6vb
There's some implementation divergence, but not as much as I originally thought.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D116388/new/

https://reviews.llvm.org/D116388

Files:
  libcxx/include/__functional/bind.h
  libcxx/test/std/utilities/function.objects/bind/func.bind/func.bind.isbind/is_bind_expression.pass.cpp
  libcxx/test/std/utilities/function.objects/bind/func.bind/func.bind.isbind/is_placeholder.pass.cpp
  libcxx/test/std/utilities/function.objects/bind/func.bind/func.bind.isbind/specialization.pass.cpp
  libcxx/test/std/utilities/function.objects/bind/func.bind/func.bind.isplace/is_placeholder.pass.cpp
  libcxx/test/std/utilities/function.objects/bind/func.bind/func.bind.isplace/specialization.pass.cpp

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