[libcxx-commits] [PATCH] D125300: type_traits: rewrite is_invocable_r in terms of argument passing.

[Aaron Jacobs via Phabricator via libcxx-commits]

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This fixes incorrect handling of non-moveable types, adding tests for this case.
See issue 55346 <https://github.com/llvm/llvm-project/issues/55346>.

The current implementation is based on is_convertible, which is
defined <https://timsong-cpp.github.io/cppwp/n4659/meta.rel#5> in terms of
validity of the following function:

  To test() {
    return declval<From>();
  }

But this doesn't work if To and From are both some non-moveable type, which the
definition <https://timsong-cpp.github.io/cppwp/n4659/conv#3> of implicit
conversions says should work due to guaranteed copy elision:

  To to = E;  // E has type From

It is this latter definition that is used in the
definition <https://timsong-cpp.github.io/cppwp/n4659/function.objects#func.require-2>
of INVOKE<R>. Make is_invocable_r use an implementation that captures the
ability to use guaranteed copy elision, making the definition correct for
non-moveable types.

Fixes llvm/llvm-project#55346.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D125300

Files:
  libcxx/include/type_traits
  libcxx/test/libcxx/type_traits/is_invocable_r.pass.cpp

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