[all-commits] [llvm/llvm-project] 0acafc: [libc++] Provide an assignment operator from pair<...

[Louis Dionne via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 0acafcd8043412b028be89ce68ce301ce329d3eb
      https://github.com/llvm/llvm-project/commit/0acafcd8043412b028be89ce68ce301ce329d3eb
  Author: Louis Dionne <ldionne.2 at gmail.com>
  Date:   2023-05-09 (Tue, 09 May 2023)

  Changed paths:
    M libcxx/include/__utility/pair.h
    M libcxx/test/std/utilities/utility/pairs/pairs.pair/assign_const_pair_U_V.pass.cpp
    M libcxx/test/std/utilities/utility/pairs/pairs.pair/assign_pair_cxx03.pass.cpp

  Log Message:
  -----------
  [libc++] Provide an assignment operator from pair<U, V> in C++03

This adds an extension to std::pair in C++03 mode where we provide an
assignment operator from a pair<U, V>. Previously, any code trying to
trigger such an assignment operator would have tried using the
`operator=(pair const&)` copy assignment operator, which would then
have tried creating a `pair const&` by using the unconstrained
pair<U, V> constructor.

After this patch, pair will instead go through operator= directly if
its member types are assignable. If they are not assignable, the extension
operator= is disabled with SFINAE and the pair(pair<U, V>) constructor
will be used. Since that constructor is unconstrained, that will either
work or give a hard error.

This should be pretty transparent to users, but this is technically a
behavior change in C++03. Indeed, if a type has both a valid cross-type
assignment operator *and* a valid cross-type constructor, the library
will now prefer the cross-type assignment instead of going through the
cross-type constructor and then using the copy-constructor. Since this
is the mandated behavior in C++11, however, one could argue that any user
broken by that needs to change their code.

The motivation for this change is to allow simplifying the definition
of std::map's value_type, which requires handling assignment to a pair
of references properly. This patch will allow removing complexity from
https://llvm.org/D121485 instead of adding complexity in that patch.

Differential Revision: https://reviews.llvm.org/D150119