[all-commits] [llvm/llvm-project] 1c1edd: [libc++] Fixes thread::id's operator<<.

[Mark de Wever via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 1c1edd1b5d00daae2efdfbc5754a9fae77597dd4
      https://github.com/llvm/llvm-project/commit/1c1edd1b5d00daae2efdfbc5754a9fae77597dd4
  Author: Mark de Wever <koraq at xs4all.nl>
  Date:   2023-07-15 (Sat, 15 Jul 2023)

  Changed paths:
    M libcxx/include/__thread/thread.h
    M libcxx/test/std/thread/thread.threads/thread.thread.class/thread.thread.id/stream.pass.cpp

  Log Message:
  -----------
  [libc++] Fixes thread::id's operator<<.

The output of

  template<class charT, class traits>
    basic_ostream<charT, traits>&
      operator<<(basic_ostream<charT, traits>& out, thread::id id);

is affected by the state of out. The wording states

[thread.thread.id]/2
  The text representation for the character type charT of an object of
  type thread::id is an unspecified sequence of charT such that, for two
  objects of type thread::id x and y, if x == y is true, the thread::id
  objects have the same text representation, and if x != y is true, the
  thread::id objects have distinct text representations.

[thread.thread.id]/9
  template<class charT, class traits>
    basic_ostream<charT, traits>&
      operator<< (basic_ostream<charT, traits>& out, thread::id id);

  Effects: Inserts the text representation for charT of id into out.

This wording changed in C++23 due to adding a formatter specialization for
thread::id. However the requirement was the same in older versions of C++.

This issue is that thread::id is an integral or pointer and affected by the
formatting manipulators for them. Thus the text representation can differ if
x == y which violates the requirements.

The fix has to hard-code some formatting style for the text
representation. It uses the Standard specified default values

Table 124: basic_ios::init() effects [tab:basic.ios.cons] flags()
  flags() skipws | dec

Fixes PR: https://llvm.org/PR62073

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D153336