[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

1 files changed, 30 insertions, 5 deletions

diff --git a/libcxx/include/__thread/thread.h b/libcxx/include/__thread/thread.h
index 3d52d87f7026..dec40c7dc1f0 100644
--- a/libcxx/include/__thread/thread.h
+++ b/libcxx/include/__thread/thread.h
@@ -24,6 +24,10 @@
 #include <iosfwd>
 #include <tuple>
 
+#ifndef _LIBCPP_HAS_NO_LOCALIZATION
+#  include <locale>
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
@@ -126,11 +130,32 @@ struct _LIBCPP_TEMPLATE_VIS hash<__thread_id>
     }
 };
 
-template<class _CharT, class _Traits>
-_LIBCPP_INLINE_VISIBILITY
-basic_ostream<_CharT, _Traits>&
-operator<<(basic_ostream<_CharT, _Traits>& __os, __thread_id __id)
-{return __os << __id.__id_;}
+#ifndef _LIBCPP_HAS_NO_LOCALIZATION
+template <class _CharT, class _Traits>
+_LIBCPP_INLINE_VISIBILITY basic_ostream<_CharT, _Traits>&
+operator<<(basic_ostream<_CharT, _Traits>& __os, __thread_id __id) {
+    // [thread.thread.id]/9
+    //   Effects: Inserts the text representation for charT of id into out.
+    //
+    // [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.
+    //
+    // Since various flags in the output stream can affect how the
+    // thread id is represented (e.g. numpunct or showbase), we
+    // use a temporary stream instead and just output the thread
+    // id representation as a string.
+
+    basic_ostringstream<_CharT, _Traits> __sstr;
+    __sstr.imbue(locale::classic());
+    __sstr << __id.__id_;
+    return __os << __sstr.str();
+}
+#endif // _LIBCPP_HAS_NO_LOCALIZATION
 
 class _LIBCPP_EXPORTED_FROM_ABI thread
 {