Proposed resolution:

This wording is relative to N4878.

1. Modify [stacktrace.syn], header <stacktrace> synopsis, as indicated:

   namespace std {
     // [stacktrace.entry], class stacktrace_entry
     class stacktrace_entry;
     
     // [stacktrace.basic], class template basic_stacktrace
     template<class Allocator>
     class basic_stacktrace;
     
     […]
     
     // [stacktrace.basic.nonmem], non-member functions
     […]
     
     string to_string(const stacktrace_entry& f);
     
     template<class Allocator>
       string to_string(const basic_stacktrace<Allocator>& st);
     
     template<class charT, class traits>
       basic_ostream<charT, traits>&
         operator<<(basic_ostream<charT, traits>& os, const stacktrace_entry& f);
       
     template<class charT, class traits, class Allocator>
       basic_ostream<charT, traits>&
         operator<<(basic_ostream<charT, traits>& os, const basic_stacktrace<Allocator>& st);
     
     […]
   }
   

2. Modify [stacktrace.basic.nonmem] as indicated:

   template<class charT, class traits>
   basic_ostream<charT, traits>&
     operator<<(basic_ostream<charT, traits>& os, const stacktrace_entry& f);
   

   -4- Effects: Equivalent to: return os << to_string(f);

   template<class charT, class traits, class Allocator>
   basic_ostream<charT, traits>&
     operator<<(basic_ostream<charT, traits>& os, const basic_stacktrace<Allocator>& st);
   

   -5- Effects: Equivalent to: return os << to_string(st);

[ 2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP. ]

[ 2022-11-07; Kona ]

Move to Immediate.

[ 2021-03-12; Reflector poll ]

Set priority to 2 and status to LEWG following reflector poll.

According to [string.io], the operator<< overloads in [stacktrace.basic.nonmem] are well-formed only if the template parameters charT and traits are char and std::char_traits<char> (that of std::string) respectively, because it is required in Effects: that these overloads behave as-if insert a std::string.

I think the declarations of these overloads should be changed to:

ostream& operator<<(ostream& os, const stacktrace_entry& f); 

template<class Allocator>
ostream& operator<<(ostream& os, const basic_stacktrace<Allocator>& st);