Proposed resolution:

This wording is relative to N3797.

1. Modify [template.bitset] p1, class template bitset synopsis, as indicated:

   namespace std {
     template <size_t N> class bitset {
     public:
       […]
       template<class charT, class traits, class Allocator>
         explicit bitset(
           const basic_string<charT,traits,Allocator>& str,
           typename basic_string<charT,traits,Allocator>::size_type pos = 0,
           typename basic_string<charT,traits,Allocator>::size_type n =
             basic_string<charT,traits,Allocator>::npos,
             charT zero = charT('0')see below, charT one = charT('1')see below);
        […]
     };
     […]
   }
   

2. Modify [bitset.cons] as indicated:

   template<class charT, class traits, class Allocator>
   explicit 
   bitset(const basic_string<charT, traits, Allocator>& str,
          typename basic_string<charT, traits, Allocator>::size_type pos = 0,
          typename basic_string<charT, traits, Allocator>::size_type n =
            basic_string<charT, traits, Allocator>::npos,
            charT zero = charT('0')see below, charT one = charT('1')see below);
   

   -?- The default values of zero and one compare equal to the character literals 0 and 1 of type charT, respectively.

   -3- Requires:: pos <= str.size().

   […]

[ Lenexa 2015-05-05: Move to Open ]

Ask for complete PR (need quoted, to string, et al.)

Will then take it up again

Expectation is that this is correct way to fix this

Example: char16_t('1') != u'1' is possible.

The numeric value of char16_t is defined to be Unicode code point, which is same to the ASCII value and UTF-8 for 7-bit chars. However, char is not guaranteed to have an encoding which is compatible with ASCII. For example, '1' in EBCDIC is 241.

I found three places in the standard casting narrow char literals: bitset::bitset, bitset::to_string and quoted.

PJ confirmed this issue and says he has a solution used in their <filesystem> implementation, and he may want to propose it to the standard.

The solution in my mind, for now, is to make those default arguments magical, where the "magic" can be implemented with a C11 _Generic selection (works in clang):

#define _G(T, literal) _Generic(T{}, \
      char: literal, \
      wchar_t: L ## literal, \
      char16_t: u ## literal, \
      char32_t: U ## literal)

  _G(char16_t, '1') == u'1'