[Bug libstdc++/80624] New: char_traits<char16_t>::eof() doesn't meet requirements

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80624

            Bug ID: 80624
           Summary: char_traits<char16_t>::eof() doesn't meet requirements
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

#include <string>

using u16_traits = std::char_traits<char16_t>;
constexpr u16_traits::char_type c = u'\uFFFF';
constexpr u16_traits::int_type e = u16_traits::eof();
static_assert( u16_traits::eq_int_type(e, c) == false, "eof() is correct" );


[char.traits.require] says eof() return "a value e such that
X::eq_int_type(e,X::to_int_type(c)) is false for all values c."

Despite being a noncharacter, u'\uFFFF' is a valid code point, see
http://www.unicode.org/faq/private_use.html#nonchar7 and
http://www.unicode.org/faq/private_use.html#nonchar8

This means there's no way to tell if std::ostream<char16_t>::put(u'\uFFFF') is
successful, because it returns to_int_type(u'\uFFFF') on success and returns
eof() on failure, which compares equal.

This might be fixable by making to_int_type(eof()) return u'\uFFFD' as
suggested by the FAQ linked to above.