In MSVC, basic_string_view<charT>::const_iterator customized type prohibits casting from a char pointer even with explicit casting.

[denchat]

When I try to use <format> using clang-target-msvc VS 2019, I got these 4 errors :

C:\Users\User\AppData\Roaming\fmt-master\include\format(611,26): error: no viable conversion from 'const char *' to
      'std::basic_format_parse_context<char>::const_iterator' (aka '_String_view_iterator<std::char_traits<char> >')
    parse_ctx.advance_to(p);
                         ^
C:\Users\User\AppData\Roaming\fmt-master\include/fmt/format.h(2152,15): note: in instantiation of member function
      'std::detail::format_handler<std::detail::arg_formatter<fmt::v5::back_insert_range<fmt::v5::internal::buffer<char> > >, char,
      std::basic_format_context<std::back_insert_iterator<fmt::v5::internal::buffer<char> >, char> >::on_replacement_field' requested here
      handler.on_replacement_field(p);
              ^
C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.20.27508\include\xstring(851,7): note: candidate constructor
      (the implicit copy constructor) not viable: no known conversion from 'const char *' to 'const std::_String_view_iterator<std::char_traits<char> > &' for 1st
      argument
class _String_view_iterator { // iterator for character buffer wrapper
      ^
C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.20.27508\include\xstring(851,7): note: candidate constructor
      (the implicit move constructor) not viable: no known conversion from 'const char *' to 'std::_String_view_iterator<std::char_traits<char> > &&' for 1st argument
C:\Users\User\AppData\Roaming\fmt-master\include\format(153,46): note: passing argument to parameter 'it' here
    constexpr void advance_to(const_iterator it);
                                             ^
C:\Users\User\AppData\Roaming\fmt-master\include\format(618,26): error: no viable conversion from 'const char *' to
      'std::basic_format_parse_context<char>::const_iterator' (aka '_String_view_iterator<std::char_traits<char> >')
    parse_ctx.advance_to(begin);
                         ^~~~~
C:\Users\User\AppData\Roaming\fmt-master\include/fmt/format.h(2161,21): note: in instantiation of member function
      'std::detail::format_handler<std::detail::arg_formatter<fmt::v5::back_insert_range<fmt::v5::internal::buffer<char> > >, char,
      std::basic_format_context<std::back_insert_iterator<fmt::v5::internal::buffer<char> >, char> >::on_format_specs' requested here
        p = handler.on_format_specs(p + 1, end);
                    ^
C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.20.27508\include\xstring(851,7): note: candidate constructor
      (the implicit copy constructor) not viable: no known conversion from 'const char *' to 'const std::_String_view_iterator<std::char_traits<char> > &' for 1st
      argument
class _String_view_iterator { // iterator for character buffer wrapper
      ^
C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.20.27508\include\xstring(851,7): note: candidate constructor
      (the implicit move constructor) not viable: no known conversion from 'const char *' to 'std::_String_view_iterator<std::char_traits<char> > &&' for 1st argument
C:\Users\User\AppData\Roaming\fmt-master\include\format(153,46): note: passing argument to parameter 'it' here
    constexpr void advance_to(const_iterator it);
                                             ^
C:\Users\User\AppData\Roaming\fmt-master\include\format(620,42): error: no viable conversion from returned value of type
      'std::basic_format_parse_context<char>::const_iterator' (aka '_String_view_iterator<std::char_traits<char> >') to function return type 'const char *'
    if (visit_format_arg(f, arg)) return parse_ctx.begin();
                                         ^~~~~~~~~~~~~~~~~
C:\Users\User\AppData\Roaming\fmt-master\include\format(628,26): error: no viable conversion from 'const char *' to
      'std::basic_format_parse_context<char>::const_iterator' (aka '_String_view_iterator<std::char_traits<char> >')
    parse_ctx.advance_to(begin);
                         ^~~~~
C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.20.27508\include\xstring(851,7): note: candidate constructor
      (the implicit copy constructor) not viable: no known conversion from 'const char *' to 'const std::_String_view_iterator<std::char_traits<char> > &' for 1st
      argument
class _String_view_iterator { // iterator for character buffer wrapper
      ^
C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.20.27508\include\xstring(851,7): note: candidate constructor
      (the implicit move constructor) not viable: no known conversion from 'const char *' to 'std::_String_view_iterator<std::char_traits<char> > &&' for 1st argument
C:\Users\User\AppData\Roaming\fmt-master\include\format(153,46): note: passing argument to parameter 'it' here
    constexpr void advance_to(const_iterator it);
                                             ^
4 errors generated.

Peeking into <format>, we are trying to convert plain char pointer to std::basic_string_view<charT>::const_iterator

  template<class charT>
  class basic_format_parse_context {
  public:
    using char_type = charT;
    using const_iterator = typename basic_string_view<charT>::const_iterator;
    using iterator = const_iterator;

In MSVC implementation, this basic_string_view<charT>::const_iterator is deliberately defined as _String_view_iterator<std::char_traits<char> > and this MSVC class template has one and only explicit private constructor from const std::char_traits<char>::chartype * which deliberately prohibits both implicit and explicit casting from const char * by outside users.

Does this mean in <format>, we must and always speak about string_view's iterators in term of basic_string_view<charT>::const_iterator when passing parameters?!?!

Does the standard have any plan to address these different approaches?

[vitaut]

Fixed in 98b3775. Thanks for catching this!

[denchat]

That's awesome. Thank you.