In MSVC, basic_string_view<charT>::const_iterator customized type prohibits casting from a char pointer even with explicit casting. #1156
Description
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?