new compilation error by upgrading from 11.0.1 to 11.1.2 with std::ostreambuf_iterator

[lano1106]

In file included from /usr/include/fmt/chrono.h:23,
                 from time_util.cpp:11:
/usr/include/fmt/format.h: In instantiation of ‘constexpr OutputIt fmt::v11::detail::write(OutputIt, T) [with Char = char; OutputIt = std::ostreambuf_iterator<char, std::char_traits<char> >; T = long int; typename std::enable_if<((is_integral<T>::value && (! std::is_same<T, bool>::value)) && (! std::is_same<T, Char>::value)), int>::type <anonymous> = 0]’:
/usr/include/fmt/compile.h:169:25:   required from ‘constexpr OutputIt fmt::v11::detail::field<Char, T, N>::format(OutputIt, const Args& ...) const [with OutputIt = std::ostreambuf_iterator<char, std::char_traits<char> >; Args = {long int, long int}; Char = char; T = long int; int N = 0]’
  169 |       return write<Char>(out, arg);
      |              ~~~~~~~~~~~^~~~~~~~~~
/usr/include/fmt/compile.h:234:21:   required from ‘constexpr OutputIt fmt::v11::detail::concat<L, R>::format(OutputIt, const Args& ...) const [with OutputIt = std::ostreambuf_iterator<char, std::char_traits<char> >; Args = {long int, long int}; L = fmt::v11::detail::field<char, long int, 0>; R = fmt::v11::detail::concat<fmt::v11::detail::code_unit<char>, fmt::v11::detail::spec_field<char, long int, 1> >]’
  234 |     out = lhs.format(out, args...);
      |           ~~~~~~~~~~^~~~~~~~~~~~~~
/usr/include/fmt/compile.h:460:19:   required from ‘constexpr OutputIt fmt::v11::format_to(OutputIt, const CompiledFormat&, const Args& ...) [with OutputIt = std::ostreambuf_iterator<char, std::char_traits<char> >; CompiledFormat = detail::concat<detail::field<char, long int, 0>, detail::concat<detail::code_unit<char>, detail::spec_field<char, long int, 1> > >; Args = {long int, long int}; typename std::enable_if<detail::is_compiled_format<CompiledFormat>::value, int>::type <anonymous> = 0]’
  460 |   return cf.format(out, args...);
      |          ~~~~~~~~~^~~~~~~~~~~~~~
/usr/include/fmt/compile.h:500:26:   required from ‘constexpr OutputIt fmt::v11::format_to(OutputIt, const S&, Args&& ...) [with OutputIt = std::ostreambuf_iterator<char, std::char_traits<char> >; S = detail::udl_compiled_string<char, 10, detail::fixed_string<char, 10>{"{}.{:06d}"}>; Args = {const long int&, const long int&}; typename std::enable_if<detail::is_compiled_string<S>::value, int>::type <anonymous> = 0]’
  500 |     return fmt::format_to(out, compiled, std::forward<Args>(args)...);
      |            ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
time_util.cpp:63:19:   required from here
   63 |     fmt::format_to(std::ostreambuf_iterator<char>{o}, "{}.{:06d}"_cf,
      |     ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   64 |                ts.tv_sec, ts.tv_usec);
      |                ~~~~~~~~~~~~~~~~~~~~~~
/usr/include/fmt/format.h:2168:30: error: no matching function for call to ‘format_decimal<char>(std::ostreambuf_iterator<char, std::char_traits<char> >&, long unsigned int&, int&)’
 2168 |   return format_decimal<Char>(out, abs_value, num_digits);
      |          ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/fmt/format.h:1199:31: note: candidate: ‘template<class Char, class UInt> constexpr Char* fmt::v11::detail::format_decimal(Char*, UInt, int)’
 1199 | FMT_CONSTEXPR FMT_INLINE auto format_decimal(Char* out, UInt value,
      |                               ^~~~~~~~~~~~~~
/usr/include/fmt/format.h:1199:31: note:   template argument deduction/substitution failed:
/usr/include/fmt/format.h:2168:31: note:   cannot convert ‘out’ (type ‘std::ostreambuf_iterator<char, std::char_traits<char> >’) to type ‘char*’
 2168 |   return format_decimal<Char>(out, abs_value, num_digits);
      |                               ^~~
/usr/include/fmt/format.h:1207:20: note: candidate: ‘template<class Char, class UInt, class OutputIt, typename std::enable_if<fmt::v11::detail::is_back_insert_iterator<OutputIt>::value, int>::type <anonymous> > constexpr OutputIt fmt::v11::detail::format_decimal(OutputIt, UInt, int)’
 1207 | FMT_CONSTEXPR auto format_decimal(OutputIt out, UInt value, int num_digits)
      |                    ^~~~~~~~~~~~~~
/usr/include/fmt/format.h:1207:20: note:   template argument deduction/substitution failed:
In file included from /usr/include/fmt/format.h:41:
/usr/include/fmt/format.h:1206:11: error: no type named ‘type’ in ‘struct std::enable_if<false, int>’
 1206 |           FMT_ENABLE_IF(is_back_insert_iterator<OutputIt>::value)>
      |           ^~~~~~~~~~~~~
make: *** [Makefile:64: time_util.o] Error 1

I have locally fixed the problem by changing the fmt/formt.h template declaration from:

template <typename Char, typename UInt, typename OutputIt,
          FMT_ENABLE_IF(is_back_insert_iterator<OutputIt>::value)>
FMT_CONSTEXPR auto format_decimal(OutputIt out, UInt value, int num_digits)
    -> OutputIt {

to

template <typename Char, typename UInt, typename OutputIt>
FMT_CONSTEXPR auto format_decimal(OutputIt out, UInt value, int num_digits)
    -> OutputIt {

[vitaut]

Please provide a godbolt repro.

[lano1106]

I am not sure why you need a godbolt repro... with my explanations, it is pretty obvious what is happening.
the format_decimal() template function is not enabled because std::ostreambuf_iterator is an output iterator and not a back_inserter_iterator.

format_decimal() does not need the iterator to be a back_inserter_iterator for the template function to work perfectly. I did not go as far as looking how it was done in 11.0.1 but I suspect that FMT_ENABLE_IF(is_back_insert_iterator::value) is an addition to 11.1

beside that, you made an excellent job in this new release by eliminating all those annoying linker warnings...

[phprus]

Maybe fixed by #4312

[lano1106]

by visual inspection, I would say that it does... I am just curious to know what the enable_if statement accomplish....

imho, it would be nice to document in a small comment...

I have solved my problem by totally removing it not knowing what was its purpose. It did not seem like the other format_decimal() overload presence could create any ambiguity for the compiler to choose which one to take...