fmt::join fails for streamable objects after 6.0.0

[Quincunx271]

For a type which only has a stream operator, i.e. std::ostream& operator<<(std::ostream&, MyType);, attempting to print it as elements of a range passed to fmt::join fails after 6.0.0:

std::vector<MyType> values = ...;
fmt::print("{}", fmt::join(values, ", "));

In 5.3.0, this compiles and calls the operator<< to print MyType. After 6.0.0, this fails to compile.

---

MCVE. Works for 5.3.0. Fails for 6.0.0.

#include <ostream>
#include <vector>

#include <fmt/format.h>
#include <fmt/ostream.h>

struct Stream { int p; };

auto operator<<(std::ostream& out, Stream s) -> std::ostream&
{
    return out << "Stream{" << s.p << "}";
}

int main() {
    fmt::print("{}\n", Stream{42});
    auto const values = std::vector{Stream{1}, Stream{2}, Stream{3}};
    fmt::print("[{}]\n", fmt::join(values, ", "));
}

Seemingly relevant parts of the compile error message:

In file included from /opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/format.h:46,
                 from <source>:4:
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h: In substitution of 'template<class T, class Context> using mapped_type_constant = fmt::v6::internal::type_constant<decltype (fmt::v6::internal::arg_mapper<Context>().map(declval<T>())), typename Context::char_type> [with T = fmt::v6::arg_join<__gnu_cxx::__normal_iterator<const Stream*, std::vector<Stream, std::allocator<Stream> > >, char>; Context = fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char> >, char>]':
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:1000:52:   required from 'constexpr long long unsigned int fmt::v6::internal::encode_types() [with Context = fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char> >, char>; Arg = fmt::v6::arg_join<__gnu_cxx::__normal_iterator<const Stream*, std::vector<Stream, std::allocator<Stream> > >, char>; Args = {}]'
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:1101:59:   required from 'constexpr const long long unsigned int fmt::v6::format_arg_store<fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char> >, char>, fmt::v6::arg_join<__gnu_cxx::__normal_iterator<const Stream*, std::vector<Stream, std::allocator<Stream> > >, char> >::types'
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:1180:54:   required from 'fmt::v6::basic_format_args<Context>::basic_format_args(const fmt::v6::format_arg_store<Context, Args ...>&) [with Args = {fmt::v6::arg_join<__gnu_cxx::__normal_iterator<const Stream*, std::vector<Stream, std::allocator<Stream> > >, char>}; Context = fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char> >, char>]'
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:1214:70:   required from 'fmt::v6::format_args::format_args(Args&& ...) [with Args = {fmt::v6::format_arg_store<fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char> >, char>, fmt::v6::arg_join<__gnu_cxx::__normal_iterator<const Stream*, std::vector<Stream, std::allocator<Stream> > >, char> >}]'
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:1409:9:   required from 'void fmt::v6::print(const S&, Args&& ...) [with S = char [6]; Args = {fmt::v6::arg_join<__gnu_cxx::__normal_iterator<const Stream*, std::vector<Stream, std::allocator<Stream> > >, char>}; typename std::enable_if<fmt::v6::internal::is_string<S>::value, int>::type <anonymous> = 0]'
<source>:17:49:   required from here
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:843:7: error: no matching function for call to 'fmt::v6::internal::arg_mapper<fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char> >, char> >::map(fmt::v6::arg_join<__gnu_cxx::__normal_iterator<const Stream*, std::vector<Stream, std::allocator<Stream> > >, char>)'
  843 | using mapped_type_constant =
      |       ^~~~~~~~~~~~~~~~~~~~
...
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:793:46: note: candidate: 'template<class T, typename std::enable_if<(std::is_constructible<fmt::v6::basic_string_view<char>, T>::value && (! fmt::v6::internal::is_string<S>::value)), int>::type <anonymous> > constexpr fmt::v6::basic_string_view<typename S::char_type> fmt::v6::internal::arg_mapper<Context>::map(const T&) [with T = T; typename std::enable_if<(std::is_constructible<fmt::v6::basic_string_view<typename S::char_type>, T>::value && (! fmt::v6::internal::is_string<T>::value)), int>::type <anonymous> = <enumerator>; Context = fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char> >, char>]'
  793 |   FMT_CONSTEXPR basic_string_view<char_type> map(const T& val) {
      |                                              ^~~
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:793:46: note:   template argument deduction/substitution failed:
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:790:13: error: no type named 'type' in 'struct std::enable_if<false, int>'
  790 |             FMT_ENABLE_IF(
      |             ^~~~~~~~~~~~~
...
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:808:43: note: candidate: 'template<class T> constexpr int fmt::v6::internal::arg_mapper<Context>::map(const T*) [with T = T; Context = fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char> >, char>]'
  808 |   template <typename T> FMT_CONSTEXPR int map(const T*) {
      |                                           ^~~
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:808:43: note:   template argument deduction/substitution failed:
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:843:7: note:   mismatched types 'const T*' and 'fmt::v6::arg_join<__gnu_cxx::__normal_iterator<const Stream*, std::vector<Stream, std::allocator<Stream> > >, char>'
  843 | using mapped_type_constant =
      |       ^~~~~~~~~~~~~~~~~~~~
...
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:828:26: note: candidate: 'template<class T, typename std::enable_if<(((! fmt::v6::internal::is_string<S>::value) && (! fmt::v6::is_char<T>::value)) && (std::is_constructible<fmt::v6::formatter<T, char, void> >::value || std::is_constructible<fmt::v6::internal::fallback_formatter<T, char, void> >::value)), int>::type <anonymous> > constexpr const T& fmt::v6::internal::arg_mapper<Context>::map(const T&) [with T = T; typename std::enable_if<(((! fmt::v6::internal::is_string<T>::value) && (! fmt::v6::is_char<T>::value)) && (std::is_constructible<typename Context::formatter_type<T> >::value || std::is_constructible<fmt::v6::internal::fallback_formatter<T, typename Context::char_type> >::value)), int>::type <anonymous> = <enumerator>; Context = fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char> >, char>]'
  828 |   FMT_CONSTEXPR const T& map(const T& val) {
      |                          ^~~
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:828:26: note:   template argument deduction/substitution failed:
/opt/compiler-explorer/libs/fmt/6.0.0/include/fmt/core.h:825:13: error: no type named 'type' in 'struct std::enable_if<false, int>'
  825 |             FMT_ENABLE_IF(!is_string<T>::value && !is_char<T>::value &&
      |             ^~~~~~~~~~~~~
...

[vitaut]

The old behavior was accidental. To make a type work with join, a formatter specialization should be provided.