Compilation errors when using _cf user-defined literal with user-defined type

[LHLaurini]

The following example (Compiler Explorer link), taken mostly unaltered from the docs

#include <type_traits>
#include <fmt/compile.h>

enum class color {red, green, blue};

template <> struct fmt::formatter<color>: formatter<string_view> {
  // parse is inherited from formatter<string_view>.

  auto format(color c, format_context& ctx) const
    -> format_context::iterator;
};

auto fmt::formatter<color>::format(color c, format_context& ctx) const
    -> format_context::iterator {
  string_view name = "unknown";
  switch (c) {
  case color::red:   name = "red"; break;
  case color::green: name = "green"; break;
  case color::blue:  name = "blue"; break;
  }
  return formatter<string_view>::format(name, ctx);
}

int main() {
  using fmt::operator""_cf;
  fmt::print("{}"_cf, color::red);
}

}

fails with

In file included from <source>:3:
In file included from /opt/compiler-explorer/libs/fmt/trunk/include/fmt/compile.h:15:
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/format.h:3601:26: error: non-const lvalue reference to type 'format_context' (aka 'fmt::context') cannot bind to a value of unrelated type 'basic_format_context<back_insert_iterator<basic_memory_buffer<char, 500, allocator<char>>>, char>' (aka 'fmt::generic_context<std::back_insert_iterator<fmt::basic_memory_buffer<char>>, char>')
 3601 |   return f.format(value, ctx);
      |                          ^~~
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/compile.h:169:14: note: in instantiation of function template specialization 'fmt::detail::write<char, std::back_insert_iterator<fmt::basic_memory_buffer<char>>, color, 0>' requested here
  169 |       return write<Char>(out, arg);
      |              ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/compile.h:460:13: note: in instantiation of function template specialization 'fmt::detail::field<char, color, 0>::format<std::back_insert_iterator<fmt::basic_memory_buffer<char>>, color>' requested here
  460 |   return cf.format(out, args...);
      |             ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/compile.h:500:17: note: in instantiation of function template specialization 'fmt::format_to<std::back_insert_iterator<fmt::basic_memory_buffer<char>>, fmt::detail::field<char, color, 0>, color, 0>' requested here
  500 |     return fmt::format_to(out, compiled, std::forward<Args>(args)...);
      |                 ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/compile.h:528:8: note: in instantiation of function template specialization 'fmt::format_to<std::back_insert_iterator<fmt::basic_memory_buffer<char>>, fmt::detail::udl_compiled_string<char, 3, detail::fixed_string<char, 3>{"{}"}>, const color &, 0>' requested here
  528 |   fmt::format_to(std::back_inserter(buffer), fmt, args...);
      |        ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/compile.h:535:3: note: in instantiation of function template specialization 'fmt::print<fmt::detail::udl_compiled_string<char, 3, detail::fixed_string<char, 3>{"{}"}>, color, 0>' requested here
  535 |   print(stdout, fmt, args...);
      |   ^
<source>:27:8: note: in instantiation of function template specialization 'fmt::print<fmt::detail::udl_compiled_string<char, 3, detail::fixed_string<char, 3>{"{}"}>, color, 0>' requested here
   27 |   fmt::print("{}"_cf, color::red);
      |        ^
<source>:14:61: note: passing argument to parameter 'ctx' here
   14 | auto fmt::formatter<color>::format(color c, format_context& ctx) const
      |                                                             ^
In file included from <source>:3:
In file included from /opt/compiler-explorer/libs/fmt/trunk/include/fmt/compile.h:15:
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/format.h:3601:10: error: no viable conversion from returned value of type 'format_context::iterator' (aka 'basic_appender<char>') to function return type 'std::back_insert_iterator<fmt::basic_memory_buffer<char>>'
 3601 |   return f.format(value, ctx);
      |          ^~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-14.2.0/lib/gcc/x86_64-linux-gnu/14.2.0/../../../../include/c++/14.2.0/bits/stl_iterator.h:689:11: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'format_context::iterator' (aka 'basic_appender<char>') to 'const back_insert_iterator<basic_memory_buffer<char>> &' for 1st argument
  689 |     class back_insert_iterator
      |           ^~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-14.2.0/lib/gcc/x86_64-linux-gnu/14.2.0/../../../../include/c++/14.2.0/bits/stl_iterator.h:689:11: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'format_context::iterator' (aka 'basic_appender<char>') to 'back_insert_iterator<basic_memory_buffer<char>> &&' for 1st argument
  689 |     class back_insert_iterator
      |           ^~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-14.2.0/lib/gcc/x86_64-linux-gnu/14.2.0/../../../../include/c++/14.2.0/bits/stl_iterator.h:704:7: note: explicit constructor is not a candidate
  704 |       back_insert_iterator(_Container& __x)
      |       ^
2 errors generated.
Compiler returned: 1

If I remove the _cf literal operator, it works fine. It also works if I rewrite it with format_as.

[vitaut]

Fixed in 2b6a786, thanks for reporting.