vformat_to_n(char *) fails to compile with GCC 8.

[toojays]

Consider the following example of formatting to a fixed size buffer before writing to stdout:

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

void vlog(fmt::string_view format, fmt::format_args args) {
  char line[1024];
  auto [_, size] = fmt::vformat_to_n(line, sizeof(line), format, args);
  write(STDOUT_FILENO, line, size);
}

template <typename S, typename... Args>
void log(const S& format, Args&&... args) {
  vlog(format, fmt::make_args_checked<Args...>(format, args...));
}

int main() {
  log("invalid squishiness: {}", 42);
}

With GCC 9 and 10 this works. But with GCC 8 this fails to compile (godbolt) with:

<source>: In function 'void vlog(fmt::v7::string_view, fmt::v7::format_args)':
<source>:6:70: error: no matching function for call to 'vformat_to_n(char [1024], long unsigned int, fmt::v7::string_view&, fmt::v7::format_args&)'
   auto [_, size] = fmt::vformat_to_n(line, sizeof(line), format, args);
                                                                      ^
In file included from /opt/compiler-explorer/libs/fmt/trunk/include/fmt/format.h:46,
                 from <source>:1:
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:1830:37: note: candidate: 'template<class OutputIt, class Char, class ... Args, typename std::enable_if<fmt::v7::detail::is_output_iterator<OutputIt, Char>::value, int>::type <anonymous> > fmt::v7::format_to_n_result<OutputIt> fmt::v7::vformat_to_n(OutputIt, size_t, fmt::v7::basic_string_view<Char>, fmt::v7::basic_format_args<fmt::v7::basic_format_context<fmt::v7::detail::buffer_appender<typename fmt::v7::type_identity<Char>::type>, typename fmt::v7::type_identity<Char>::type> >)'
 inline format_to_n_result<OutputIt> vformat_to_n(
                                     ^~~~~~~~~~~~
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:1830:37: note:   template argument deduction/substitution failed:
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:1829:11: error: no type named 'type' in 'struct std::enable_if<false, int>'
           FMT_ENABLE_IF(detail::is_output_iterator<OutputIt, Char>::value)>
           ^~~~~~~~~~~~~
ASM generation compiler returned: 1
<source>: In function 'void vlog(fmt::v7::string_view, fmt::v7::format_args)':
<source>:6:70: error: no matching function for call to 'vformat_to_n(char [1024], long unsigned int, fmt::v7::string_view&, fmt::v7::format_args&)'
   auto [_, size] = fmt::vformat_to_n(line, sizeof(line), format, args);
                                                                      ^
In file included from /opt/compiler-explorer/libs/fmt/trunk/include/fmt/format.h:46,
                 from <source>:1:
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:1830:37: note: candidate: 'template<class OutputIt, class Char, class ... Args, typename std::enable_if<fmt::v7::detail::is_output_iterator<OutputIt, Char>::value, int>::type <anonymous> > fmt::v7::format_to_n_result<OutputIt> fmt::v7::vformat_to_n(OutputIt, size_t, fmt::v7::basic_string_view<Char>, fmt::v7::basic_format_args<fmt::v7::basic_format_context<fmt::v7::detail::buffer_appender<typename fmt::v7::type_identity<Char>::type>, typename fmt::v7::type_identity<Char>::type> >)'
 inline format_to_n_result<OutputIt> vformat_to_n(
                                     ^~~~~~~~~~~~
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:1830:37: note:   template argument deduction/substitution failed:
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:1829:11: error: no type named 'type' in 'struct std::enable_if<false, int>'
           FMT_ENABLE_IF(detail::is_output_iterator<OutputIt, Char>::value)>
           ^~~~~~~~~~~~~

Unfortunately GCC 8 is still relevant to me as it is the most recent upstream-built compiler for Red Hat / CentOS 6 (via devtoolset-8).

[alexezeder]

Looks like GCC 8.x is the only compiler that fails with the current implementation of void_t - https://godbolt.org/z/zbK5q3.

fmt/include/fmt/core.h

Lines 1382 to 1385 in 640acba

	// A workaround for gcc 4.8 to make void_t work in a SFINAE context.
	template <typename... Ts> struct void_t_impl { using type = void; };
	template <typename... Ts>
	using void_t = typename detail::void_t_impl<Ts...>::type;

But this failure on GCC 8 can be solved by removing that workaround for GCC 4.8-4.9 and using straightforward implementation - https://godbolt.org/z/v4q8P6.