add fuzzers for chrono timepoint and localtime,gmtime

[pauldreik]

This adds fuzzers for previously unfuzzed parts of the fmt api. I have run them locally without finding anything.

[vitaut]

Why doesn't it compile?

[pauldreik]

I get the below compile error, so I just assumed it was not supported.

../include/fmt/core.h:1592:3: error: static_assert failed due to requirement '!std::is_same<const fmt::v8::detail::unformattable &, const fmt::v8::detail::unformattable &>::value' "Cannot format an argument. To make type T formattable provide a formatter<T> specialization: https://fmt.dev/latest/api.html#udt"
  static_assert(
  ^
../include/fmt/core.h:1718:23: note: in instantiation of function template specialization 'fmt::v8::detail::make_arg<true, fmt::v8::basic_format_context<fmt::v8::appender, char>, fmt::v8::detail::type::custom_type, std::chrono::time_point<std::chrono::_V2::steady_clock, std::chrono::duration<long, std::ratio<1, 1000000000>>> &, 0>' requested here
        data_{detail::make_arg<
                      ^
../include/fmt/core.h:1737:10: note: in instantiation of function template specialization 'fmt::v8::format_arg_store<fmt::v8::basic_format_context<fmt::v8::appender, char>, std::chrono::time_point<std::chrono::_V2::steady_clock, std::chrono::duration<long, std::ratio<1, 1000000000>>>>::format_arg_store<std::chrono::time_point<std::chrono::_V2::steady_clock, std::chrono::duration<long, std::ratio<1, 1000000000>>> &>' requested here
  return {std::forward<Args>(args)...};
         ^
../include/fmt/core.h:2913:28: note: in instantiation of function template specialization 'fmt::v8::make_format_args<fmt::v8::basic_format_context<fmt::v8::appender, char>, std::chrono::time_point<std::chrono::_V2::steady_clock, std::chrono::duration<long, std::ratio<1, 1000000000>>> &>' requested here
  return vformat(fmt, fmt::make_format_args(args...));
                           ^
../test/fuzzing/chrono-timepoint.cc:25:30: note: in instantiation of function template specialization 'fmt::v8::format<std::chrono::time_point<std::chrono::_V2::steady_clock, std::chrono::duration<long, std::ratio<1, 1000000000>>> &>' requested here
  std::string message = fmt::format(format_str.get(), timepoint);
                             ^
../test/fuzzing/chrono-timepoint.cc:41:7: note: in instantiation of function template specialization 'doit<std::chrono::_V2::steady_clock>' requested here
      doit<std::chrono::steady_clock>(data,size);
      ^
In file included from ../test/fuzzing/chrono-timepoint.cc:3:
In file included from ../include/fmt/chrono.h:17:
In file included from ../include/fmt/format.h:44:
../include/fmt/core.h:1202:14: error: call to deleted constructor of 'fmt::v8::detail::fallback_formatter<fmt::v8::detail::unformattable, char, void>'
    auto f = Formatter();
             ^
../include/fmt/core.h:1189:21: note: in instantiation of function template specialization 'fmt::v8::detail::value<fmt::v8::basic_format_context<fmt::v8::appender, char>>::format_custom_arg<fmt::v8::detail::unformattable, fmt::v8::detail::fallback_formatter<fmt::v8::detail::unformattable, char, void>>' requested here
    custom.format = format_custom_arg<
                    ^
../test/fuzzing/chrono-timepoint.cc:25:30: note: in instantiation of function template specialization 'fmt::v8::format<std::chrono::time_point<std::chrono::_V2::steady_clock, std::chrono::duration<long, std::ratio<1, 1000000000>>> &>' requested here
  std::string message = fmt::format(format_str.get(), timepoint);
                             ^
../test/fuzzing/chrono-timepoint.cc:41:7: note: in instantiation of function template specialization 'doit<std::chrono::_V2::steady_clock>' requested here
      doit<std::chrono::steady_clock>(data,size);
      ^
../include/fmt/core.h:979:3: note: 'fallback_formatter' has been explicitly marked deleted here
  fallback_formatter() = delete;
  ^
2 errors generated.
ninja: build stopped: subcommand failed.

[pauldreik]

followed your requests, rebased and force pushed.