<format>: Call to next_arg_id may result in unexpected error (regression)

[JMazurkiewicz]

When user-defined formatter calls next_arg_id and format-spec is empty we will get compiler error.

Example:

#include <print>

struct FormatNextArg {};

template <> struct std::formatter<FormatNextArg> {
public:
  template <class ParseContext> constexpr auto parse(ParseContext &ctx) {
    auto it = ctx.begin();
    if (it != ctx.end() && *it != '}') {
      throw std::format_error{"Expected empty spec"};
    }

    arg_id = ctx.next_arg_id();
    return it;
  }

  template <class FormatContext>
  auto format(FormatNextArg, FormatContext &ctx) const {
    return std::format_to(ctx.out(), "arg-id: {}", arg_id);
  }

private:
  size_t arg_id;
};

int main() {
  // std::println("{}, {}", FormatNextArg{}, 0, FormatNextArg{}, "1"); // Does not compile since VS 17.10
  std::println("{:}, {:}", FormatNextArg{}, 2, FormatNextArg{}, "3"); // Works, replacement field is not empty
}

Introduced by: #4078
Compiler explorer (VS 17.9): https://godbolt.org/z/rsje4j1hP