truncating_iterator is no std::output_iterator

[burnpanck]

My formatter for a custom type has C++20 delegates to a function that has a requirement template <std::output_iterator<char> It>. Unfortunately, that one fails when used within fmt::format_to_n, because the iterator found in ctx.out() is of type fmt::v6::internal::truncating_iterator<char*, std::false_type>. That one fails to be a std::output_iterator, apparently because it is missing default construction. Potentially, there are other requirements missing.

Is there fundamentally preventing that object from being an output iterator, or has the requirement simply not come up before?

[vitaut]

Is there fundamentally preventing that object from being an output iterator, or has the requirement simply not come up before?

It hasn't come up before. A PR to make truncating_iterator satisfy output_iterator requirements is welcome.