Thanks for the PR. We shouldn't modify base.h for this.

How about reimplementing similar functions in xchar.h?

I think what we need here as an overload of format_to_n that takes wformat_string in xchar.h. Existing runtime format string machinery should take care about the rest as it already does for other overloads.

what about char16_t?

Let's focus on wchar_t in this PR.

Let's focus on wchar_t in this PR.

I use wformat_string in the new implementation. This may remove previous support of char type not in char, wchar. Tell me if it is not desired.

I modified the original overload. I also added new tests for char16_t, char32_t that calls the original overload.

Please give me feedback about the new test cases. This is new for this repository.

Ok, I will revert it to 1dcff42.

Merged, thanks!