I'd rather not. Most of the time, you don't need it, and when you do, it's for a very small part of the input. And `wchar_t` is an abomination (it's UTF-32 on Linux, UTF-16 on Windows, and all of that is allowed); you probably really want `char32_t`, and again, not for the whole of the input; streaming such data a single rune/codepoint at a time is probably fine as well for most uses.

On the other hand, if your parser combinators process char-by-char, then maintaining a small "is this valid UTF-8 so far" context on the side should be pretty simple, so providing it would be an useful option, but actually decoding? Please don't.