Not too happy with this band-aid; will try a different approach.

Edit: Nevermind. value_type is required by the library. I'll keep working on a better type trait for string types, but don't want to rush it.
@gregmarr raised concerns that this might be a breaking change. string_t::value_type is used in a few places and string types without a value_type member wouldn't have compiled in 3.10.4 and 3.10.5 either. To be 100% sure, I'd have to grep the release headers for string_t::value_type.