The reason I'd want "frozen-size vector" is to replace pairs of data members of the form `T* foos; size_t foos_len;` without paying another 8 bytes to store a useless capacity that's never going to change.

But I don't think that makes such a container worth adding to the STL. So far, it hasn't even been worth writing in our own code. But that's the reason I've thought about writing it.