valarray_after.h (_Expr<>::operator[](slice)): Don't assume the closure implements general indexing...
* include/bits/valarray_after.h (_Expr<>::operator[](slice)):
Don't assume the closure implements general indexing, as a matter
of fact, most of them don't.
(_Expr<>::operator[](const gslice&)): Likewise.
(_Expr<>::operator[](const valarray<bool>&)): Likewise.
(_Expr<>::operator[](const valarray<size_t>&)): Likewise.
(_Expr<>::shift): Fix thinko.
(_Expr<>::cshift): Likewise.
(_Expr<>::apply): Likewise.