add_cv_t is an alias to the return type of the bsl::add_cv meta-function. Note, that the add_cv_t avoids the ::type suffix and typename prefix when we want to use the result of the bsl::add_cv in templates.
Declared in <bslmf_addcv.h>
template<class t_TYPE>
using add_cv_t = add_cv<t_TYPE>::type;