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