enable_if_t is an alias to the return type of the bsl::enable_if 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_enableif.h>
template<
bool t_COND,
class t_TYPE = void>
using enable_if_t = t_TYPE;