conditional_t is an alias to the return type of the bsl::conditional meta-function. Note, that the conditional_t avoids the ::type suffix and typename prefix when we want to use the result of the meta-function in templates.
Declared in <bslmf_conditional.h>
template<
bool t_COND,
class t_TRUE_TYPE,
class t_FALSE_TYPE>
using conditional_t = conditional<t_COND, t_TRUE_TYPE, t_FALSE_TYPE>::type;