The implementation
provides a definition that has the base characteristic of
integral_constant<int, J> if
T is the type of
std::placeholders::_J, otherwise it has a
base characteristic of
integral_constant<int, 0>.A program
may specialize this template for a program-defined type
T to
have a base characteristic of
integral_constant<int, N>
with
N > 0 to indicate that
T should be
treated as a placeholder type
.