A customisation point that notifies an operation state object to start its associated operation.
constexpr unspecified start = unspecified;
execution::start denotes a customisation point object.
for some subexpression
is expression-equivalent to:
R.start(), if that expression is valid.
start(R), if that expression is valid, with overload resolution performed in a context that includes the declaration
void start();and that does not include a declaration of