The name start denotes a customization point object
that starts ([exec.async.ops])
the asynchronous operation associated with the operation state object.
For a subexpression op,
the expression start(op) is ill-formed
if op is an rvalue.
Otherwise, it is expression-equivalent to
MANDATE-NOTHROW(op.start()).
If op.start() does not start ([exec.async.ops])
the asynchronous operation associated with the operation state op,
the behavior of calling start(op) is undefined.