#
The operation_state concept defines the requirements of an operation state type ([exec.async.ops]).
namespace std::execution { template<class O> concept operation_state = derived_from<typename O::operation_state_concept, operation_state_t> && requires (O& o) { start(o); }; }
2
#
If an operation_state object is destroyed during the lifetime of its asynchronous operation ([exec.async.ops]), the behavior is undefined.
[Note 1: 
The operation_state concept does not impose requirements on any operations other than destruction and start, including copy and move operations.
Invoking any such operation on an object whose type models operation_state can lead to undefined behavior.
— end note]
3
#
The program is ill-formed if it performs a copy or move construction or assignment operation on an operation state object created by connecting a library-provided sender.

33.8.2 execution​::​start [exec.opstate.start]

1
#
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().
2
#
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.

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