34 Execution control library [exec]

34.8 Operation states [exec.opstate]

34.8.1 General [exec.opstate.general]

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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> && is_object_v<O> && requires (O& o) { { start(o) } noexcept; }; }
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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]
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The program is ill-formed if it performs a copy or move construction or assigment operation on an operation state object created by connecting a library-provided sender.

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

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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()).
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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.