If T has a nested type cancellation_slot_type, returns t.get_cancellation_slot(). Otherwise returns type().

static decltype(auto) get(
    const T & t);
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If T has a nested type cancellation_slot_type, returns t.get_cancellation_slot(). Otherwise returns s.

static decltype(auto) get(
    const T & t,
    const CancellationSlot & s);
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