constexpr cancellation_type_t operator()() const;
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template<
    typename E,
    typename... Args>
constexpr constraint_t< !is_same< decay_t< E >, asio::error_code >::value &&!is_same< decay_t< E >, std::exception_ptr >::value, cancellation_type_t > operator()(
    const E & ,
    Args && ...) const;
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template<
    typename E,
    typename... Args>
constexpr constraint_t< is_same< decay_t< E >, asio::error_code >::value||is_same< decay_t< E >, std::exception_ptr >::value, cancellation_type_t > operator()(
    const E & e,
    Args && ...) const;
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