Move-assign a basic_writable_pipe from another.

basic_writable_pipe & operator=(
    basic_writable_pipe && other);
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Move-assign a basic_writable_pipe from a pipe of another executor type.

template<
    typename Executor1>
constraint< is_convertible< Executor1, Executor >::value, basic_writable_pipe & >::type operator=(
    basic_writable_pipe< Executor1 > && other);
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