Move-assign a stream handle from another.

basic_stream_handle & operator=(
    basic_stream_handle && other);
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Move-assign a stream handle from a handle of another executor type.

template<
    typename Executor1>
constraint_t< is_convertible< Executor1, Executor >::value, basic_stream_handle & > operator=(
    basic_stream_handle< Executor1 > && other);
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