Move-assign an object handle from another.

basic_object_handle & operator=(
    basic_object_handle && other);
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Move-assign an object handle from a handle of another executor type.

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