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### Buffer-oriented asynchronous read stream requirements

A type `X` meets the `AsyncReadStream` requirements if it satisfies the requirements listed below.

In the table below, `a` denotes a value of type `X`, `mb` denotes a (possibly const) value satisfying the `MutableBufferSequence` requirements, and `t` is a completion token.

Table 4. AsyncReadStream requirements

operation | type | semantics, pre/post-conditions
--- | --- | ---
`a.get_executor()` | A type satisfying the Executor requirements. | Returns the associated I/O executor.
`a.async_read_some(mb,t)` | The return type is determined according to the requirements for an asynchronous operation. | Meets the requirements for a read operation and an asynchronous operation with completion signature `void(error_code ec, size_t n)`. If `buffer_size(mb) > 0`, initiates an asynchronous operation to read one or more bytes of data from the stream `a` into the buffer sequence `mb`. If successful, `ec` is set such that `!ec` is `true`, and `n` is the number of bytes read. If an error occurred, `ec` is set such that `!!ec` is `true`, and `n` is 0. If all data has been read from the stream, and the stream performed an orderly shutdown, `ec` is `stream_errc::eof` and `n` is 0. If `buffer_size(mb) == 0`, the operation completes immediately. `ec` is set such that `!ec` is `true`, and `n` is 0.