Buffer-oriented synchronous read stream requirements

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### Buffer-oriented synchronous read stream requirements

In the table below, `a` denotes a synchronous read stream object, `mb` denotes an object satisfying mutable buffer sequence requirements, and `ec` denotes an object of type `error_code`.

Table 20. Buffer-oriented synchronous read stream requirements

operation | type | semantics, pre/post-conditions
--- | --- | ---

`a.read_some(mb);`

`size_t`

Equivalent to:

    error_code ec;
    size_t s = a.read_some(mb, ec);
    if (ec) throw system_error(ec);
    return s;

`a.read_some(mb, ec);`

`size_t`

Reads one or more bytes of data from the stream `a`.
The mutable buffer sequence `mb` specifies memory where the data should be placed. The `read_some` operation shall always fill a buffer in the sequence completely before proceeding to the next.
If successful, returns the number of bytes read and sets `ec` such that `!ec` is true. If an error occurred, returns `0` and sets `ec` such that `!!ec` is true.
If the total size of all buffers in the sequence `mb` is `0`, the function shall return `0` immediately.