Convertible to mutable buffer requirements

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### Convertible to mutable buffer requirements

A type that meets the requirements for convertibility to a mutable buffer must meet the requirements of `CopyConstructible` types (C++ Std, 20.1.3), and the requirements of `Assignable` types (C++ Std, 23.1).

In the table below, `X` denotes a class meeting the requirements for convertibility to a mutable buffer, `a` and `b` denote values of type `X`, and `u`, `v` and `w` denote identifiers.

Table 5. ConvertibleToMutableBuffer requirements

expression | postcondition
--- | ---

    mutable_buffer u(a);
    mutable_buffer v(a);

    buffer_cast<void*>(u) == buffer_cast<void*>(v)
      && buffer_size(u) == buffer_size(v)

    mutable_buffer u(a);
    mutable_buffer v = a;

    buffer_cast<void*>(u) == buffer_cast<void*>(v)
      && buffer_size(u) == buffer_size(v)

    mutable_buffer u(a);
    mutable_buffer v; v = a;

    buffer_cast<void*>(u) == buffer_cast<void*>(v)
      && buffer_size(u) == buffer_size(v)

    mutable_buffer u(a);
    const X& v = a;
    mutable_buffer w(v);

    buffer_cast<void*>(u) == buffer_cast<void*>(w)
      && buffer_size(u) == buffer_size(w)

    mutable_buffer u(a);
    X v(a);
    mutable_buffer w(v);

    buffer_cast<void*>(u) == buffer_cast<void*>(w)
      && buffer_size(u) == buffer_size(w)

    mutable_buffer u(a);
    X v = a;
    mutable_buffer w(v);

    buffer_cast<void*>(u) == buffer_cast<void*>(w)
      && buffer_size(u) == buffer_size(w)

    mutable_buffer u(a);
    X v(b); v = a;
    mutable_buffer w(v);

    buffer_cast<void*>(u) == buffer_cast<void*>(w)
      && buffer_size(u) == buffer_size(w)