Why do these warnings and errors happen?

Test.cs

// case 1
public struct A : System.IDisposable
{
	public void Test()
	{
		var a = new A();
		using (a)
			a[0] = 0; // warning CS0728: Possibly incorrect assignment to local 'a' which is the argument to a using or lock statement. The Dispose call or unlocking will happen on the original value of the local.
	}

	public int this[int index]
	{
		get {return 0;}
		set {}
	}

	public void Dispose() {}
}

// case 2
public struct B : System.IDisposable
{
	public void Test()
	{
		using (var b = new B())
			b[0] = 0; // error CS1654: Cannot modify members of 'b' because it is a 'using variable'
	}

	public int this[int index]
	{
		get {return 0;}
		set {}
	}

	public void Dispose() {}
}

foo.cs(9,4): warning CS0728: Possibly incorrect assignment to local 'b' which is the argument to a using or lock statement. The Dispose call or unlocking will happen on the original value of the local.