structではできるのに,classでできないのはなぜだ?

gistfile1.d

struct S
{
	this(in int val)inout
	{
		val_ = val;
	}
	private int val_;
}

class C
{
	this(in int val)inout
	{
		val_ = val;
	}
	private int val_;
}

class C2
{
	this(in int val)
	{
		val_ = val;
	}
	this(in int val)immutable
	{
		val_ = val;
	}
	private int val_;
}

void main()
{
	S ms = S(0);//OK
	immutable S ims = S(0);//OK

	C mc = new C(0);//NG
	immutable C imc = new immutable C(0);//NG

	C2 mc2 = new C2(0);//OK
	immutable C2 imc2 = new immutable C2(0);//OK
}