nullify

import std.typecons : Nullable;

assert(null.isNull);

assert((int[]).init.isNull);
immutable int[2] x = [1, 2];
assert(!x[].isNull);

alias Ni = Nullable!int;
assert(Ni.init.isNull);

Ni ni = 3;
assert(!ni.isNull);

ni.nullify();
assert(ni.isNull);

const Ni ni2 = 3;
assert(!ni2.isNull);

struct S
{
	uint value;
	static immutable nullValue = S(value.max);
}
S s;
assert(!s.isNull);
s.nullify();
assert(s.isNull);

class C
{
	@safe pure nothrow
	this(int value)
	{
		this.value = value;
	}
	int value;
}

static assert(isNullable!C);

const x = C.init;
assert(x.isNull);

const y = new C(42);
assert(!y.isNull);