Array slices and copy on write

[simendsjo]

D will copy the original content if a slice expands, but is the following behavior
implementation specific, or part of the specification?

	auto a = [0,1,2];
	auto b = a[0..2];
	a.length = 2; // is a[3] already marked for collection by the gc?
	assert(a.ptr == b.ptr);
	b.length += 1; // as b cannot reuse it even though noone is using it
	assert(a.ptr == b.ptr); // ooops, fails.. b is reallocated