Append const to array

[Yuxuan Shui via Digitalmars-d-learn]

struct A {
        ulong[] x;
}
struct B {
        ulong x;
}
void main() {
        B[] b;
        const(B) xx = B(1);
        b ~= xx; // Works

        A[] c;
        const(A) yy = A([1]);
        c ~= yy; // Does not
}

What gives?