The copy conversion from const S to S should work, because the field S.cx.arr has same qualified type const(int[]).

struct X
{
    int[] arr;
}
struct S
{
    const(X) cx;
}
void main()
{
    static assert(is(typeof((const S).init.cx.arr) == const(int[])));
    static assert(is(typeof((      S).init.cx.arr) == const(int[])));
    const S sc;
    S sm = sc;      // fails, but should work
    static assert(is(const S : S));     // fails, but should work
}