More accurately:
```d
T[] copyArray(T)(inout(T)[] arr) {
T[] copy = new T[arr.length];
static if (is(T == U[], U) && !is(T == immutable(Y)[], Y)) {
foreach(i, ref v; copy) {
v = copyArray(arr[i]);
}
} else static if (is(T == immutable(W)[], W)) {
for (size_t i = 0; i < arr.length; i++) {
copy[i] = arr[i].idup;
}
} else {
for (size_t i = 0; i < arr.length; i++) {
// it doesn't work with postblit, only with modern
copy ctor
T elem = arr[i];
copy[i] = elem;
}
}
return copy;
}
```
