struct S1 { int a; float b; }
struct S2 { char[] a; union { S1 b; S1 * c; } }
alias R = RepresentationTypeTuple!S2;
assert(R.length == 4
    && is(R[0] == char[]) && is(R[1] == int)
    && is(R[2] == float) && is(R[3] == S1*));