This is probably not a minimal test-case, and I haven't tested with more types, but it shows the problem.

// This works
struct S {
    this(int i) {
        a = i;
    }
    union {
        int a = void;
        int b = void;
    }
}
unittest
{
    auto s = S(1);
    assert(&s.a == &s.b);
    enum s2 = S(1);
    static assert(S(1).a == 1);
}

// But this doesn't
struct S2 {
    this(int i) { // error at this line
        a[] = i;
    }
    union {
        int[1] a = void;
        int[1] b = void;
    }
}
unittest
{
    auto s = S2(1);
    assert(s.a.ptr == s.b.ptr);
    // bug.d(19): Error: uninitialized variable 'b' cannot be returned from CTFE
    enum s2 = S2(1);
}

void main() {
}

Filed a bug. http://d.puremagic.com/issues/show_bug.cgi?id=9932