Eigen Compiler Suite

I undestand how a record (struct) type has to be represented, but I didn't find out yet how to represent a union type.

The C++ compiler so far seems neither to render unions nor structs. If I enter something like

union X {
    int a;
    char b;
};

void test(X x) {
}

in the sandbox, I get

.code "test(X)" 

    loc    "input", 6, 6
    void

    push    fun $lnk
    enter    0
    sym    +2, "x", ptr [$fp + 32]
    loc    "input", 6, 13
    type    "X" 

    pop    ptr $fp
    ret