The following vala code produces invalid C, without error from Vala.

test.vala.c:29: error: invalid operands to binary == (have ‘GValue’ and ‘int’)

static void main () {
    Value? v = Value (typeof(int));
    v.set_int (1);
    if (v == 2) {
        debug ("yep");
    }
}

And also this one produce another invalid C code.
 
static Value? get_value (Value ?v) {
    return v;
}

static void main () {
    Value? v = Value (typeof(int));
    v.set_int (1);
    if (get_value (v) == 2) {                                                                                                                                                                       
        debug ("yep");
    }
}