Quartus eliminates intermediate signal causing an overflow.

Not that I'm following this well but in your original post you said that it appears that 'f' is being optimized away. I would expect this to happen if you don't use it anyway but the assignment into 'e'.

I recommend sticking to your code fragment at the start of your post and tell us what you were using for inputs b, c, and d and we can then tell you exactly how the logic should behave. Were any of the inputs hardcoded by any chance?