I think that the [answer of thesecretmaster](https://cseducators.stackexchange.com/a/5907/1293) is correct but let me add a bit of advice to an instructor who would do this. Just as you wonder yourself, the rule doesn't seem to make a lot of sense and it won't make sense to students either. It may cause resentment. 

So, if an instructor wants to use a rule like this then, I think that a general rule agains using things not yet taught is foolish. However, you can achieve the same result by carefully stating the problem you want solved. So, for example, following thesecretmaster, instead of asking for the students to compute the absolute value ask them to compute the absolute value using only the bitwise operators. In other words, put the specific restriction into the question itself. 

This brings up an important teaching and learning technique called "creativity under constraint". Some wood working artists, for example, use only hand tools, forgoing power tools. Furniture makers I've known do this. The restricted set of tools forces them to make better use of the tools that the _do_ use, becoming more skilled in the process. But, to impose it on someone else requires that you make it clear why it is a good idea to do this. 

I've discussed Creativity Under Constraint here in the past in other contexts. See [this post](https://cseducators.stackexchange.com/a/1111/1293) and [this other one](https://cseducators.stackexchange.com/a/2883/1293).