1. God is all-knowing (omniscient).
2. God is all-good (omnibenevolent).
3. If (1) then God always knows what it is the he will do.
4. If (2) then God always does that which promotes or allows for the greatest good.
If (1) and (2), are true then from (3) and (4) we deduce:
5. God is not free.
So, the proof (1-5) holds that God can only do that which he knows to be good and consequently he already knows that which he is going to do. If that is true, despite the obvious obscurity about whether or not God can choose that which is future or possible, then it must be the case that God is not free with regards to some action. The additional result is that God can not even choose amid some set of options; like: a set of choice's say, A and B. As per A is always better than B or B must always be better than A for God.
Some may make the objection that some set of options say, C and D are exactly equal both quantitatively and qualitatively, then God could make the choice amid C and D base upon some ability of preference. However, for those who make this claim they must understand that if C and D are numerically identical (in terms of properties) then they run into Leibniz's law, which states: if any two objects A and B possess exactly the same properties then A and B are exactly the same thing. This means that if A=B and B=A then the result if some type if tautology such as: A=b and B=A the AB=BA and BA=AB --> A=A and B=B (B&A+A&B)=(A&B+B&A). So it does not matter which is chose as per A and B are the same thing.