[Restrikted]

My Philosophy professor emailed our class this question to see if we could figure it out, and we'll get bonus points if we answer it correct. But I've been going crazy trying to figure it, and I honestly don't think it's possible. Without further ado:

Imagine you are going down a river in search of a treasure that you know is guarded by two guardsmen. You know one of these guardsmen always tells lies (whatever he says is false), while the other always tells the truth (whatever he says is true). They are standing by a fork in the river. If you follow one of these forks, the treasure is yours. If you follow the other, you will meet with certain death. You are allowed to ask only one of the guards only one question, and you cannot be sure whether the guard you are asking is the truth-teller or the liar.

What one question can you ask that will guarantee that you will get the treasure rather than meeting with certain death?

Does anyone have a clue? I don't know how it's possible to figure out who is the liar while simultaneously figuring out which way to go without asking a compound question, which isn't allowed.