If you're given a "confirm my idea is good" project by a lead who doesn't want disconfirming evidence, ask them before you start "what kind of information would change your mind?".

Because before you start, if they can't answer that question, then it's obvious even to themselves they're being disingenuous.

Now you have a roadmap of a line or reasoning to try to show why it's not a good idea, that they will be forced to accept if you can because previous them said it would be convincing.