This fixes an issue where immediate invocations were queued inside of decltype, resulting in decltype's operand being evaluated.
Thanks Tyker, you're right, there is a general problem that needs handled, if this is to be handled; I had a bit of tunnel vision here. However, I'm retracting/abandoning this patch entirely as upon further review, it's not standard compliant. The current situation, is actually the correct one (for now?).
From [7.7.13] of the working draft:
Note: A manifestly constant-evaluated expression is evaluated even in an unevaluated operand.