I think I've got a good idea of what's happening here. @jtbandes was spot-on with the idea that the problem was the LinkerExprAnalyzer. It was traversing the AST and didn't handle Force-unwrapping, so when it encountered such an expression it recurred into the inner optional expression until it found a declref, then recorded its type. Naturally, that means it thought the result of the linked expression was `Double?` here instead of simply `Double` and tried to propagate this nonsense outward to the rest of the system causing this bogus diagnostic. The fix is 3 lines telling the walker to stop dead.