I was looking at implementations of LUB and AND on demand signatures and I found this interesting case: lubStr (SProd s1) (SProd s2) | length s1 == length s2 = mkSProd (zipWith lubArgStr s1 s2) | otherwise = HeadStr The "otherwise" case is interesting, I'd expect that to be an error. I'm trying to come up with an example, let's say I have a case expression: case x of P1 -> RHS1 P2 -> RHS2 and y is used in RHS1 with S(SS) and in RHS2 with S(SSS). This seems to me like a type error leaked into the demand analysis. Same thing applies to `bothDmd` too. Funnily, it has this extra comment on this same code: bothStr (SProd s1) (SProd s2) | length s1 == length s2 = mkSProd (zipWith bothArgStr s1 s2) | otherwise = HyperStr -- Weird Does "Weird" here means "type error and/or bug" ? Should I try replacing these cases with panics and try to validate?