because UndecidableInstances is definitely required for this and I know it's a problematic one.
Hi Juan, I'll knock that on the head at once. `UndecidableInstances` is not "problematic" (or at least it's far less problematic than others you list). Although we're lacking a proof that it can't lead to incoherence/type unsafety, nobody's demonstrated unsafety due to `UndecidableInstances` alone -- that is, providing the program compiles (i.e. instance resolution terminates). OTOH `FlexibleInstances` can give `OverlappingInstances` -- maybe overlapping with those in some other module, thus giving the dreaded Orphan instances problems. I'd be much more concerned about them.
instance (Ord a, forall b c. (Ord b, Ord c)) => Class1 a where fun1 = (<)
Why does that even mention `b` or `c`? There's no FunDep from `a`, to get a FunDep there'd be a constraint `D a b c` or something. They seem totally redundant.
completely overlooked by the compiler
Yes. Quite. What do you expect the compiler to do? Even if the class decl gave a signature for `fun1` mentioning `b`, `c`, those would be _distinct_ tyvars, because they're not scoped in the class head.
is there any way I can make this work?
Sorry, I don't know what you want to "work". Please at least show a class decl with a FunDep. From your second message:
it is possible in principle that (Ord a, Ord b) produces a functional dependency between a and b
No it isn't possible, even in principle: `(..., ...)` is a tuple constructor, not a class; therefore no FunDep could apply. AntC