I have another interesting application of guarded impredicativity that I want to bring up. Currently, GHC #16140 [1] makes it rather inconvenient to use quantified constraints in type synonyms. For instance, GHC rejects the following example by default: type F f = (Functor f, forall a. Eq (f a)) This is because F is a synonym for a constraint tuple, so mentioning a quantified constraint in one of its arguments gets flagged as impredicative. In the discussion for #16140, we have pondered doing a major rewrite of the code in TcValidity to permit F. But perhaps we don't need to! After all, the quantified constraint in the example above appears directly underneath a type constructor (namely, the type constructor for the constraint 2-tuple), which should be a textbook case of guarded impredicativity. I don't have the guarded impredicativity branch built locally, so I am unable to test if this hypothesis is true. In any case, I wanted to mention it as another motivating use case. Ryan S. ----- [1] https://gitlab.haskell.org/ghc/ghc/issues/16140