Using a type family to build a compound constraint that includes several constraints produced by applying another type family results in a type checker error like,

Could not deduce (ConFun l a, ConFun l b)
      arising from a use of ‘bar’
    from the context (ConFun l a, ConFun l b)
      bound by the type signature for
                 baz :: (ConFun l a, ConFun l b) => Proxy l -> HList '[a, b] -> ()

An example program is given at, https://gist.github.com/acowley/a001bd092599b5914bad