Specialization optimization assumes that all type variables are at the front

[lukaszcz]

The specialization optimization in Core assumes that all type variables come first.

This assumption is not true, e.g., for for and rfor here:

instance
foldableRangeI {N} {{Ord N}} {{Natural N}} : Foldable (Range N) N :=
  mkFoldable@{
    for {B : Type} (f : B -> N -> B) (ini : B) : Range N -> B
      | (mkRange low high step) :=
        let
          {-# specialize-by: [f] #-}
          terminating
          go (acc : B) (next : N) : B :=
            if
              | next > high := acc
              | else := go (f acc next) (next + step);
        in go ini low;
    rfor {B : Type} (g : B -> N -> B) (ini : B) : Range N -> B
      | (mkRange low high step) :=
        let
          {-# specialize-by: [g] #-}
          terminating
          go (base : B) (next : N) : B :=
            if
              | next <= high := g (go base (next + step)) next
              | else := base;
        in go ini low
  };