"addC: malformed constraint: cconsE" in closed-world class

[amigalemming]

Consider this module:

module Data.Array.Comfort.Shape where

import Control.DeepSeq (NFData, rnf)


data Lower = Lower
data Upper = Upper

class TriangularPart part where
   switchTriangularPart :: f Lower -> f Upper -> f part
instance TriangularPart Lower where switchTriangularPart f _ = f
instance TriangularPart Upper where switchTriangularPart _ f = f

data Triangular part size =
   Triangular {
      triangularPart :: part,
      triangularSize :: size
   } deriving (Show)

newtype Flip f b a = Flip {getFlip :: f a b}

instance (TriangularPart part, NFData size) => NFData (Triangular part size) where
   rnf (Triangular part sz) =
      rnf
         (flip getFlip part $
            switchTriangularPart (Flip $ \Lower -> ()) (Flip $ \Upper -> ()),
          sz)

LiquidHaskell/GHC-9.6.3 complains:

src/Data/Array/Comfort/Shape.hs:26:11: error:
    Uh oh.
    addC: malformed constraint:
cconsE: 
 t = lq_tmp$x##1052:{lq_tmp$x##1083 : (Data.Array.Comfort.Shape.Flip {lq_tmp$x##1080 : (GHC.Prim.FUN {lq_tmp$x
...
   |
26 |          (flip getFlip part $
   |           ^^^^^^^^^^^^^^^^^

This is a simplified version of:
https://hackage.haskell.org/package/comfort-array-0.5.4.2/docs/Data-Array-Comfort-Shape.html

[Facundo Domínguez]

First stop: figuring out what the error message means 🙈
I'm attaching the full error output.

[Niki Vazou]

It comes from here: https://github.com/ucsd-progsys/liquidhaskell/blob/develop/liquidhaskell-boot/src/Language/Haskell/Liquid/Constraint/Generate.hs#L360

LH is adding a subtype constraint with the two huge types that are printed in the error and checks that these two types have the same Haskell type. This error message says that they do not have the same Haskell type.