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Tried to debug unrelated build failure of constraints-0.4.1.3 and found this in today's ghc-HEAD:

$ inplace/bin/ghc-stage2 -fforce-recomp -c C.hs -O0
WARNING: file compiler/stgSyn/CoreToStg.hs, line 250
  $fCategoryConstraint:- True False

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE GADTs #-}

module C () where
import Control.Category
import GHC.Types (Constraint)

data Dict :: Constraint -> * where
  Dict :: a => Dict a

infixr 9 :-

newtype a :- b = Sub (a => Dict b)

instance Category (:-) where
  id  = refl
  (.) = trans

infixl 1 \\

(\\) :: a => (b => r) -> (a :- b) -> r
r \\ Sub Dict = r

trans :: (b :- c) -> (a :- b) -> a :- c
trans f g = Sub (Dict \\ f \\ g)

refl :: a :- a
refl = Sub Dict

{-
$ inplace/bin/ghc-stage2 -fforce-recomp -c C.hs -O0

WARNING: file compiler/stgSyn/CoreToStg.hs, line 250
  $fCategoryConstraint:- True False
-}

Original file it was factored out is: https://github.com/ekmett/constraints/blob/c550b7653e88d58882873d11f05538d783313a05/src/Data/Constraint.hs

Might be a result of D1889

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	osa1
Operating system
Architecture