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Created by Ryan Scott@RyanGlScottMaintainer

Compile-time stack overflow exception in GHC HEAD only

If I compile the following program in GHC HEAD:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeOperators #-}
module Bug where

import Data.Kind
import Data.Type.Equality

data F (a :: k) :: (a ~~ k) => Type where
  MkF :: F a

Then it simply loops until the stack overflows:

$ ~/Software/ghc5/inplace/bin/ghc-stage2 --interactive Bug.hs
GHCi, version 8.9.0.20190620: https://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
*** Exception: stack overflow

This isn't strictly a regression since writing equality constraints in kinds (like in the kind of F) is a featurette that GHC has only really supported properly as of HEAD, although it is worth noting that previous versions of GHC did not exhibit this behavior.