GHC panic using TypeInType with minimal source code (#15709) · Issues · Glasgow Haskell Compiler / GHC

GHC panic using TypeInType with minimal source code

The following source code causes GHC 8.4.3 on 64bit Linux to panic:

{-# LANGUAGE TypeInType #-}

module Lib where

import Data.Kind

class Contravariant f where
    contramap :: (a -> b) -> f b -> f a

dimap :: (Contravariant (p :: * -> b -> p * b), Functor (p a)) => (z -> a) -> (b -> c) -> p a b -> p z c
dimap f g = contramap f . fmap g

I have no idea if it should compile or not, but it shouldn't do this:

ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.3 for x86_64-unknown-linux):
	piResultTy
  k_a34u[tau:1]
  *
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:947:35 in ghc:Type

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

The following source code causes GHC 8.4.3 on 64bit Linux to panic:

```haskell
{-# LANGUAGE TypeInType #-}

module Lib where

import Data.Kind

class Contravariant f where
    contramap :: (a -> b) -> f b -> f a

dimap :: (Contravariant (p :: * -> b -> p * b), Functor (p a)) => (z -> a) -> (b -> c) -> p a b -> p z c
dimap f g = contramap f . fmap g
```

I have no idea if it should compile or not, but it shouldn't do this:

```
ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.3 for x86_64-unknown-linux):
	piResultTy
  k_a34u[tau:1]
  *
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:947:35 in ghc:Type

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
```

<details><summary>Trac metadata</summary>

| Trac field             | Value        |
| ---------------------- | ------------ |
| Version                | 8.4.3        |
| Type                   | Bug          |
| TypeOfFailure          | OtherFailure |
| Priority               | normal       |
| Resolution             | Unresolved   |
| Component              | Compiler     |
| Test case              |              |
| Differential revisions |              |
| BlockedBy              |              |
| Related                |              |
| Blocking               |              |
| CC                     |              |
| Operating system       |              |
| Architecture           |              |

</details>