Couldn't match kind ‘k1’ with ‘k1’
The following program gives a hopelessly confusing error message on GHC 8.6.3 and HEAD:
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE PolyKinds #-}
module Bug where
import Data.Kind
type Const a b = a
type SameKind (a :: k) (b :: k) = (() :: Constraint)
class SameKind a b => C (k :: Const Type a) (b :: k)$ /opt/ghc/8.6.3/bin/ghc Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
Bug.hs:11:18: error:
• Couldn't match kind ‘k1’ with ‘k1’
• In the second argument of ‘SameKind’, namely ‘b’
In the class declaration for ‘C’
|
11 | class SameKind a b => C (k :: Const Type a) (b :: k)
| ^I imagine that the real issue is that SameKind a b would force a :: k, which would be ill-scoped. But figuring that out from this strange error message requires a lot of thought.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.6.3 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |