GHC HEAD type family regression involving invisible arguments
The following code compiles on GHC 8.0.2 through 8.6.2:
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
module Bug where
import Data.Kind
class C k where
type S :: k -> Type
data D :: Type -> Type
data SD :: forall a. D a -> Type
instance C (D a) where
type S = SDBut fails to compile on GHC HEAD (commit 73cce63f):
$ ~/Software/ghc2/inplace/bin/ghc-stage2 Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
Bug.hs:15:3: error:
• Type indexes must match class instance head
Expected: S @(D a)
Actual: S @(D a1)
• In the type instance declaration for ‘S’
In the instance declaration for ‘C (D a)’
|
15 | type S = SD
| ^^^^^^^^^^^This regression prevents the stitch library from compiling.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.7 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | highest |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |