Spurious untouchable type variable in connection with rank-2 type and constraint family
The following code cannot be compiled:
{-# LANGUAGE ConstraintKinds, Rank2Types, TypeFamilies #-}
import GHC.Exts (Constraint)
type family F a b :: Constraint
data T b c = T
f :: (forall b . F a b => T b c) -> a
f _ = undefined
GHC outputs the following error message:
Untouchable.hs:9:6: error:
• Couldn't match type ‘c0’ with ‘c’
‘c0’ is untouchable
inside the constraints: F a b
bound by the type signature for:
f :: F a b => T b c0
at Untouchable.hs:9:6-37
‘c’ is a rigid type variable bound by
the type signature for:
f :: forall a c. (forall b. F a b => T b c) -> a
at Untouchable.hs:9:6
Expected type: T b c0
Actual type: T b c
• In the ambiguity check for ‘f’
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
In the type signature:
f :: (forall b. F a b => T b c) -> a
Trac metadata
Trac field | Value |
---|---|
Version | 8.2.1-rc2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |