Unhelpful error messages about lifted and unlifted types

I wrote this code:

{-# LANGUAGE MagicHash #-}
import GHC.Prim
import GHC.Types

main = do
    let primDouble = 0.42## :: Double#
    let double = 0.42 :: Double
    IO (\s -> mkWeakNoFinalizer# double () s)

and I get this error message:

WeakDouble.hs:8:15: error:
    • Couldn't match a lifted type with an unlifted type
      Expected type: (# State# RealWorld, Weak# () #)
        Actual type: (# State# RealWorld, Weak# () #)
    • In the expression: mkWeakNoFinalizer# double () s
      In the first argument of ‘IO’, namely
        ‘(\ s -> mkWeakNoFinalizer# double () s)’
      In a stmt of a 'do' block:
        IO (\ s -> mkWeakNoFinalizer# double () s)

with -fprint-explicit-kinds. (Without the flag, it looks the same, but tells me to use this flag…).

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	goldfire
Operating system
Architecture