QuantifiedConstraints: Problems with Typeable

This compiles

{-# Language GADTs, ConstraintKinds, ScopedTypeVariables, QuantifiedConstraints #-}

import Data.Typeable

data Ex cls where
  MkEx :: cls xx => xx -> Ex cls

a :: Ex Typeable -> TypeRep
a (MkEx (_::xx)) = typeRep @_ @xx Proxy

-- b :: (forall xx. cls xx => Typeable xx) => Ex cls -> TypeRep
-- b (MkEx (_::xx)) = typeRep @_ @xx Proxy

c :: (forall xx. cls xx => Show xx) => Ex cls -> String
c (MkEx xx) = show xx

but uncommenting b is too much:

$ qc hs/189-bug.hs
GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( hs/189-bug.hs, interpreted )

hs/189-bug.hs:12:20: error:
    • Could not deduce (Typeable xx) arising from a use of ‘typeRep’
      from the context: forall xx. cls xx => Typeable xx
        bound by the type signature for:
                   b :: forall (cls :: * -> Constraint).
                        (forall xx. cls xx => Typeable xx) =>
                        Ex cls -> TypeRep
        at hs/189-bug.hs:11:1-60
      or from: cls xx
        bound by a pattern with constructor:
                   MkEx :: forall xx (cls :: * -> Constraint). cls xx => xx -> Ex cls,
                 in an equation for ‘b’
        at hs/189-bug.hs:12:4-15
    • In the expression: typeRep @_ @xx Proxy
      In an equation for ‘b’: b (MkEx (_ :: xx)) = typeRep @_ @xx Proxy
   |
12 | b (MkEx (_::xx)) = typeRep @_ @xx Proxy
   |                    ^^^^^^^^^^^^^^^^^^^^
Failed, no modules loaded.
Prelude> 

This is maybe because of the Typeable solver.

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture