Type inference breaks on constraint-kinded parameter used by a Rank-2 callback

Apologies in advance if the terminology in the summary is incorrect, I'm not really sure of the proper terms to use here

The following code:

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}

module Scratch where

import Prelude

data Foo
  = FooA Int
  | FooB String

useFoo :: forall cls a . (cls Int, cls String) => (forall k . cls k => k -> a) -> Foo -> a
useFoo f (FooA a) = f a
useFoo f (FooB b) = f b

seems like it should be able to infer the cls type parameter to useFoo based on the first argument, but instead calling with kshow gives the following error message:

>>> useFoo show (FooA 1)

<interactive>:52:1: error:
    • Could not deduce: (cls0 Int, cls0 String)
        arising from a use of ‘useFoo’
    • In the expression: useFoo show (FooA 1)
      In an equation for ‘it’: it = useFoo show (FooA 1)

<interactive>:52:8: error:
    • Couldn't match type ‘a’ with ‘String’
        ‘a’ is untouchable
          inside the constraints: cls0 k
          bound by a type expected by the context:
                     forall k. cls0 k => k -> a
          at <interactive>:52:1-20
      ‘a’ is a rigid type variable bound by
        the inferred type of it :: a at <interactive>:52:1-20
      Possible fix: add a type signature for ‘it’
      Expected type: k -> a
        Actual type: k -> String
    • In the first argument of ‘useFoo’, namely ‘show’
      In the expression: useFoo show (FooA 1)
      In an equation for ‘it’: it = useFoo show (FooA 1)
    • Relevant bindings include it :: a (bound at <interactive>:52:1)

Type-applying useFoo works as expected:

>>> :set -XTypeApplications
>>> useFoo @Show show (FooA 1)
"1"

This broke both with and without ScopedTypeVariables

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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