Report an error for a missing class instance before an error for type family instances of an associated type.

The following code

{-# LANGUAGE FlexibleContexts, TypeFamilies, DataKinds #-}

import GHC.Generics
import GHC.TypeLits

type family RepHasNoInstance (f :: * -> *) ::  *

-- edit 1
-- type instance RepHasNoInstance f = Int

-- edit 2
-- class RepHasNoInstanceC (f :: * -> *)
-- foo :: (Generic a, RepHasNoInstanceC (Rep a)) => a -> Int
-- foo = const 1
foo :: (Generic a, RepHasNoInstance (Rep a) ~ Int) => a -> Int
foo = const 1

data NotGeneric = NotGeneric

bar :: NotGeneric -> Int
bar = foo

main :: IO ()
main = return ()

gives an error like

associated-type-families-test.hs:21:7: error:
    • Couldn't match type ‘RepHasNoInstance (Rep NotGeneric)’
                     with ‘Int’
        arising from a use of ‘foo’
    • In the expression: foo
      In an equation for ‘bar’: bar = foo
   |
21 | bar = foo
   |       ^^^

in ghcs: 8.0.2, 8.2.1, master(a few days old).

The error message is for ambiguous types in 7.10.3

Uncommenting edit 1, the error changes to:

associated-type-families-test.hs:16:7: error:
    • No instance for (Generic NotGeneric) arising from a use of ‘foo’
    • In the expression: foo
      In an equation for ‘bar’: bar = foo
   |
16 | bar = foo
   | 

I think this is a much better error message, since there is no hope for the associated type to have an instance for anything unless there is an instance (Generic NotGeneric).

Uncommenting edit 2, (and commenting the existing foo) gives the "No instance for (Generic NotGeneric)" message.

Trac metadata

Trac field	Value
Version	8.2.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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