Puzzling error message: No instance for ?Generic s?

[ddssff]

I sometimes get error messages that resemble GHC missing instance messages but I don't understand what they actually mean. For example, the declaration

import Control.Lens (ReifiedLens(Lens))
foo :: HasPosition i s t a b => ReifiedLens s t a b
foo = Lens (position0 @i)

Gives this error:

src/Data/Generics/Product/Positions.hs:109:18: error:
    * | No instance for ?Generic s?
      |   arising from a generic lens focusing on the field at
      |   position ?i? of type ?a? in ?s?
      
    * In the first argument of `Lens', namely `position0'
      In the expression: Lens (position0 @i)
      In an equation for `foo': foo = Lens (position0 @i)

Could someone explain what this question mark bracketing means? There is already a Generic constraint on s.

[aavogt]

generic lens core hardcodes unicode quotes, whereas ghc's own messages fall back to ascii.

If you're consistent with the 0 suffix it compiles:

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
foo0 :: forall i s t a b. (HasPosition0 i s t a b) => ReifiedLens s t a b
foo0 = Lens (position0 @i)
foo :: forall i s t a b. (HasPosition i s t a b) => ReifiedLens s t a b
foo = Lens (position @i)

Adding a Generic s to fooErr's type signature confusingly has no effect on the error message.

fooErr :: forall i s t a b. (Generic s, HasPosition i s t a b) => ReifiedLens s t a b
fooErr = Lens (position0 @i)
{- main.hs:60:14: error: [GHC-64725]
    • | No instance for ‘Generic s’
      |   arising from a generic lens focusing on the field at
      |   position ‘i’ of type ‘a’ in ‘s’
      
    • In the first argument of ‘Lens’, namely ‘(position0 @i)’
      In the expression: Lens (position0 @i)
      In an equation for ‘fooErr’: fooErr = Lens (position0 @i)
-}

Instead of saying that the instance will always fix the problem, type family NoGeneric could instead make a suggestion that usually works:

    • | Suggested fix: `deriving instance Generic s'
      |   to satisfy a generic lens focusing on the field at
      |   position ‘i’ of type ‘a’ in ‘s’
      
    • In the first argument of ‘Lens’, namely ‘(position0 @i)’
      In the expression: Lens (position0 @i)
      In an equation for ‘fooErr’: fooErr = Lens (position0 @i)

The original message is slightly better in that it doesn't suggest an unnecessary -XStandaloneDeriving.