Unclear type error when HasType constraint is missing

[f-f]

Thanks for the great talk today 🙂

I found that when calling the/typed on a type that is missing the HasType constraint - e.g. when wiring up some code using a Reader and using this library for pulling values from there - produces less-than-desiderable type errors.

Minimal repro:

foo :: env -> Int
foo env = env ^. the @Int

..which produces this type error:

error:
    • Overlapping instances for HasType Int env
        arising from a use of ‘the’
      Matching instances:
        instance generic-lens-core-2.0.0.0:Data.Generics.Product.Internal.Typed.Context
                   a s =>
                 HasType a s
          -- Defined in ‘Data.Generics.Product.Typed’
        instance [overlapping] HasType a a
          -- Defined in ‘Data.Generics.Product.Typed’
        ...plus one instance involving out-of-scope types
        (use -fprint-potential-instances to see them all)
      (The choice depends on the instantiation of ‘env’
       To pick the first instance above, use IncoherentInstances
       when compiling the other instance declarations)
    • In the second argument of ‘(^.)’, namely ‘the @Int’
      In the expression: env ^. the @Int
      In an equation for ‘foo’: foo env = env ^. the @Int

I would expect this to be:

• about "missing constraint" instead of "overlapping instance"
• a bit more actionable - e.g. I'd at least like GHC to suggest me to add the HasType constraint to the env, as it usually happens when trying to use a typeclass method on something that doesn't have the constraint.

Is there anything we could do about this here, or is this an inherent GHC limitation?