Re: [Haskell-cafe] Injective type classes?

On Tue, 30 Mar 2021, Tom Smeding wrote:

Note that: - In my actual use case, 'C' is of course not empty. - In my actual use case, my type class instances _are_ in fact injective, even   though I do enable FlexibleInstances to be able to write e.g.   'instance C (T a Int)'. - Above, the dictionary for 'C (a, b)' includes a dictionary for 'C b', doesn't   it? So if inference can resolve 'C b', then the compilation to Core can find   the right dictionary, I think? (Not sure about this part.)

How shall GHC find the method implementations for 'b' if it only has the methods for '(a,b)'?