[Haskell-cafe] Injective type classes?

[Henning Thielemann]

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)'?