Provide an additional reason with the 'cannot derive instance' error

[hdgarrood]

Currently the following attempts to derive instances all produce a compiler error "cannot derive a type class instance for X" and nothing else:

-- Eq has just one type argument
derive instance eqIntInt :: Eq Int Int

-- missing the wildcard type argument
derive instance genericFoo :: Data.Generic.Rep.Generic Foo

-- cannot derive this class
class Foo
derive instance foo :: Foo

I think it would be nice to provide a little more detail in this error message. Possible reasons the compiler might have failed to derive an instance that I can think of:

• It's not aware of the class
• The instance was given the wrong number of type arguments (in particular, I think derive instance genericFoo :: Generic Foo missing the second wildcard might be a common mistake)

It's also possible that we might later add more reasons to this list as the compiler is able to derive more instances.

[garyb]

Ah yeah, I figured the latter one would be fixed by @LiamGoodacre's recent work on instance arity, but of course it won't here as this is arises during in desugaring and doesn't reach the typechecker.

👍

[hdgarrood]

I just noticed #2519. Perhaps these two should be addressed at the same time - maybe this issue subsumes that one, even?

[garyb]

The errors mentioned in there are caught during typechecking instead of desugaring, but the suggestion is to use a message more like when it fails during desugaring, so I think they are different despite the desired result being the same 😄