Confusing error writing an instance of a non-class type

Summary

Messing up an instance head can lead to very confusing errors.

Steps to reproduce

I was trying to write a Functor instance for a type, but accidentally omitted the word Functor from the instance head.

data Foo m a

instance Foo m where
  fmap _ x = case x of

GHC told me

error:
    ‘fmap’ is not a (visible) method of class ‘Foo’

Huh?

Expected behavior

I would expect GHC to tell me that Foo is not a class and therefore cannot have any instances. I would imagine this could be handled in the renamer.

Environment

• GHC version used: 9.4.3

Optional:

• Operating System:
• System Architecture: