I got bitten by this recently, and while I can see GHC's reasoning, I wish it told me something different and more useful.

class X a where
  foo :: a

  {-# MINIMAL foo #-}
  foo = undefined

instance X Int

For this program, ghc says:

GHCi, version 8.6.3: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( a.hs, interpreted )

a.hs:7:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘foo’
    • In the instance declaration for ‘X Int’
  |
7 | instance X Int
  |          ^^^^^

This is arguably "correct", since I made foo part of MINIMAL; but it's very confusing, because I know I added a default definition.

I wish GHC instead said something like:

You made `foo` MINIMAL, but also gave an explicit definition for it.

I can see the logic behind the current error message, but it was rather confusing. The background is that my class had many other methods and a MINIMAL pragma. Much later I realized I could give a default definition of one of those methods but I forgot to remove it from the MINIMAL list. So, I had to puzzle a while at the error messages that were coming from far away modules that had nothing to do with the class definition itself.

This is not a showstopper by any means, but unless there are good reasons to do otherwise, it would be nice to get a warning right where the class is defined, as opposed to where it is instantiated.