A conditional +1 from me, the condition being to allow the same monad instance code to compile at least with GHC versions 7.8, 7.10, and 8.0 without any CPP directives. I think that can be accomplished by an extra clarification that the meaning of "override" in
**Phase 1** *(GHC 8.0)*: Implement new warning in GHC which gets triggered when `Monad` instances explicitly override the default `return` method implementation.
does *not* include a re-definition of return = pure Yes, I realize this corner case would be a wart in the next language standard. We could fix that by disabling even this re-definition in case {#- LANGUAGE Haskell2020 #-} or equivalent is declared. Perhaps there should be two pragmas, {#- LANGUAGE Haskell2020+ #-} and {#- LANGUAGE Haskell2010-2020 #-}?