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Summary

Currently we can use DerivingVia to derive an instance via the very type that we're trying to define an instance for:

❯ ghci
GHCi, version 8.10.3: https://www.haskell.org/ghc/  :? for help
Prelude> :set -XDerivingVia
Prelude> data Color = Red | Green | Blue deriving Show via Color
Prelude> show Red
"

This is nonsensical and just leads to infinite recursion at runtime.

While there are plenty of other ways one can shoot themselves in their foot, I suggest that we at least notice this case. I just accidentally introduced this into some code at work when I forget to provide a newtype. I meant to write:

data Foo = ...
  deriving (FromField) via ReadShow Foo

but instead wrote

data Foo = ...
  deriving (FromField) via Foo

Expected behavior

At least trigger a warning, if there are legitimate reasons we might want to have data Foo = ... deriving C via Foo. If there are no legitimate reasons to write this, a compile time error would be emitted.

Environment

• GHC version used: 8.10.3