Potentially unnecessary warning about missing deriving strategies
Summary
I'm not sure if this is a bug or not. I ran into it when compiling code using GHC 8.8.1 and -Weverything
. I was surprised to find that -Wmissing-deriving-strategies
fires even when there is no ambiguity about which deriving strategy should be used. In other words, with no language extensions enabled, (I think) stock
is the only valid deriving strategy. Therefore I would not expect this warning to fire. I would expect this warning to fire if I had GeneralizedNewtypeDeriving
, DeriveAnyClass
, DerivingStrategies
, or DerivingVia
enabled.
Steps to reproduce
$ ghc -Wmissing-deriving-strategies -e 'data T = C deriving Eq'
<interactive>:0:12: warning: [-Wmissing-deriving-strategies]
No deriving strategy specified. Did you want stock, newtype, or anyclass?
Use DerivingStrategies to specify a strategy.
Expected behavior
I expected no warning because the deriving
clause is not ambiguous.
Environment
- GHC version used: 8.8.1
Optional:
- Operating System: Windows 10
- System Architecture: x86_64