GHC does not inform me about -Wdeprecated-flags
If I say
{-# LANGUAGE DatatypeContexts #-}
module Bug where
I get
Bug.hs:1:14: warning:
-XDatatypeContexts is deprecated: It was widely considered a misfeature, and has been removed from the Haskell language.
|
1 | {-# LANGUAGE DatatypeContexts #-}
| ^^^^^^^^^^^^^^^^
I had assumed for some time that this was an unsuppressable warning. A bit annoying to have an unsuppressable warning, but nothing I felt compelled to take action about.
But this was wrong.
If instead I say
{-# LANGUAGE DatatypeContexts #-}
{-# OPTIONS_GHC -Wno-deprecated-flags #-}
module Bug where
I get no warning. This is because the deprecation warning is actually controlled by -Wdeprecated-flags
(on by default), but GHC unhelpfully does not tell me this when printing the warning.
Happily, there is a fix: !5207 (57a98976), originally part of !5207 (closed), but I believe will be committed separately, in the end.