Dear all, As some of you might know, for the past few months I have been working on changing GHC's diagnostic messages from plain SDocs to richer Haskell types. As part of this work, I have added a mechanism to embed hints into diagnostics, defined in `GHC.Types.Hint` in `HEAD`. One of the main workhorse of this `GhcHint` type is the `SuggestExtension LangExt.Extension` constructor, which embeds the extension to enable to use a particular feature. The `LangExt.Extension` type comes from `GHC.LanguageExtensions.Types`, and up until now there has always been a 1:1 mapping between the language pragma for the extension and the type itself. Today I was working on turning this error into a proper Haskell type: badPun :: Located RdrName -> TcRnMessage badPun fld = TcRnUnknownMessage $ mkPlainError noHints $ vcat [text "Illegal use of punning for field" <+> quotes (ppr fld), text "Use NamedFieldPuns to permit this"] I was ready to yield a `SuggestExtension LangExt.NamedFieldPuns` when I discovered that there is no `NamedFieldPuns` constructor. Rather, there is a `RecordPuns` , which refer to a deprecated flag, and we simply map `NamedFieldPuns` back to it in `GHC.Driver.Session`: ... depFlagSpec' "RecordPuns" LangExt.RecordPuns (deprecatedForExtension "NamedFieldPuns"), ... flagSpec "NamedFieldPuns" LangExt.RecordPuns, ... This is problematic for the `GhcHint` type, because now if I was to yield `SuggestExtension LangExt.RecordPuns` to the user, I could still pretty-print the suggestion to turn `RecordPuns` into `NamedFieldPuns`, but this means that IDEs or third-party library would have access to the "raw" Haskell datatype, and at that point they will be stuck with a suggestion to enable a deprecated extension! (or best case scenario they will have to transform the suggestion into something more sensible, which partially defeats the point of this refactoring work I have been doing). I am not sure this behaviour is unique for just `NamedFieldPuns`, but my question is: 1. What prevents us from adding `NamedFieldPuns` as a proper constructor for the `Extension` type and in principle remove `RecordPuns`? Backward compatibility I assume? Many thanks, Alfredo