Document GHC-97044 #445
Document GHC-97044 #445
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| This restriction applies to the following type classes: | ||
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| - [Coercible](https://hackage.haskell.org/package/base/docs/Data-Coerce.html#t:Coercible) - This class does not have regular instances; instead they are created on-the-fly during type-checking. | ||
| - [HasField](https://hackage.haskell.org/package/base/docs/GHC-Records.html#t:HasField) - This class is used by [OverloadedRecordDot](https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/overloaded_record_dot.html) and related extensions. |
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At least for the HasField typeclass it is possible to create user-written instances. See for example my library here: https://github.com/BinderDavid/tuple-fields/blob/main/src/Data/Tuple/Fields.hs
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You're right. Seems like I was misled by browsing around GHC codebase and I interpreted this
to mean that HasField can also trigger this warning.
But after deeper look it seems to be related to other "validations" done on instances of this type class.
As for other typeclasses mentioned, I'm pretty sure they should be there as I saw them mentioned in warnings in ghc test suite: https://github.com/search?q=repo%3Aghc%2Fghc%2097044&type=code
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