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The following code

{-# LANGUAGE FlexibleContexts      #-}
{-# LANGUAGE TypeFamilies          #-}
module TorXakis.Test
( -- * Referable
  Referable(..)
)
where
import           Data.Hashable
import qualified Data.HashMap         as HashMap

-- | A referable class
class Referable a where
    type Ref a
    toRef :: a -> Ref a

-- | Map of Referable objects.
data (Referable a, Ord (Ref a), Hashable (Ref a)) =>
            RefMap a = RefMap { -- | the HashMap
                                toHashMap :: HashMap.Map (Ref a) a
                              } deriving (Eq, Ord, Show, Read)

gives the following errors

C:\TorXakis\sys\txs-basics\src\TorXakis\Test.hs:20:43: error:
    * Could not deduce (Ord (Ref a))
        arising from the 'deriving' clause of a data type declaration
      from the context: (Eq a, Referable a)
        bound by the deriving clause for `Eq (RefMap a)'
        at src\TorXakis\Test.hs:20:43-44
      Possible fix:
        use a standalone 'deriving instance' declaration,
          so you can specify the instance context yourself
    * When deriving the instance for (Eq (RefMap a))
   |
20 |                               } deriving (Eq, Ord, Show, Read)
   |                                           ^^

C:\TorXakis\sys\txs-basics\src\TorXakis\Test.hs:20:47: error:
    * Could not deduce (Hashable (Ref a))
        arising from the 'deriving' clause of a data type declaration
      from the context: (Ord a, Referable a)
        bound by the deriving clause for `Ord (RefMap a)'
        at src\TorXakis\Test.hs:20:47-49
      Possible fix:
        use a standalone 'deriving instance' declaration,
          so you can specify the instance context yourself
    * When deriving the instance for (Ord (RefMap a))
   |
20 |                               } deriving (Eq, Ord, Show, Read)
   |                                               ^^^

C:\TorXakis\sys\txs-basics\src\TorXakis\Test.hs:20:52: error:
    * Could not deduce (Ord (Ref a))
        arising from the 'deriving' clause of a data type declaration
      from the context: (Show a, Referable a)
        bound by the deriving clause for `Show (RefMap a)'
        at src\TorXakis\Test.hs:20:52-55
      Possible fix:
        use a standalone 'deriving instance' declaration,
          so you can specify the instance context yourself
    * When deriving the instance for (Show (RefMap a))
   |
20 |                               } deriving (Eq, Ord, Show, Read)
   |                                                    ^^^^

C:\TorXakis\sys\txs-basics\src\TorXakis\Test.hs:20:58: error:
    * Could not deduce (Ord (Ref a))
        arising from the 'deriving' clause of a data type declaration
      from the context: (Read a, Referable a)
        bound by the deriving clause for `Read (RefMap a)'
        at src\TorXakis\Test.hs:20:58-61
      Possible fix:
        use a standalone 'deriving instance' declaration,
          so you can specify the instance context yourself
    * When deriving the instance for (Read (RefMap a))
   |
20 |                               } deriving (Eq, Ord, Show, Read)
   |                                                          ^^^^

--  While building package txs-basics-0.1.0.0 using:
      C:\sr\setup-exe-cache\x86_64-windows-integersimple\Cabal-simple_Z6RU0evB_2.0.1.0_ghc-8.2.2.exe --builddir=.stack-w
ork\dist\67675594 build lib:txs-basics --ghc-options " -ddump-hi -ddump-to-file"
    Process exited with code: ExitFailure 1

yet the requirements such as Ord (Ref a) are clearly given to the data definition!

possibly related to #16319

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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