[GHC] #16323: Cannot deduce X error with X provided in TypeFamilies

[GHC]

#16323: Cannot deduce X error with X provided in TypeFamilies
---------------------------------+----------------------------------------
        Reporter:  pjljvdlaar    |                Owner:  (none)
            Type:  bug           |               Status:  new
        Priority:  normal        |            Milestone:
       Component:  Compiler      |              Version:  8.2.2
      Resolution:                |             Keywords:  Deriving
Operating System:  Windows       |         Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:                |  Differential Rev(s):
       Wiki Page:                |
---------------------------------+----------------------------------------

Comment (by pjljvdlaar):

 replacing data by newtype give slightly different errors {{{Eq (Ref a)}}}
 is also required.
 Adding this to the definition, i.e.
 {{{#!hs
 newtype (Referable a, Eq (Ref a), Ord (Ref a), Hashable (Ref a)) =>
             RefMap a = RefMap { -- | the HashMap
                                 toHashMap :: HashMap.Map (Ref a) a
                               } deriving (Eq, Ord, Show, Read)
 }}}
 still give the same errors for newtype (yet different from data):

 {{{
 C:\TorXakis\sys\txs-basics\src\TorXakis\Test.hs:20:43: error:
     * Could not deduce (Eq (Ref a))
         arising from the 'deriving' clause of a data type declaration
       from the context: Eq 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 (Ord (Ref a))
         arising from the 'deriving' clause of a data type declaration
       from the context: Ord 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
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16323#comment:3>
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