[GHC] #13952: Liberal coverage condition fails if TypeInType is enabled

10 Jul 2017

      #13952: Liberal coverage condition fails if TypeInType is enabled
-------------------------------------+-------------------------------------
           Reporter:  mpickering     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following program compiles with 8.0.2 but fails with 8.2.1-rc2. It
 succeeds if the `TypeInType` extension is disabled.

 {{{#!hs
 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE DataKinds,
   FlexibleContexts, FlexibleInstances, FunctionalDependencies,
   MultiParamTypeClasses,  TypeFamilies, TypeOperators #-}
 -- Removing TypeInType causes compilation to succeed
 {-# LANGUAGE TypeInType #-}
 module Bookkeeper.Internal where

 import GHC.Generics
 import GHC.TypeLits (Symbol, KnownSymbol, TypeError, ErrorMessage(..))

 class FromGeneric a book | a -> book where
   fromGeneric :: a x ->  book

 type family Expected a where
   Expected (l :+: r) = TypeError ('Text "Cannot convert sum types into
 Books")
   Expected U1        = TypeError ('Text "Cannot convert non-record types
 into Books")

 instance (book ~ Expected U1) => FromGeneric U1 book where
   fromGeneric = error "impossible"
 }}}

 {{{
 src/Bookkeeper/Internal.hs:18:10: error:
     • Illegal instance declaration for ‘FromGeneric U1 book’
         The coverage condition fails in class ‘FromGeneric’
           for functional dependency: ‘a -> book’
         Reason: lhs type ‘U1’ does not determine rhs type ‘book’
         Un-determined variable: book
     • In the instance declaration for ‘FromGeneric U1 book’
    |
 18 | instance (book ~ Expected U1) => FromGeneric U1 book where
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 Perhaps this is related to #12803 but I made another ticket so that it can
 be diagnosed separately.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13952
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