[GHC] #16322: "deriving newtype instance" generates an infinite loop

[GHC]

#16322: "deriving newtype instance" generates an infinite loop
-------------------------------------+-------------------------------------
        Reporter:  paf31             |                Owner:  (none)
            Type:  bug               |               Status:  infoneeded
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 If it makes you feel any better, the code you were trying to write will be
 rejected with an error message in GHC 8.8 due to a slight tweak in the
 code generation strategy for `GeneralizedNewtypeDeriving` (notice the
 extra visible type applications):

 {{{
 λ> deriving instance C a b => C (X String a) (X String b)

 ==================== Derived instances ====================
 Derived class instances:
   instance Ghci1.C a b =>
            Ghci1.C
              (Ghci2.X GHC.Base.String a) (Ghci2.X GHC.Base.String b) where
     Ghci1.c
       = GHC.Prim.coerce
           @(Data.Proxy.Proxy (Ghci2.X GHC.Base.String a_a1I3)
             -> GHC.Types.Int)
           @(Data.Proxy.Proxy (Ghci2.X GHC.Base.String a_a1I3)
             -> GHC.Types.Int)
           (Ghci1.c @(Ghci2.X GHC.Base.String a_a1I3) @b_a1I4) ::
           Data.Proxy.Proxy (Ghci2.X GHC.Base.String a_a1I3) ->
 GHC.Types.Int


 Derived type family instances:



 <interactive>:13:1: error:
     • Occurs check: cannot construct the infinite type: b ~ X String b
         arising from a functional dependency between constraints:
           ‘C a b’ arising from a use of ‘c’ at <interactive>:13:1-54
           ‘C a b1’
             arising from the instance declaration at <interactive>:13:1-54
     • In the third argument of ‘GHC.Prim.coerce’, namely
         ‘(c @(X String a) @b)’
       In the expression:
           GHC.Prim.coerce
             @(Proxy (X String a) -> Int)
             @(Proxy (X String a) -> Int)
             (c @(X String a) @b) ::
             Proxy (X String a) -> Int
       In an equation for ‘c’:
           c = GHC.Prim.coerce
                 @(Proxy (X String a) -> Int)
                 @(Proxy (X String a) -> Int)
                 (c @(X String a) @b) ::
                 Proxy (X String a) -> Int
       When typechecking the code for ‘c’
         in a derived instance for ‘C (X String a) (X String b)’:
         To see the code I am typechecking, use -ddump-deriv

 <interactive>:13:1: error:
     • Occurs check: cannot construct the infinite type: a ~ X String a
         arising from a functional dependency between constraints:
           ‘C (X String a) b1’
             arising from a use of ‘c’ at <interactive>:13:1-54
           ‘C a b1’
             arising from the instance declaration at <interactive>:13:1-54
     • In the third argument of ‘GHC.Prim.coerce’, namely
         ‘(c @(X String a) @b)’
       In the expression:
           GHC.Prim.coerce
             @(Proxy (X String a) -> Int)
             @(Proxy (X String a) -> Int)
             (c @(X String a) @b) ::
             Proxy (X String a) -> Int
       In an equation for ‘c’:
           c = GHC.Prim.coerce
                 @(Proxy (X String a) -> Int)
                 @(Proxy (X String a) -> Int)
                 (c @(X String a) @b) ::
                 Proxy (X String a) -> Int
       When typechecking the code for ‘c’
         in a derived instance for ‘C (X String a) (X String b)’:
         To see the code I am typechecking, use -ddump-deriv
     • Relevant bindings include
         c :: Proxy (X String a) -> Int (bound at <interactive>:13:1)
 }}}

 -----

 If this discussion answers your question, can this issue be closed?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16322#comment:11>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler