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

14 Feb 2019

      #16322: "deriving newtype instance" generates an infinite loop
-------------------------------------+-------------------------------------
        Reporter:  paf31             |                Owner:  (none)
            Type:  bug               |               Status:  new
        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:                    |
-------------------------------------+-------------------------------------
Description changed by paf31:

Old description:
...
I was trying to create a newtype instance for the
`SqlSelect`(https://hackage.haskell.org/package/esqueleto-2.5.3/docs
/Database-Esqueleto-Internal-Sql.html#t:SqlSelect) class from the
`esqueleto` library. It has multiple type arguments so I was using
StandaloneDeriving:
{{{
newtype X a b = X { unX :: b }
deriving newtype instance SqlSelect b c => SqlSelect (X a b) (X a c)
}}}

...
Unfortunately, this generates code which must include an infinite loop,
because the compiled code spins, but replacing it with the obvious
handwritten instance seems to fix the problem.
Apologies if this has been reported or fixed elsewhere, I was unable to
find any matching issues.
New description:

 I was trying to create a newtype instance for the
 `SqlSelect`(https://hackage.haskell.org/package/esqueleto-2.5.3/docs
 /Database-Esqueleto-Internal-Sql.html#t:SqlSelect) class from the
 `esqueleto` library. It has multiple type arguments so I was using
 StandaloneDeriving and DerivingStrategies:

 {{{
 newtype X a b = X { unX :: b }

 deriving newtype instance SqlSelect b c => SqlSelect (X a b) (X a c)
 }}}

 Unfortunately, this generates code which must include an infinite loop,
 because the compiled code spins, but replacing it with the obvious
 handwritten instance seems to fix the problem.

 Apologies if this has been reported or fixed elsewhere, I was unable to
 find any matching issues.

--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16322#comment:1
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler