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

[GHC]

#16322: "deriving newtype instance" generates an infinite loop
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        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:                    |
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Comment (by RyanGlScott):

 Replying to [comment:5 paf31]:
 > I understand that only the last argument is coerced, but even so, that
 would require an instance for `C (X String a) b`, which in this specific
 case instantiates to `C (X String Int) Int`, and the functional dependency
 should (I think) force this to fail via `X String Int ~ Int`. No?

 If you had written `instance C (X String a) b => C (X String a) (X String
 b)`, then that would be the case. But you didn't—you specifically wrote
 `instance C a b => C (X String a) (X String b)`, which has no functional
 dependency issues.

 > But I still think it shouldn't loop at runtime.

 For the same reasons I explained in comment:4, the code that gets
 generated is `c = coerce c`, where the two occurrences of `c` have the
 same type, i.e., an infinite loop.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16322#comment:7>
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