[GHC] #13272: DeriveAnyClass regression involving a rigid type variable

[GHC]

#13272: DeriveAnyClass regression involving a rigid type variable
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:  Generics
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * cc: simonpj (added)


Comment:

 FWIW, in the example in comment:1, the loop happens upon the first call to
 `solveWantedsAndDrop`
 [http://git.haskell.org/ghc.git/blob/0e7601749d53d59df528ede996d8b54352051498:/compiler/typecheck/TcDerivInfer.hs#l645
 here].

 I'm a bit clueless as to where to begin searching for a place to debug
 this. Simon, do you have any ideas?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13272#comment:3>
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