[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:  closed
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:  fixed             |             Keywords:  Generics
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  deriving/should_compile/T13272,
                                     |  T13272a
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * milestone:  8.4.1 => 8.2.1


Comment:

 Many thanks Simon, you're a lifesaver.

 For my edification, which part in particular was causing the reported
 error? Was it the fact that we were creating new variables in
 `inferConstraintsDAC` without using `pushTcLevelM`? Or was it due to not
 using explicit unification variables (via `newMetaTyVarsX`)?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13272#comment:11>
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