[GHC] #12660: singletons doesn't compile with GHC 8.0.2 snapshot

[GHC]

#12660: singletons doesn't compile with GHC 8.0.2 snapshot
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Compiler          |              Version:
      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 bgamari):

 `TcCanonical.canEvVar` calls `TcCanonical.canEvNC` which has no cost
 center but does have a `traceTcS`. Compiling `singletons` with `-ddump-tc-
 trace` shows that the compiler indeed seems to be looping in the
 typechecker,

 {{{
 ...
 Following filled tyvar
   s_aov1[fuv:20] = n1_alUY[ssk] GHC.TypeLits.- n0_alUX[ssk]
 flattenTyVarFinal
   x2_alWb[sk] :: Nat
   Nat
   <Nat>_N :: Nat ~ Nat
 flattenTyVar1
   (x2_alWb[sk] :: Nat)
   <Nat>_N :: Nat ~ Nat
 flattenTyVar2
   n1_alUY[ssk]
   x2_alWb[sk]
 flattenTyVarFinal
   x1_alWa[sk] :: Nat
   Nat
   <Nat>_N :: Nat ~ Nat
 flattenTyVar1
   (x1_alWa[sk] :: Nat)
   <Nat>_N :: Nat ~ Nat
 flattenTyVar2
   n0_alUX[ssk]
   x1_alWa[sk]
 flatten/flat-cache hit
   GHC.TypeLits.- [x2_alWb[sk], x1_alWa[sk]]
   s_aov1[fuv:20]
 Following filled tyvar
   s_aov1[fuv:20] = n1_alUY[ssk] GHC.TypeLits.- n0_alUX[ssk]
 ...
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12660#comment:2>
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