[GHC] #13264: GHC panic with (->) generalization branch while compiling lens

[GHC]

#13264: GHC panic with (->) generalization branch while compiling lens
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      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 goldfire):

 I see what's going on here.

 `tcRepSplitTyConApp_maybe` rightly needs to know the `RuntimeRep` of the
 argument type `([] |> <*>_N ->_N Sym {alzj}) a_alzd`. This `RuntimeRep` is
 buried in the from-type of the coercion hole `{alzj}`. We can see in the
 trace that `{alzj}` has from-type `k_agxC`. Except this has already been
 filled in with `*`. So the problem is that we haven't zonked yet -- and
 that's it.

 `can_eq_nc'` works over non-flat (that is, unzonked) types and also over
 flat (zonked) types. See `Note [Canonicalising equalities]` immediately
 above. Your decomposition will work on the zonked types. So returning
 `Nothing`, as you propose, works, as it forces `can_eq_nc'` to zonk and
 then try again, succeeding. Is `tcRepSplitTyConApp_maybe` used anywhere
 else? If so, returning `Nothing` in this case may be wrong.... except that
 returning `Nothing` in this case makes `tcRepSplitTyConApp_maybe` strictly
 more well-defined, so I suppose nothing can actually go wrong with this
 change.

 A detailed `Note` about the choice for `tcRepSplitTyConApp_maybe` not to
 panic is called for.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13264#comment:5>
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