[GHC] #15648: Core Lint error with source-level unboxed equality

[GHC]

#15648: Core Lint error with source-level unboxed equality
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15209            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Replying to [comment:1 goldfire]:
 > 1. Why does GHC type check `Jank` at type `JankyEquality a b`? There
 must be something in the type checker which treats arguments of type `a ~#
 b` as invisible. But these shouldn't be -- they are not `Constraint`s. To
 fix: find this code and kill it.

 I think there is a simpler explanation for why this happens: when we kind-
 check the type `(a ~# b) -> c`, everything works since `(->)` is levity
 polymorphic and `a ~# b` has kind `TYPE (TupleRep '[])`. This means that
 resulting `Type` is `FunTy (TyConApp (~#) [TyVarTy a, TyVarTy b]) (TyVarTy
 c)`—which GHC believes to be `(a ~# b) => c`—so things like `Jank` are
 seemingly typechecked as if the value of type `a ~# b` were invisible.

 From a certain perspective, this is a natural consequence of making `a ~#
 b` have kind `TYPE (TupleRep '[])`. This suggests that one way to work
 around this issue would be to change its kind.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15648#comment:5>
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