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

[GHC]

#15648: Core Lint error with source-level unboxed equality
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Good news—the original program is now rejected:

 {{{
 $ ~/Software/ghc/inplace/bin/ghc-stage2 Bug.hs[1 of 2] Compiling Foo
 ( Foo.hs, Foo.o )
 [2 of 2] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:23:21: error:
     • Couldn't match type ‘(a0 ~ b0) -> JankyEquality a0 b0’
                      with ‘JankyEquality a a’
       Expected type: JankyEquality a b
         Actual type: (a0 ~ b0) -> JankyEquality a0 b0
     • Probable cause: ‘Jank’ is applied to too few arguments
       In the expression: Jank
       In an equation for ‘legitToJank’: legitToJank Legit = Jank
     • Relevant bindings include
         legitToJank :: LegitEquality a b -> JankyEquality a b
           (bound at Bug.hs:23:1)
    |
 23 | legitToJank Legit = Jank
    |                     ^^^^

 Bug.hs:30:10: error:
     • Couldn't match expected type ‘(a ~ b) -> b ~ a’
                   with actual type ‘b ~ a’
     • In the expression: unJank $ legitToJank $ mkLegit @b @a
       In an equation for ‘ueqSym’:
           ueqSym = unJank $ legitToJank $ mkLegit @b @a
     • Relevant bindings include
         ueqSym :: (a ~ b) -> b ~ a (bound at Bug.hs:30:1)
    |
 30 | ueqSym = unJank $ legitToJank $ mkLegit @b @a
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 You can still use `$ueqT a b` as a visible argument to a function, but it
 appears to be functionally inert now, since you can't bring equalities
 into scope with it anymore (at least, not as far as I can tell).

 Should we claim victory on this ticket?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15648#comment:14>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler