[GHC] #16246: GHC HEAD-only Core Lint error with unboxed equality (Non-CoVar has coercion type)

[GHC]

#16246: GHC HEAD-only Core Lint error with unboxed equality (Non-CoVar has coercion
type)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.10.1
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15648            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Simpler example:

 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}
 module Bug where

 import Foo (ueqT)

 f :: $ueqT a b -> $ueqT a b
 f x = x
 }}}
 {{{
 $ ~/Software/ghc5/inplace/bin/ghc-stage2 --interactive Bug.hs -dcore-lint
 GHCi, version 8.7.20190115: https://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 2] Compiling Foo              ( Foo.hs, interpreted )
 [2 of 2] Compiling Bug              ( Bug.hs, interpreted )
 *** Core Lint errors : in result of Desugar (before optimization) ***
 Bug.hs:7:3: warning:
     [in body of lambda with binder x_a4vD :: a_a4vV ~# b_a4vW]
     Non-CoVar has coercion type x_a4vD :: a_a4vV ~# b_a4vW
 *** Offending Program ***
 Rec {
 $trModule :: Module
 [LclIdX]
 $trModule = Module (TrNameS "main"#) (TrNameS "Bug"#)

 f :: forall a b. (a ~# b) -> a ~# b
 [LclIdX]
 f = \ (@ a_a4vV) (@ b_a4vW) (x_a4vD :: a_a4vV ~# b_a4vW) ->
       break<0>() x_a4vD
 end Rec }

 *** End of Offense ***
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16246#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler