[GHC] #14149: Tyepchecker generates top-level unboxed coercion

[GHC]

#14149: Tyepchecker generates top-level unboxed coercion
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider this code
 {{{
 {-# OPTIONS_GHC -fdefer-out-of-scope-variables #-}

 module Foo where

 import Data.Coerce

 f :: Bool
 f = coerce (k :: Int)
 }}}
 It generates a lint error:
 {{{
 *** Core Lint errors : in result of Desugar (after optimization) ***
 <no location info>: warning:
     [RHS of cobox_a11N :: (Int :: *) ~R# (Bool :: *)]
     The type of this binder is unlifted: cobox_a11N
     Binder's type: (Int :: *) ~R# (Bool :: *)
 *** Offending Program ***
 $trModule :: Module
 [LclIdX]
 $trModule = Module (TrNameS "main"#) (TrNameS "Foo"#)

 cobox_a11N :: (Int :: *) ~R# (Bool :: *)
 [LclId[CoVarId]]
 cobox_a11N
   = typeError
       @ ('TupleRep '[])
       @ ((Int :: *) ~R# (Bool :: *))
       "Foo.hs:8:5: error:\n\
       \    \\226\\128\\162 Couldn't match representation of type
 \\226\\128\\152Int\\226\\128\\153 with that of
 \\226\\128\\152Bool\\226\\128\\153\n\
       \        arising from a use of
 \\226\\128\\152coerce\\226\\128\\153\n\
       \    \\226\\128\\162 In the expression: coerce (k :: Int)\n\
       \      In an equation for \\226\\128\\152f\\226\\128\\153: f =
 coerce (k :: Int)\n\
       \(deferred type error)"#

 f :: Bool
 [LclIdX]
 f = (typeError
        @ 'LiftedRep
        @ Int
        "Foo.hs:8:13: error: Variable not in scope: k :: Int\n\
        \(deferred type error)"#)
     `cast` (cobox_a11N :: (Int :: *) ~R# (Bool :: *))

 *** End of Offense ***
 }}}
 Reason: this rather hacky test in `TcUnify.buildImplication`
 {{{
        ; if null skol_tvs && null given && (not deferred_type_errors ||
                                             not (isTopTcLevel tc_lvl))
 }}}
 did take account of `Opt_DeferOutOfScopeVariables`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14149>
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