Tyepchecker generates top-level unboxed coercion

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.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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