Don't use unsafe coerce when desugaring lazy pattersn

Christiaan Baaij asks (on ghc-devs): when I ask for the desugaring of:

module PatError where

paterror :: Maybe Int -> Int
paterror (Just i) = i

I get the following:

PatError.paterror =
  \ (ds_dIS :: Data.Maybe.Maybe GHC.Types.Int) ->
    break<1>()
    case ds_dIS of _ [Occ=Dead] {
      __DEFAULT ->
        (\ _ [Occ=Dead, OS=OneShot] ->
           Control.Exception.Base.patError
             @ GHC.Types.Int "PatError.hs:4:1-21|function paterror"#)
          GHC.Prim.void#;
      Data.Maybe.Just i_aqG -> break<0>(i_aqG) i_aqG
    }

Where Control.Exception.Base.patError is applied to Int.

However, when I compile the almost identical code with an irrefutable pattern:

module PatError2 where

patError :: Maybe Int -> Int
patError ~(Just i) = i

I get the following core:

PatError2.paterror =
  \ (ds_dIT :: Data.Maybe.Maybe GHC.Types.Int) ->
    break<1>()
    let {
      i_aqG :: GHC.Types.Int
      [LclId, Str=DmdType]
      i_aqG =
        case ds_dIT of _ [Occ=Dead] {
          __DEFAULT ->
            (\ _ [Occ=Dead, OS=OneShot] ->
               (Control.Exception.Base.irrefutPatError
                  @ () "PatError2.hs:4:1-22|(Data.Maybe.Just i)"#)
               `cast` (UnivCo representational () GHC.Types.Int
                       :: () ~# GHC.Types.Int))
              GHC.Prim.void#;
          Data.Maybe.Just i_aqG -> i_aqG
        } } in
    break<0>(i_aqG) i_aqG

Where Control.Exception.Base.patError is first applied to () after which it is casted to GHC.Types.Int. Out of the [Note] for the UnivCo coercion:

The UnivCo ("universal coercion") serves two rather separate functions:
 - the implementation for unsafeCoerce#
 - placeholder for phantom parameters in a TyConAppCo

It seems to be used in the unsafeCoerce# role here. My question is: why can't the irrefutable pattern be translated to `Control.Exception.Base.irrefutPatError' applied to 'Int'? Instead of using this 'cast'? I find the cast-less version much more readable.

Greetings,

Christiaan

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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