Error thunks not being exposed with "B" strictness

At the moment GHC often floats error "urk" to the top level (which it should), after the strictness analyser. But since it's after strictness analysis, the exported thing doesn't have a strictness signature saying "I am a diverging term", which in turn loses useful optimisations in importing modules.

An example is test T3286. If you compile it with -O --ddump-simpl, you'll see stuff like

     case T3286b.$fFractionalLogFloat3
          `cast` (CoUnsafe T3286b.LogFloat GHC.Prim.Double#
                  :: T3286b.LogFloat ~ GHC.Prim.Double#)
     of ww2_aGk { __DEFAULT ->
     (GHC.Types.D# ww2_aGk)
     `cast` (sym T3286b.NTCo:LogFloat
             :: GHC.Types.Double ~ T3286b.LogFloat)

But if you look at T3286b.$fFractionalLogFloat3, it turns out to be bottom, so the case should be eliminated.

This is a long-standing infelicity; I'm making a ticket so I don't forget it.

Trac metadata

Trac field	Value
Version	6.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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