"Can't splice the polymorphic local variable" check looks dead

While I was looking for TH checks which might want to be checked when Quotes are enabled ala #10382 (closed), I found the "Can't splice the polymorphic local variable" check in checkCrossStageLifting in TcExpr. I thought this was a bit odd, because there is another implementation of this very function in RnSplice.

So I went ahead and removed this check from the compiler, and it validated fine.

I then went and tried to see if I could tickle the problem. An old mailing list suggested the following test program to induce the error:

module TH_polymorphic where

import Language.Haskell.TH
import Language.Haskell.TH.Syntax

-- See https://mail.haskell.org/pipermail/template-haskell/2006-April/000552.html

test2 () = runQ [| foldr f z xs |]  
    where (f,z,xs) = undefined

but I get this error:

TH_polymorphic.hs:8:17: error:
    Could not deduce (Lift t0) arising from a use of ‘lift’
    from the context: Quasi m
      bound by the inferred type of test2 :: Quasi m => () -> m Exp
      at TH_polymorphic.hs:(8,1)-(9,30)  
    The type variable ‘t0’ is ambiguous
    Note: there are several potential instances:
      instance (Lift a, Lift b) => Lift (Either a b)
        -- Defined in ‘Language.Haskell.TH.Syntax’
      instance Lift a => Lift (Maybe a)
        -- Defined in ‘Language.Haskell.TH.Syntax’
      instance Lift Int16 -- Defined in ‘Language.Haskell.TH.Syntax’
      ...plus 24 others
    In the expression: lift xs
    In the first argument of ‘runQ’, namely
      ‘[| foldr f z xs |]
       pending(rn) [<xs, lift xs>, <z, lift z>, <f, lift f>]’
    In the expression:
      runQ
        [| foldr f z xs |]  
        pending(rn) [<xs, lift xs>, <z, lift z>, <f, lift f>]

which seems to be induced the check in RnSplice.

So is it dead? If so, let's remove it!

Trac metadata

Trac field	Value
Version	7.11
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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