[GHC] #10384: "Can't splice the polymorphic local variable" check looks dead

[GHC]

#10384: "Can't splice the polymorphic local variable" check looks dead
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        Reporter:  ezyang            |                   Owner:  ezyang
            Type:  task              |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Template Haskell  |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Changes (by ezyang):

 * owner:   => ezyang


Comment:

 Looking at the Core of:

 {{{
 {-# LANGUAGE TemplateHaskell, FlexibleInstances, IncoherentInstances #-}
 module A where
 import Language.Haskell.TH.Syntax
 instance Lift a where
     lift = undefined
 x = \y -> [|| y ||]
 }}}

 we can see that it goes through `Lift`, and then an `unsafeTExpCoerce`, so
 there's no need for a separate `tlift` typeclass. (`tcTypedBracket` is
 responsible for typechecking the expression and then coercing it.)

 But that was enough of a hint to let me figure out how to tickle this
 case.

 {{{
 {-# LANGUAGE TemplateHaskell, RankNTypes, ScopedTypeVariables #-}
 module A where
 x = \(y :: forall a. a -> a) -> [|| y ||]
 }}}

 {{{
 A.hs:3:37:
     Can't splice the polymorphic local variable ‘y’
     In the Template Haskell quotation [|| y ||]
     In the expression: [|| y ||]
     In the expression: \ (y :: forall a. a -> a) -> [|| y ||]
 }}}

 I'll add a test-case for it.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10384#comment:2>
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