[GHC] #11098: TH mishandles type variables that begin with an underscore

[GHC]

#11098: TH mishandles type variables that begin with an underscore
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        Reporter:  goldfire          |                Owner:  jstolarek
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Template Haskell  |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by darchon):

 Indeed, I was just about to comment that it is not just TH. When I compile

 {{{
 {-# LANGUAGE PartialTypeSignatures, NamedWildCards, ExplicitForAll #-}
 module Bar where

 foo :: _a -> _a
 foo x = x

 bar :: forall _a . _a -> _a
 bar x = x
 }}}

 I get:

 {{{
 $ ghc -fforce-recomp Bar.hs
 [1 of 1] Compiling Bar              ( Bar.hs, Bar.o )

 Bar.hs:4:8: warning:
     Found type wildcard '_a' standing for 't'
     Where: 't' is a rigid type variable bound by
                the inferred type of foo :: t -> t at Bar.hs:5:1
     In the type signature for:
       foo :: _a -> _a

 Bar.hs:7:20: warning:
     Found type wildcard '_a' standing for 't0'
     Where: 't0' is an ambiguous type variable
     In the type signature for:
       bar :: forall _a. _a -> _a
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11098#comment:4>
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