[GHC] #12597: -Wmissing-signatures uses forall even when invalid in source

[GHC]

#12597: -Wmissing-signatures uses forall even when invalid in source
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           Reporter:  nomeata        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The `-Wmissing-signatures` warning is great to learn about the signatures
 that things should have, and is especially useful when teaching Haskell to
 beginners.

 Unfortunately, it uses explicit quantification (`forall`) even when in the
 compiled module, this is not valid:

 {{{
 $ echo 'foo x = x' > Foo.hs
 $ ghci -Wall Foo.hs
 GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( Foo.hs, interpreted )

 Foo.hs:1:1: warning: [-Wmissing-signatures]
     Top-level binding with no type signature: foo :: forall t. t -> t
 Ok, modules loaded: Main.
 *Main>
 $ echo -e 'foo :: forall t. t -> t\nfoo x = x' > Foo.hs
 $ ghci -Wall Foo.hs
 GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( Foo.hs, interpreted )

 Foo.hs:1:16: error:
     Illegal symbol '.' in type
     Perhaps you intended to use RankNTypes or a similar language
     extension to enable explicit-forall syntax: forall <tvs>. <type>
 Failed, modules loaded: none.
 }}}

 The type signature provided by `-Wmissing-signatures` should be in a form
 that is valid in the context of the function that is missing the
 signature.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12597>
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