[GHC] #13791: Document allowed syntax in WARNING and DEPRECATED pragmas

[GHC]

#13791: Document allowed syntax in WARNING and DEPRECATED pragmas
-------------------------------------+-------------------------------------
           Reporter:  phischu        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  lowest         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Documentation
  Unknown/Multiple                   |  bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 GHC accepts the following deprecation pragma:

 {{{#!hs
 {-# DEPRECATED f ["f is deprecated","use g instead"] #-}
 f :: a -> a
 f x = x
 }}}

 The deprecation message here is a list of strings. I didn't expect this to
 be allowed because
 [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html
 #warning-and-deprecated-pragmas] only shows examples where the deprecation
 message is a single string.

 Are there other forms of deprecation message allowed by GHC?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13791>
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