[GHC] #15166: Bad error message when `!` is used without BangPatterns extension

[GHC]

#15166: Bad error message when `!` is used without BangPatterns extension
-------------------------------------+-------------------------------------
           Reporter:  sighingnow     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 For

 {{{#!hs
 f :: a -> a
 f !x = x
 }}}

 ghc-HEAD gives

 {{{
 P.hs:22:1: error:
     The type signature for ‘f’ lacks an accompanying binding
    |
 22 | f :: a -> a
    | ^
 }}}

 Here ghc should suggest `BangPatterns` extension.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15166>
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