[GHC] #15423: Ungrammatical error message involving a pattern binding

[GHC]

#15423: Ungrammatical error message involving a pattern binding
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           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           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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 Take a look at this error message from GHC's `Typeable1` test case:

 {{{
 Typeable1.hs:22:5: error: [-Winaccessible-code (in -Wdefault), -Werror
 =inaccessible-code]
     • Couldn't match type ‘ComposeK’ with ‘a3 b3’
       Inaccessible code in
         a pattern with pattern synonym:
           App :: forall k2 (t :: k2).
                  () =>
                  forall k1 (a :: k1 -> k2) (b :: k1).
                  (t ~ a b) =>
                  TypeRep a -> TypeRep b -> TypeRep t,
         in a pattern binding in
              'do' block
 }}}

 In particular, note the "in a pattern binding in 'do' block" part. I think
 that should be "in **a** 'do' block".

 I believe fixing this is a simple matter of applying this patch:

 {{{#!diff
 diff --git a/compiler/hsSyn/HsExpr.hs b/compiler/hsSyn/HsExpr.hs
 index 96d86c8..a5c65fb 100644
 --- a/compiler/hsSyn/HsExpr.hs
 +++ b/compiler/hsSyn/HsExpr.hs
 @@ -2804,7 +2804,7 @@ pprMatchContextNoun PatBindGuards   = text "pattern
 binding guards"
  pprMatchContextNoun LambdaExpr      = text "lambda abstraction"
  pprMatchContextNoun ProcExpr        = text "arrow abstraction"
  pprMatchContextNoun (StmtCtxt ctxt) = text "pattern binding in"
 -                                      $$ pprStmtContext ctxt
 +                                      $$ pprAStmtContext ctxt
  pprMatchContextNoun PatSyn          = text "pattern synonym declaration"

  -----------------
 }}}

 Patch incoming.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15423>
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