[GHC] #15659: Wacky error message when RULE mentions out-of-scope variable

[GHC]

#15659: Wacky error message when RULE mentions out-of-scope variable
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:  newcomer       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 If I compile the following program with GHC 8.4 or later:

 {{{#!hs
 module Foo where

 {-# RULES "test" forall x. f x = x #-}
 }}}

 The error message I get is somewhat eyebrow-raising:

 {{{
 $ /opt/ghc/8.4.3/bin/ghc Foo.hs
 [1 of 1] Compiling Foo              ( Foo.hs, Foo.o )

 Foo.hs:3:11: error:
     Rule "test":
         Not in scope: OutOfScope(f)
         in left-hand side: f x
     LHS must be of form (f e1 .. en) where f is not forall'd
   |
 3 | {-# RULES "test" forall x. f x = x #-}
   |           ^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 I'm not sure what this `OutOfScope` business is, but I doubt we want to be
 referring to it in error messages. The error in GHC 8.2 and earlier was
 much less confusing:

 {{{
 $ /opt/ghc/8.2.2/bin/ghc Foo.hs
 [1 of 1] Compiling Foo              ( Foo.hs, Foo.o )

 Foo.hs:3:11: error:
     Rule "test":
         Not in scope: f
         in left-hand side: f x
     LHS must be of form (f e1 .. en) where f is not forall'd
   |
 3 | {-# RULES "test" forall x. f x = x #-}
   |           ^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15659>
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