[GHC] #13947: GHC 8.2 gives misleading error message for out-of-scope infix type constructor

[GHC]

#13947: GHC 8.2 gives misleading error message for out-of-scope infix type
constructor
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc2
           Keywords:                 |  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:
-------------------------------------+-------------------------------------
 {{{#!hs
 {-# LANGUAGE TypeOperators #-}
 module Bug where

 f :: () -> Int :~: Int
 f = undefined
 }}}

 GHC 8.0.2 gives a reasonable error message:

 {{{
 $ /opt/ghc/8.0.2/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:4:12: error: Not in scope: type constructor or class ‘:~:’
 }}}

 But GHC 8.2.1's error message is much more confusing:

 {{{
 $ /opt/ghc/8.2.1/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:4:6: error:
     Precedence parsing error
         cannot mix ‘(->)’ [infixr 0] and ‘:~:’ [infixl 0] in the same
 infix expression
   |
 4 | f :: () -> Int :~: Int
   |      ^^^^^^^^^^^^^^^^^

 Bug.hs:4:12: error:
     Not in scope: type constructor or class ‘:~:’
     A data constructor of that name is in scope; did you mean DataKinds?
   |
 4 | f :: () -> Int :~: Int
   |            ^^^^^^^^^^^
 }}}

 I was thrown akilter by the claim that `:~:` was `infixl 0`, since it was
 displayed before the real cause of the error (that it's out-of-scope).
 Adding `import Data.Type.Equality` fixes the error, but GHC really
 shouldn't be making claims about the fixities of out-of-scope type
 constructors in the first place.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13947>
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