[GHC] #15986: Poor error message source location reporting with unsaturated type family

[GHC]

#15986: Poor error message source location reporting with unsaturated type family
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Compiler       |           Version:  8.6.2
           Keywords:  TypeFamilies,  |  Operating System:  Unknown/Multiple
  TypeErrors                         |
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 If you compile this program:

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 module Bug where

 newtype WrapChar f = MkWrapChar (f Char)
 type family F a
 type family T a
 type instance T Int = WrapChar F
 }}}

 Then you'll get this error message:

 {{{
 $ /opt/ghc/8.6.2/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:7:15: error:
     • The type family ‘F’ should have 1 argument, but has been given none
     • In the type instance declaration for ‘T’
   |
 7 | type instance T Int = WrapChar F
   |               ^
 }}}

 The thing is, the location of that caret is rather unfocused. It's
 pointing at `T`, but the real culprit is `F`!

 I know that GHC can do better here because other error messages actually
 get this right. For instance, if you change the `T Int` instance to this:

 {{{#!hs
 type instance T Int = F
 }}}

 Then GHC will actually point to `F` in the resulting error message:

 {{{
 $ /opt/ghc/8.6.2/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:7:23: error:
     • Expecting one more argument to ‘F’
       Expected a type, but ‘F’ has kind ‘* -> *’
     • In the type ‘F’
       In the type instance declaration for ‘T’
   |
 7 | type instance T Int = F
   |                       ^
 }}}

 If GHC can point to the right place in the source code in this situation,
 then it ought to get the earlier situation right as well.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15986>
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