[GHC] #14719: Caret diagnostics for GADT constructor error message don't span the whole constructor

25 Jan 2018

      #14719: Caret diagnostics for GADT constructor error message don't span the whole
constructor
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           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:
-------------------------------------+-------------------------------------
 Inspecting the caret diagnostics in the error message for this program:

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

 data Foo1 where
   MkFoo1 :: Bool

 newtype Foo2 where
   MkFoo2 :: Foo2
 }}}
 {{{
 $ /opt/ghc/8.2.2/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:5:3: error:
     • Data constructor ‘MkFoo1’ returns type ‘Bool’
         instead of an instance of its parent type ‘Foo1’
     • In the definition of data constructor ‘MkFoo1’
       In the data type declaration for ‘Foo1’
   |
 5 |   MkFoo1 :: Bool
   |   ^

 Bug.hs:8:3: error:
     • The constructor of a newtype must have exactly one field
         but ‘MkFoo2’ has none
     • In the definition of data constructor ‘MkFoo2’
       In the newtype declaration for ‘Foo2’
   |
 8 |   MkFoo2 :: Foo2
   |   ^
 }}}

 We notice something fishy: the carets don't span the entirey of the
 constructor! Contrast this with the carets for the error message in this
 program:

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

 data Foo where
   MkFoo, MkFoo :: Foo
 }}}
 {{{
 $ /opt/ghc/8.2.2/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:5:3: error:
     Multiple declarations of ‘MkFoo’
     Declared at: Bug.hs:5:3
                  Bug.hs:5:3
   |
 5 |   MkFoo, MkFoo :: Foo
   |   ^^^^^^^^^^^^^^^^^^^
 }}}

 Where the carets span everything that is relevant.

 I know what is causing this: patch incoming.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14719
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