[GHC] #15621: Error message involving type families points to wrong location

[GHC]

#15621: Error message involving type families points to wrong location
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.10.1
       Component:  Compiler (Type    |              Version:  8.4.3
  checker)                           |
      Resolution:                    |             Keywords:  TypeFamilies
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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Comment (by pacak):

 One more example, figuring out why ghc complains on an innocent looking
 line took a bit of time.

 {{{
 % cat foo.hs
 }}}

 {{{#!haskell
 module P (o_O) where

 import GHC.Types

 o_O :: Maybe Any
 o_O = do
     present <- pure True
     if present
         then undefined :: Maybe Int
         else undefined :: Maybe Int
 }}}

 {{{
 % ghc -O foo.hs
 [1 of 1] Compiling P                ( foo.hs, foo.o )

 foo.hs:7:5: error:
     • Couldn't match type ‘Any’ with ‘Int’
       Expected type: Maybe Any
         Actual type: Maybe Int
     • In a stmt of a 'do' block: present <- pure True
       In the expression:
         do present <- pure True
            if present then undefined :: Maybe Int else undefined :: Maybe
 Int
       In an equation for ‘o_O’:
           o_O
             = do present <- pure True
                  if present then undefined :: Maybe Int else undefined ::
 Maybe Int
   |
 7 |     present <- pure True
   |     ^^^^^^^^^^^^^^^^^^^^
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15621#comment:12>
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