[GHC] #15149: Identical distinct type family fields miscompiled

14 May 2018

      #15149: Identical distinct type family fields miscompiled
-------------------------------------+-------------------------------------
           Reporter:  NeilMitchell   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Given the code:

 {{{#!hs
 -- An.hs
 {-# LANGUAGE TypeFamilies #-}
 module An where
 data family An c :: *

 -- AnInt.hs
 {-# LANGUAGE TypeFamilies #-}
 module AnInt where
 import An
 data instance An Int = AnInt {an :: Int} deriving Show

 -- AnDouble.hs
 {-# LANGUAGE TypeFamilies #-}
 module AnDouble where
 import An
 data instance An Double = AnDouble {an :: Double} deriving Show

 -- Main.hs
 {-# LANGUAGE DisambiguateRecordFields #-}
 module Main where
 import AnInt
 import AnDouble
 main = print (AnDouble{an=1}, AnInt{an=1})
 }}}

 I would expect this code to work. In reality it fails at runtime with GHC
 8.2.2:

 {{{
 Main.hs:4:15-28: warning: [-Wmissing-fields]
     * Fields of `AnDouble' not initialised: an
     * In the expression: AnDouble {an = 1}
       In the first argument of `print', namely
         `(AnDouble {an = 1}, AnInt {an = 1})'
       In the expression: print (AnDouble {an = 1}, AnInt {an = 1})
   |
 6 | main = print (AnDouble{an=1}, AnInt{an=1})
   |               ^^^^^^^^^^^^^^

 *** Exception: Main.hs:4:15-28: Missing field in record construction an
 }}}

 And fails at compile time in GHC 8.4.2:

 {{{
 Main.hs:4:31-41: error:
     * Constructor `AnInt' does not have field `an'
     * In the expression: AnInt {an = 1}
       In the first argument of `print', namely
         `(AnDouble {an = 1}, AnInt {an = 1})'
       In the expression: print (AnDouble {an = 1}, AnInt {an = 1})
   |
 6 | main = print (AnDouble{an=1}, AnInt{an=1})
   |                               ^^^^^^^^^^^
 }}}

 This code was extracted from a real example, where this bug is pretty
 fatal, as I haven't been able to find any workarounds (without just
 avoiding clashing record fields).

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15149
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