[GHC] #16211: Recompiling results in a missing instance

[GHC]

#16211: Recompiling results in a missing instance
-------------------------------------+-------------------------------------
           Reporter:  crockeea       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The attached program compiles without error using cabal.

 {{{
 import Bar

 main :: IO ()
 main = do
   let x = True   -- line 5
   --x <- True    -- line 6
   foo x bar

 foo :: Bool -> IO () -> IO ()
 foo _ _ = undefined
 }}}

 However:
  1. `cabal build`. This succeeds. [This step isn't required to reproduce
 the bug, but it shows that the program should compile.]
  2. Replace line 5 with line 6 in Main.hs
  3. `cabal build`. GHC correctly reports a compile error (`Couldn't match
 expected type 'IO Bool' with actual type 'Bool'`)
  4. Fix the problem by reinstating line 5 and removing line 6.
  5. `cabal build`. This step *should* compile without errors, but instead
 I get a GHC error about a missing instance related to the `monad-classes`
 library.
  6. `cabal build` all day. The `missing instance` error still occurs until
 you clean temporary files and start from scratch.

 I suspect the error in step 5 is itself a (different?) GHC bug, but for
 now it is reported [https://github.com/strake/monad-classes.hs/issues/4
 here]. This GHC bug report is specifically about compiling with identical
 source code (steps 1 and 5) and getting different outcomes (step 1
 compiles, step 5 has an error), which shouldn't happen regardless of other
 bugs.

 [Note I can produce the `missing instance` issue without having to
 recompile GHC, which is why I think it might be independent of this bug.]

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16211>
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