[GHC] #11577: GHCi accepts invalid programs when recompiling

[GHC]

#11577: GHCi accepts invalid programs when recompiling
-------------------------------------+-------------------------------------
           Reporter:  crockeea       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  GHCi           |           Version:  7.10.2-rc2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC accepts
  Unknown/Multiple                   |  invalid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This ticket may or may not be a duplicate of #9729, but I ran into it
 again in a different context, so I'm reporting it. My GHC version is
 7.10.2.20151030. The example is as minimal as I could make it.

 File Bar.hs
 {{{
 {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-}

 module Bar where

 class Bar a b where
   (*^) :: a -> b -> b
 }}}

 File Foo.hs
 {{{
 {-# LANGUAGE MultiParamTypeClasses #-}

 module Foo where

 import Bar
 import GHC.Prim

 newtype Foo fp = Foo [fp]

 instance {-# OVERLAPS #-}
   Bar (Foo fp) [fp]
 }}}

 File Main.hs
 {{{
 {-# LANGUAGE MultiParamTypeClasses #-}

 import Foo
 import Bar

 newtype LW a = LW [a]

 instance Bar (Foo fp) (LW fp) where
   r *^ (LW xs) = LW $ r *^ xs
 }}}

 If I start GHCi with `ghci Main`, everything compiles as it should. Then I
 remove the `{-# OVERLAPS #-}` pragma in Foo.hs, which should break
 Main.hs. But GHCi happily recompiles with `:r`.

 A couple of notes: *with* the pragma, `:i Bar` gives the instances
 {{{
 instance Bar (Foo fp) (LW fp) -- Defined at Main.hs:8:10
 instance Bar a b => Bar a [b] -- Defined at Bar.hs:8:10
 instance [overlap ok] Bar (Foo fp) [fp] -- Defined at Foo.hs:22:3
 }}}

 while after (successfully, but incorrectly) recompiling without the
 pragma, I get the instances
 {{{
 instance Bar (Foo fp) (LW fp) -- Defined at Main.hs:8:10
 instance Bar a b => Bar a [b] -- Defined at Bar.hs:8:10
 instance Bar (Foo fp) [fp] -- Defined at Foo.hs:22:3
 }}}
 (i.e., without the `[overlap ok]`).

 Another strange thing is that the bug is only triggered if I import
 specific (superfluous) modules in Foo.hs. For example, if I replace
 GHC.Prim with Control.Monad or Data.Maybe, the bug is not triggered (i.e.,
 GHCi correctly detects when I remove the pragma).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11577>
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