[GHC] #14923: Recompilation avoidance fails after a LANGUAGE change

[GHC]

#14923: Recompilation avoidance fails after a LANGUAGE change
-------------------------------------+-------------------------------------
           Reporter:  akio           |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.4.1
           Keywords:                 |  Operating System:  Linux
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 It looks like the recompilation checker can be confused when a LANGUAGE
 pragma is added or removed.

 The following shell script demonstrates the issue:

 {{{
 #!/bin/sh
 cat >Foo.hs <<END
 {-# LANGUAGE UndecidableInstances #-}
 module Foo where
 END
 rm -f Foo.hi Foo.o

 ghc -c Foo.hs; ghc --show-iface Foo.hi | grep flag

 sed -i 's/^{-#/--&/' Foo.hs # comment out the pragma

 ghc -c Foo.hs; ghc --show-iface Foo.hi | grep flag
 ghc -c Foo.hs; ghc --show-iface Foo.hi | grep flag
 ghc -c Foo.hs; ghc --show-iface Foo.hi | grep flag
 }}}

 I get this output:

 {{{
 % ./bug.sh
   flag hash: b1ada48735eec2ef16d5070887aa7f10
   flag hash: b1ada48735eec2ef16d5070887aa7f10
   flag hash: e7b294090e04f1a84f0bfc9b8cc49322
 compilation IS NOT required
   flag hash: e7b294090e04f1a84f0bfc9b8cc49322
 }}}

 In the third invocation to GHC, I expect the recompilation checker to
 detect that compilation is not required, because nothing has changed since
 the second invocation. Also it seems suspicious that the flag hash changes
 beteen the second run and the third run.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14923>
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