[commit: ghc] ghc-7.8: Fix desguaring of bang patterns (Trac #8952) (69fa6a6)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : ghc-7.8
Link       : http://ghc.haskell.org/trac/ghc/changeset/69fa6a67ad99d425d184262ed726bf14c330a6b1/ghc

>---------------------------------------------------------------

commit 69fa6a67ad99d425d184262ed726bf14c330a6b1
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Thu Apr 3 13:30:59 2014 +0100

    Fix desguaring of bang patterns (Trac #8952)
    
    A palpable bug, although one that will rarely bite
    
    (cherry picked from commit 3671d0027329804a31a628a5bee355e0640a2045)


>---------------------------------------------------------------

69fa6a67ad99d425d184262ed726bf14c330a6b1
 compiler/deSugar/Match.lhs                                   |   10 +++++-----
 testsuite/tests/deSugar/should_run/T8952.hs                  |    8 ++++++++
 .../{rts/T7037.stdout => deSugar/should_run/T8952.stdout}    |    0
 testsuite/tests/deSugar/should_run/all.T                     |    1 +
 4 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/compiler/deSugar/Match.lhs b/compiler/deSugar/Match.lhs
index 0433d87..e0a5d4a 100644
--- a/compiler/deSugar/Match.lhs
+++ b/compiler/deSugar/Match.lhs
@@ -586,8 +586,6 @@ tidy1 _ non_interesting_pat
 
 --------------------
 tidy_bang_pat :: Id -> SrcSpan -> Pat Id -> DsM (DsWrapper, Pat Id)
--- BangPatterns: Pattern matching is already strict in constructors,
--- tuples etc, so the last case strips off the bang for those patterns.
 
 -- Discard bang around strict pattern
 tidy_bang_pat v _ p@(ListPat {})   = tidy1 v p
@@ -596,8 +594,7 @@ tidy_bang_pat v _ p@(PArrPat {})   = tidy1 v p
 tidy_bang_pat v _ p@(ConPatOut {}) = tidy1 v p
 tidy_bang_pat v _ p@(LitPat {})    = tidy1 v p
 
--- Discard lazy/par/sig under a bang
-tidy_bang_pat v _ (LazyPat (L l p))     = tidy_bang_pat v l p
+-- Discard par/sig under a bang
 tidy_bang_pat v _ (ParPat (L l p))      = tidy_bang_pat v l p
 tidy_bang_pat v _ (SigPatOut (L l p) _) = tidy_bang_pat v l p
 
@@ -607,7 +604,10 @@ tidy_bang_pat v l (AsPat v' p)  = tidy1 v (AsPat v' (L l (BangPat p)))
 tidy_bang_pat v l (CoPat w p t) = tidy1 v (CoPat w (BangPat (L l p)) t)
 
 -- Default case, leave the bang there:
--- VarPat, WildPat, ViewPat, NPat, NPlusKPat
+-- VarPat, LazyPat, WildPat, ViewPat, NPat, NPlusKPat
+-- For LazyPat, remember that it's semantically like a VarPat
+--  i.e.  !(~p) is not like ~p, or p!  (Trac #8952)
+
 tidy_bang_pat _ l p = return (idDsWrapper, BangPat (L l p))
   -- NB: SigPatIn, ConPatIn should not happen
 \end{code}
diff --git a/testsuite/tests/deSugar/should_run/T8952.hs b/testsuite/tests/deSugar/should_run/T8952.hs
new file mode 100644
index 0000000..42eeb25
--- /dev/null
+++ b/testsuite/tests/deSugar/should_run/T8952.hs
@@ -0,0 +1,8 @@
+{-# LANGUAGE BangPatterns #-}
+
+module Main where
+
+main = print (case Nothing of
+                  !(~(Just x)) -> "ok"
+                  Nothing   -> "bad")
+
diff --git a/testsuite/tests/rts/T7037.stdout b/testsuite/tests/deSugar/should_run/T8952.stdout
similarity index 100%
copy from testsuite/tests/rts/T7037.stdout
copy to testsuite/tests/deSugar/should_run/T8952.stdout
diff --git a/testsuite/tests/deSugar/should_run/all.T b/testsuite/tests/deSugar/should_run/all.T
index 352a652..233f648 100644
--- a/testsuite/tests/deSugar/should_run/all.T
+++ b/testsuite/tests/deSugar/should_run/all.T
@@ -40,3 +40,4 @@ test('mc08', normal, compile_and_run, [''])
 test('T5742', normal, compile_and_run, [''])
 test('DsLambdaCase', when(compiler_lt('ghc', '7.5'), skip), compile_and_run, [''])
 test('DsMultiWayIf', when(compiler_lt('ghc', '7.5'), skip), compile_and_run, [''])
+test('T8952', normal, compile_and_run, [''])