[commit: ghc] master: Improve instanceCantMatch (6e618d7)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/6e618d77d64255c32bef543a3f9635abce24a66d/ghc

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

commit 6e618d77d64255c32bef543a3f9635abce24a66d
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Fri Jul 24 10:42:05 2015 +0100

    Improve instanceCantMatch
    
    When staring at instanceCantMatch I realised that it
    was returning False (safe but inefficient) when it could
    validly return True, on arguments like
       [Nothing,   Just Int]
       [Just Bool, Just Bool]
    
    This patch makes it a bit cleverer.


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

6e618d77d64255c32bef543a3f9635abce24a66d
 compiler/types/InstEnv.hs | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/compiler/types/InstEnv.hs b/compiler/types/InstEnv.hs
index e93d707..b8a3e6a 100644
--- a/compiler/types/InstEnv.hs
+++ b/compiler/types/InstEnv.hs
@@ -267,8 +267,12 @@ instanceCantMatch :: [Maybe Name] -> [Maybe Name] -> Bool
 -- (instanceCantMatch tcs1 tcs2) returns True if tcs1 cannot
 -- possibly be instantiated to actual, nor vice versa;
 -- False is non-committal
-instanceCantMatch (Just t : ts) (Just a : as) = t/=a || instanceCantMatch ts as
-instanceCantMatch _             _             =  False  -- Safe
+instanceCantMatch (mt : ts) (ma : as) = itemCantMatch mt ma || instanceCantMatch ts as
+instanceCantMatch _         _         =  False  -- Safe
+
+itemCantMatch :: Maybe Name -> Maybe Name -> Bool
+itemCantMatch (Just t) (Just a) = t /= a
+itemCantMatch _        _        = False
 
 {-
 Note [When exactly is an instance decl an orphan?]