[commit: ghc] master: Do not do type-class defaulting if there are insoluble constraints (b9fccbc)

Repository : ssh://darcs.haskell.org//srv/darcs/ghc

On branch  : master

http://hackage.haskell.org/trac/ghc/changeset/b9fccbc8bc377cde8e75bce8c78470e3c4fa4018

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

commit b9fccbc8bc377cde8e75bce8c78470e3c4fa4018
Author: Simon Peyton Jones <[email protected]>
Date:   Thu Oct 4 17:55:38 2012 +0100

    Do not do type-class defaulting if there are insoluble constraints
    
    Type-class defaulting won't help with those insolubles,
    and it does make arbitrary choices that obfuscate the
    error messages from the insolubles.

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

 compiler/typecheck/TcSimplify.lhs |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index c0ff59d..09a5b11 100644
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -79,7 +79,9 @@ simplifyTop wanteds
            ; simpl_top_loop wc_first_go }
     
     simpl_top_loop wc
-      | isEmptyWC wc 
+      | isEmptyWC wc || insolubleWC wc
+             -- Don't do type-class defaulting if there are insolubles
+             -- Doing so is not going to solve the insolubles
       = return wc
       | otherwise
       = do { wc_residual <- nestTcS (solve_wanteds_and_drop wc)