[commit: ghc] master: Comments only (fe2d807)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/fe2d80768641f66eca8a86c69d3a3bc5f04f88ee/ghc

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

commit fe2d80768641f66eca8a86c69d3a3bc5f04f88ee
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Thu Jul 31 13:49:47 2014 +0100

    Comments only


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

fe2d80768641f66eca8a86c69d3a3bc5f04f88ee
 compiler/typecheck/TcCanonical.lhs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/compiler/typecheck/TcCanonical.lhs b/compiler/typecheck/TcCanonical.lhs
index 43cbb2c..d58d5db 100644
--- a/compiler/typecheck/TcCanonical.lhs
+++ b/compiler/typecheck/TcCanonical.lhs
@@ -1186,6 +1186,9 @@ canEqTyVar2 dflags ev swapped tv1 xi2 co2
        ; case mb of
            Nothing     -> return ()
            Just new_ev -> emitInsoluble (mkNonCanonical new_ev)
+                          -- If we have a ~ [a], it is not canonical, and in particular
+                          -- we don't want to rewrite existing inerts with it, otherwise
+                          -- we'd risk divergence in the constraint solver
        ; return Stop }
   where
     xi1 = mkTyVarTy tv1