[commit: ghc] master: Be careful when combining two CFunEqCans, in the (82879d9)

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

On branch  : master

http://hackage.haskell.org/trac/ghc/changeset/82879d95c8e941191bfa329604716f78ec4faa83

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

commit 82879d95c8e941191bfa329604716f78ec4faa83
Author: Simon Peyton Jones <[email protected]>
Date:   Mon Oct 22 09:09:21 2012 +0100

    Be careful when combining two CFunEqCans, in the
    case where one has a unification variable on the right.
    
    See Note [Carefully solve the right CFunEqCan].
    This makes the error message in SimplFail16 come out
    the right way round.

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

 compiler/typecheck/TcInteract.lhs |   31 +++++++++++++++++++++++++++++--
 1 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/compiler/typecheck/TcInteract.lhs b/compiler/typecheck/TcInteract.lhs
index ac0439d..d6044d0 100644
--- a/compiler/typecheck/TcInteract.lhs
+++ b/compiler/typecheck/TcInteract.lhs
@@ -708,7 +708,8 @@ doInteractWithInert ii@(CFunEqCan { cc_ev = ev1, cc_fun = tc1
                                   , cc_tyargs = args1, cc_rhs = xi1, cc_loc = d1 }) 
                     wi@(CFunEqCan { cc_ev = ev2, cc_fun = tc2
                                   , cc_tyargs = args2, cc_rhs = xi2, cc_loc = d2 })
-  | fl1 `canSolve` fl2 
+  | i_solves_w && (not (w_solves_i && isMetaTyVarTy xi1))
+               -- See Note [Carefully solve the right CFunEqCan]
   = ASSERT( lhss_match )   -- extractRelevantInerts ensures this
     do { traceTcS "interact with inerts: FunEq/FunEq" $ 
          vcat [ text "workItem =" <+> ppr wi
@@ -753,10 +754,36 @@ doInteractWithInert ii@(CFunEqCan { cc_ev = ev1, cc_fun = tc1
     fl1 = ctEvFlavour ev1
     fl2 = ctEvFlavour ev2
     
-doInteractWithInert _ _ = return (IRKeepGoing "NOP")
+    i_solves_w = fl1 `canSolve` fl2 
+    w_solves_i = fl2 `canSolve` fl1 
+    
 
+doInteractWithInert _ _ = return (IRKeepGoing "NOP")
 \end{code}
 
+Note [Carefully solve the right CFunEqCan]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider the constraints
+  c1 :: F Int ~ a      -- Arising from an application line 5
+  c2 :: F Int ~ Bool   -- Arising from an application line 10
+Suppose that 'a' is a unification variable, arising only from 
+flattening.  So there is no error on line 5; it's just a flattening
+variable.  But there is (or might be) an error on line 10.
+
+Two ways to combine them, leaving either (Plan A)
+  c1 :: F Int ~ a      -- Arising from an application line 5
+  c3 :: a ~ Bool       -- Arising from an application line 10
+or (Plan B)
+  c2 :: F Int ~ Bool   -- Arising from an application line 10
+  c4 :: a ~ Bool       -- Arising from an application line 5
+
+Plan A will unify c3, leaving c1 :: F Int ~ Bool as an error
+on the *totally innocent* line 5.  An example is test SimpleFail16
+where the expected/actual message comes out backwards if we use
+the wrong plan.
+
+The second is the right thing to do.  Hence the isMetaTyVarTy
+test when solving pairwise CFunEqCan.
 
 Note [Shadowing of Implicit Parameters]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~