From: simonpj@microsoft.com <unknown>
Date: Tue, 9 Jan 2007 13:47:19 +0000 (+0000)
Subject: Fix egregious bug in implication constraints
X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=commitdiff_plain;h=d90023f2e54e2c0b5aad169d0f55748a152fecce

Fix egregious bug in implication constraints

Build an implication constraint if there is a non-trivial
refinement, even if there are no other 'given' constraints.

Test = gadt/set.hs
---

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 708b8e4..7b9f1f1 100644
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -849,10 +849,11 @@ bindIrredsR loc qtvs co_vars reft givens irreds
 		-- The givens can include methods
 		-- See Note [Pruning the givens in an implication constraint]
 
-	   -- If there are no 'givens', then it's safe to 
+	   -- If there are no 'givens' *and* the refinement is empty
+	   -- (the refinement is like more givens), then it's safe to 
 	   -- partition the 'wanteds' by their qtvs, thereby trimming irreds
 	   -- See Note [Freeness and implications]
-	; irreds' <- if null givens'
+	; irreds' <- if null givens' && isEmptyRefinement reft
 	     	     then do
 	     	    	{ let qtv_set = mkVarSet qtvs
 	     	    	      (frees, real_irreds) = partition (isFreeWrtTyVars qtv_set) irreds