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 (file)
--- 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