Fix constraint handling for lazy patterns

[ghc-hetmet.git] / compiler / typecheck / TcPat.lhs

diff --git a/compiler/typecheck/TcPat.lhs b/compiler/typecheck/TcPat.lhs
index a5d4209..f945292 100644 (file)
--- a/compiler/typecheck/TcPat.lhs
+++ b/compiler/typecheck/TcPat.lhs
@@ -332,11 +332,26 @@ tc_pat pstate (BangPat pat) pat_ty thing_inside
 --
 -- Nor should a lazy pattern bind any existential type variables
 -- because they won't be in scope when we do the desugaring
+--
+-- Note [Hopping the LIE in lazy patterns]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+-- In a lazy pattern, we must *not* discharge constraints from the RHS
+-- from dictionaries bound in the pattern.  E.g.
+--     f ~(C x) = 3
+-- We can't discharge the Num constraint from dictionaries bound by
+-- the pattern C!  
+--
+-- So we have to make the constraints from thing_inside "hop around" 
+-- the pattern.  Hence the getLLE and extendLIEs later.
+
 tc_pat pstate lpat@(LazyPat pat) pat_ty thing_inside
-  = do { (pat', pat_tvs, res) <- tc_lpat pat pat_ty pstate $ \ _ ->
-                                 thing_inside pstate
-                                       -- Ignore refined pstate',
-                                       -- revert to pstate
+  = do { (pat', pat_tvs, (res,lie)) 
+               <- tc_lpat pat pat_ty pstate $ \ _ ->
+                  getLIE (thing_inside pstate)
+               -- Ignore refined pstate', revert to pstate
+       ; extendLIEs lie
+       -- getLIE/extendLIEs: see Note [Hopping the LIE in lazy patterns]
+
        -- Check no existentials
        ; if (null pat_tvs) then return ()
          else lazyPatErr lpat pat_tvs