Fix an obscure bug in rule-matching

This bug is the cause of Trac #1092.  The fix is easy
by making the RnEnv2 implementation do the right thing.
See Note [rnBndrLR] in VarEnv.

Test case is simplCore/should_compile/rule1

diff --git a/compiler/basicTypes/VarEnv.lhs b/compiler/basicTypes/VarEnv.lhs
index 0b2553a..fc2dbf7 100644 (file)
--- a/compiler/basicTypes/VarEnv.lhs
+++ b/compiler/basicTypes/VarEnv.lhs
@@ -226,20 +226,34 @@ rnBndr2 (RV2 { envL = envL, envR = envR, in_scope = in_scope }) bL bR
 rnBndrL, rnBndrR :: RnEnv2 -> Var -> (RnEnv2, Var)
 -- Used when there's a binder on one side or the other only
 -- Useful when eta-expanding
+-- 
 rnBndrL (RV2 { envL = envL, envR = envR, in_scope = in_scope }) bL
   = (RV2 { envL     = extendVarEnv envL bL new_b
-        , envR     = envR
+        , envR     = extendVarEnv envR new_b new_b     -- Note [rnBndrLR]
         , in_scope = extendInScopeSet in_scope new_b }, new_b)
   where
     new_b = uniqAway in_scope bL
 
 rnBndrR (RV2 { envL = envL, envR = envR, in_scope = in_scope }) bR
-  = (RV2 { envL     = envL
+  = (RV2 { envL     = extendVarEnv envL new_b new_b    -- Note [rnBndrLR]
         , envR     = extendVarEnv envR bR new_b
         , in_scope = extendInScopeSet in_scope new_b }, new_b)
   where
     new_b = uniqAway in_scope bR
 
+-- Note [rnBndrLR] 
+-- ~~~~~~~~~~~~~~~
+-- Notice that in rnBndrL, rnBndrR, we extend envR, envL respectively
+-- with a binding [new_b -> new_b], where new_b is the new binder.
+-- This is important when doing eta expansion; e.g. matching (\x.M) ~ N
+-- In effect we switch to (\x'.M) ~ (\x'.N x'), where x' is new_b
+-- So we must add x' to the env of both L and R.  (x' is fresh, so it
+-- can't capture anything in N.)  
+--
+-- If we don't do this, we can get silly matches like
+--     forall a.  \y.a  ~   v
+-- succeeding with [x -> v y], which is bogus of course 
+
 rnOccL, rnOccR :: RnEnv2 -> Var -> Var
 -- Look up the renaming of an occurrence in the left or right term
 rnOccL (RV2 { envL = env }) v = lookupVarEnv env v `orElse` v