[project @ 2002-03-12 09:13:08 by simonpj]

-----------
	Fix mkSynTy
	-----------

Fix a nasty and long-lived bug in mkSynTy which meant that in:

	newtype A a = A a
	type B = A
	f :: B Double

the type (B Double) wasn't regarded properly as (A Double).  This bug
has lasted for a long time because the type inference engine is fairly
forgiving about missing the invariant that a TyConApp always looks
like one right at the top level.  In fact, it's so forgiving that I
don't know how to tickle this bug into showing up.  (It showed up in
5.02, becuase the context-simplification for 'deriving' is done by a
different engine as all the other context simplification.)

Still, the invariant should hold, and this fix makes it so.

diff --git a/ghc/compiler/types/Type.lhs b/ghc/compiler/types/Type.lhs
index dc642d0..526cf3d 100644 (file)
--- a/ghc/compiler/types/Type.lhs
+++ b/ghc/compiler/types/Type.lhs
@@ -358,7 +358,10 @@ mkSynTy tycon tys
   | n_args == arity    -- Exactly saturated
   = mk_syn tys
   | n_args >  arity    -- Over-saturated
-  = case splitAt arity tys of { (as,bs) -> foldl AppTy (mk_syn as) bs }
+  = case splitAt arity tys of { (as,bs) -> mkAppTys (mk_syn as) bs }
+       -- Its important to use mkAppTys, rather than (foldl AppTy),
+       -- because (mk_syn as) might well return a partially-applied
+       -- type constructor; indeed, usually will!
   | otherwise          -- Un-saturated
   = TyConApp tycon tys
        -- For the un-saturated case we build TyConApp directly