[project @ 2006-01-05 13:10:55 by simonpj]
MERGE TO STABLE This commit fixes a nasty problem discovered by Volker Stolz. The problem is described in Note [Multiple instantiation] in TcExpr, which is reproduced below. (Core Lint identifies the problem, incidentally.) tc200 is a test case. Note [Multiple instantiation] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We are careful never to make a MethodInst that has, as its meth_id, another MethodInst. For example, consider f :: forall a. Eq a => forall b. Ord b => a -> b At a call to f, at say [Int, Bool], it's tempting to translate the call to f_m1 where f_m1 :: forall b. Ord b => Int -> b f_m1 = f Int dEqInt f_m2 :: Int -> Bool f_m2 = f_m1 Bool dOrdBool But notice that f_m2 has f_m1 as its meth_id. Now the danger is that if we do a tcSimplCheck with a Given f_mx :: f Int dEqInt, we may make a binding f_m1 = f_mx But it's entirely possible that f_m2 will continue to float out, because it mentions no type variables. Result, f_m1 isn't in scope. Here's a concrete example that does this (test tc200): class C a where f :: Eq b => b -> a -> Int baz :: Eq a => Int -> a -> Int instance C Int where baz = f Current solution: only do the "method sharing" thing for the first type/dict application, not for the iterated ones. A horribly subtle point.