[project @ 2004-04-02 13:16:07 by simonpj]

[project @ 2004-04-02 13:16:07 by simonpj]
* Improve error message for overlapping instances

* Improve handling of type-variable-only constraints like (Foo a).
  Previously we never looked them up in the instance envt, *except* if
  -fallow-undecideable-instances was on, because that allows
	instance (...) => Foo a
  But -fallow-undecideable-instances might be on in the module with the
  instance decl, but off in the importing module.  Also it's really a per-class
  thing.  So now we just record in the instance env whether there are any such
  strange instances, a kind of short-cut for the lookup.

* Arrange that we are a bit more eager about resolving overloading in
  the case of existential pattern matching [George Russel suggestion]
  Here's the example (see comments in InstEnv)
	-- The key_tys can contain skolem constants, and we can guarantee that those
	-- are never going to be instantiated to anything, so we should not involve
	-- them in the unification test.  Example:
	--	class Foo a where { op :: a -> Int }
	--	instance Foo a => Foo [a] 	-- NB overlap
	--	instance Foo [Int]		-- NB overlap
	-- 	data T = forall a. Foo a => MkT a
	--	f :: T -> Int
	--	f (MkT x) = op [x,x]
	-- The op [x,x] means we need (Foo [a]).  Without the filterVarSet we'd
	-- complain, saying that the choice of instance depended on the instantiation
	-- of 'a'; but of course it isn't *going* to be instantiated.