Without -XImpredicativeTypes, the typing rules say that a function
should be instantiated only at a *monotype*. In implementation terms,
that means that a unification variable should not unify with a type
involving foralls. But we were not enforcing that rule, and that
gave rise to some confusing error messages, such as
Trac #7264, #6069
This patch adds the test for foralls. There are consequences
* I put the test in occurCheckExpand, since that is where we see if a
type can unify with a given unification variable. So
occurCheckExpand has to get DynFlags, so it can test for
-XImpredicativeTypes
* We want this to work
foo :: (forall a. a -> a) -> Int
foo = error "foo"
But that means instantiating error at a polytype! But error is special
already because you can instantiate it at an unboxed type like Int#.
So I extended the specialness to allow type variables of openTypeKind
to unify with polytypes, regardless of -XImpredicativeTypes.
Conveniently, this works in TcUnify.matchExpectedFunTys, which generates
unification variable for the function arguments, which can be polymorphic.
* GHC has a special typing rule for ($) (see Note [Typing rule
for ($)] in TcExpr). It unifies the argument and result with a
unification variable to exclude unboxed types -- but that means I
now need a kind of unificatdion variable that *can* unify with a
polytype.
So for this sole case I added PolyTv to the data type TcType.MetaInfo.
I suspect we'll find mor uses for this, and the changes are tiny,
but it still feel a bit of a hack. Well the special rule for ($)
is a hack!
There were some consequential changes in error reporting (TcErrors).