id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
22	Mask mutability constraints on fresh data	benl		"In a type like

{{{
 fun :: forall %r1 %r2
     .  Int %r1 -> Int %r2
     :- Mutable %r2
}}}

The returned Int is fresh, that its region does not appear on the lhs of an arrow or in a closure. In this case it does not matter that the data is Mutable, and this fetter can be masked. 

We'll want to use the closure information the core language for this, so make sure the core type checker works.

"	enhancement	new	project		Source Type Inferencer