Reword reference article about type projection so that it highlights what is allowed

docs/_docs/reference/changed-features/type-projection.md

@@ -0,0 +1,21 @@
+---
+layout: doc-page
+title: "Tightened Type Projection"
+nightlyOf: https://docs.scala-lang.org/scala3/reference/changed-features/type-projection.html
+---
+
+Scala 2 allowed general type projection `T#A` where `T` is an arbitrary type and `A` names a type member of `T`.
+This turns out to be [unsound](https://github.com/scala/scala3/issues/1050) (at least when combined with other Scala 3 features).
+
+To remedy this, Scala 3 only allows type projection if `T` is a concrete type (any type which is not abstract), an example for such a type would be a class type (`class T`).
+A type is abstract if it is:
+* An abstract type member (`type T` without `= SomeType`)
+* A type parameter (`[T]`)
+* An alias to an abstract type (`type T = SomeAbstactType`).
+There are no restriction on `A` appart from the fact it has to be a member type of `T`, for example a subclass (`class T { class A }`).
+
+To rewrite code using type projections on abstract types, consider using
+path-dependent types or implicit parameters.
+
+This restriction rules out the [type-level encoding of a combinator
+calculus](https://michid.wordpress.com/2010/01/29/scala-type-level-encoding-of-the-ski-calculus/).