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Change in Scoping Rules for Wildcard Placeholders in Types
A wildcard in a type now binds to the closest enclosing type application.
For example List[List[_]] is now equivalent to the existential type
List[List[t] forSome { type t }]In version 2.7.0, the type expanded instead to
List[List[t]] forSome { type t }The new convention corresponds exactly to the way wildcards in Java are interpreted.

No Contractiveness Requirement for Implicits
The contractiveness requirement for implicit method definitions has been dropped. Instead it is checked for each implicit expansion individually that the expansion does not result in a cycle or a tree of infinitely growing types.

Read more http://www.scala-lang.org/index.html