15.8.4 Qualified this
Any lexically enclosing instance can be referred to by explicitly qualifying the
keyword this.
Let C be the class denoted by ClassName. Let n be an integer such that C is the
nth lexically enclosing class of the class in which the qualified this expression
appears. The value of an expression of the form ClassName.this is the nth lexically
enclosing instance of this (§8.1.3). The type of the expression is C. It is a
compile-time error if the current class is not an inner class of class C or C itself.

spec里翻了好半天才找到