Enable type parameter lower-bound syntax

TypeScript Version: 2.1.1

Code

class Animal {}
class Cat extends Animal {}
class Kitten extends Cat{}

function foo<A super Kitten>(a: A) { /* */ }

Expected behavior:
The type parameter A has the type Kitten as lower-bound.

Actual behavior:
Compilation failure. The syntax is unsupported.

Discussion:
The upper-bound counterpart of the failed code works fine:

class Animal {}
class Cat extends Animal {}
class Kitten extends Cat{}

function foo<A extends Animal>(a: A) { /* */ }

People in issue #13337 have suggested to use

function foo <X extends Y, Y>(y: Y) { /* */ }

to lower-bound Y with X. But this does not cover the case where X is an actual type (instead of a type parameter).

首先查看提议的 A super Kitten 语法以及 issue #13337 中的讨论，因为这里没有确定任何实现文件或测试。确定所需的语言和类型检查更改，然后添加覆盖，证明带下界的类型参数能够编译并按描述运行。