Noncrashing property access through non-null assertion operator should narrow that property to NonNullable

TypeScript Version: 2.3 (Playground)

Code

Using strictNullChecks.

interface Foo {
    optional?: number;
}

interface Bar {
    foo?: Foo;
}

function test(bar: Bar) {
    if (bar.foo!.optional) {
        let num: number = bar.foo.optional;
    }

    if (bar.foo && bar.foo.optional) {
        let num: number = bar.foo.optional;
    }
}

Expected behavior:
No errors or warnings.

Actual behavior:

• num in first if block is number | undefined
• bar.foo in first if block can be undefined

在启用 strictNullChecks 的 TypeScript 2.3 Playground 中重现所提供的示例，并比较 non-null assertion 情况与显式的 bar.foo 检查。调查第一个 if 块中的属性访问是如何被缩小范围的；当两个赋值都能在没有错误或警告的情况下被接受时，即表示完成。