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

Reproduce the provided example in the TypeScript 2.3 Playground with strictNullChecks enabled, comparing the non-null assertion case with the explicit bar.foo check. Investigate how property access is narrowed in the first if block; done means both assignments are accepted without errors or warnings.