TypeScript Version: 2.7.1-insiders.20180127

Search Terms: strictNullChecks mapped types property undefined generated types

Code

interface Foo {
    bar?: {
        baz: string;
    }
}

type ShouldBeString = Foo['bar']['baz'];

Expected behavior: Type ShouldBeString is a string type. This is useful especially for generated types where it isn't possible to write { baz: string } as a separate type.

Actual behavior: Accessing 'baz' errors with Property 'baz' does not exist on type '{ baz: string; } | undefined'.

Playground Link:
Make sure to toggle strictNullChecks on: https://www.typescriptlang.org/play/#src=interface%20Foo%20%7B%0D%0A%20%20%20%20bar%3F%3A%20%7B%0D%0A%20%20%20%20%20%20%20%20baz%3A%20string%3B%0D%0A%20%20%20%20%7D%0D%0A%7D%0D%0A%0D%0Atype%20ShouldBeString%20%3D%20Foo%5B'bar'%5D%5B'baz'%5D

Related Issues: #16108 #6606