Typescript incorrectly infer the types for Readonly optional properties.

[dryganets]

TypeScript Version: 2.5.3) / 2.6.0-dev.20170930

Code

https://github.com/dryganets/typescript-bug/blob/master/index.ts
I expect this code to compile.
This example is
Expected behavior:
type guard

if (this.props.a) {
}

reduces the type from string|undefined to string.

Actual behavior:
type is still string|undefined after check.

The bug doesn't repro if I use the slightly different syntax:

export class Test extends ComponentBase<Props, {}> {

But we have some places in the app where the former syntax is required.

[mhegazy]

This is a duplicate of #15376. this.prop.a is a generic type, P["a"], and there is no way currently to narrow a generic type.

It is worth noting that the compiler does the right thing on property access/ index access/ call thanks to #15576. but the type of the symbol does not change.

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.