undefined not assignable error within "if" statement, when member of Readonly<T>

[captain-igloo]

TypeScript Version: 3.3.0-dev.20181212

readonly if

Code

interface IProps {
    foo1?: string;
    foo2: string;
}

class Foo<P> {
    protected props: Readonly<P>;

    constructor(props: Readonly<P>) {
        this.props = props;
    }
}

class Bar<P extends IProps> extends Foo<P> {

    private test1() {
        const { foo1, foo2 } = this.props;

        if (foo1) {
            foo1.toUpperCase(); // fine
            this.test2(foo1); // error "Type undefined is not assignable to type string"
        }
        this.test2(foo2); // fine
    }

    private test2(foo: string): void {
    }
}

Expected behavior:
I expect the above to compile without error
Actual behavior:
There's an error - Type 'undefined' is not assignable to type 'string'

"foo1" above can't be undefined because it's within an "if (foo1) {}".

Playground
link

[jakebailey]

This one was fixed back in #43183, but probably deserves a test (I didn't see any that were similar in the PR).