Excess properties in tagged union not checked if type is optional and literal

[OliverJAsh]

TypeScript Version: 3.4.1, 3.5.1

Search Terms: tagged union optional literal excess properties props

Code

type Button = { tag: 'button'; type?: 'submit'; };
type Anchor = { tag: 'a'; type?: string; href: string };

type Union = Button | Anchor;
const obj: Union = {
    tag: 'button',
    type: 'submit',

    // no error, bad! :-(
    href: 'foo',
    // error, good! :-)
    href2: 'foo',
};

If I replace Button with:

type Button = { tag: 'button'; type?: string };

That is, if I replace the optional string literal with an optional string, then it works as expected (I get an error for the excess prop href), suggesting this is down to the fact that we're using string literals?

[RyanCavanaugh]

This works as expected if you turn strict null checks off 🙃

[sandersn]

Right now the discriminable property check gives up if more than one type in the union matches any discriminant property. Unfortunately, type: 'submit' matches both Button.type and Anchor.type.

I'm going to try making the check stricter and see if it breaks anything.

Not sure why strictNullChecks makes this work.