No excess property error for computed property

TypeScript Version: 3.7.5 and 3.9.0-dev.20200220

Search Terms: excess property, computed property, index signature

Code

interface Foo {
	a: string;
} 

const bar: Foo = {
	a: "",
	z: "" // error, excess prop
}

const baz: Foo = {
	a: "",
	["Z".toLowerCase()]: "" // this is okay I guess?
}

Expected behavior:
The computed property in the definition of baz should maybe produce an excess property error, since Foo has no string index signature and "Z".toLowerCase() is only known to be a string.

Actual behavior:
The computed property is allowed.

Playground Link: Here

Related Issues: #22427

This issue is a re-opening of #22427, which was marked as a bug and closed as "fixed", but I don't see an actual fix referenced anywhere in that issue and the behavior described in that issue persists. Any ideas? Thanks!

There's an SO question asking this, which brought me to #22427, which has me scratching my head 😕.

Start with the TypeScript playground reproduction in the issue and compare the computed-property case with the direct excess-property case. Read related issue #22427 to understand the prior discussion; done means the expected excess-property behavior is resolved or the intended behavior is documented by updated compiler tests.