tsserver signals TS7017 incorrectly when using a symbol-indexed property

[argv-minus-one]

When declaring a symbol-indexed property on an existing interface, attempting to use it gives error TS7017 in VSCode. tsc, however, compiles it without complaint, implying a bug in tsserver.

TypeScript Version: 3.3.0-dev.20190119

Search Terms: TS7017 tsserver symbol property

Code

const additionalProp = Symbol();

interface Document {
	[additionalProp]?: number;
}

document[additionalProp] = 42; // TS7017 in tsserver/vscode (but not tsc)
console.log(document[additionalProp]); // same

tsconfig.json:

{
"compilerOptions": {
	"strict": true,
	"charset": "UTF-8",
	"target": "es6",
	"module": "es6",
	"lib": ["es6", "dom"]
}
}

Expected behavior:

The above code does not have any error highlights.

Actual behavior:

There is an error highlight. The hover says: “Element implicitly has an 'any' type because 'Document' has no index signature. ts (7017)”

Playground Link — The Playground also shows the incorrect error highlight.

Related Issues: None, as far as I can tell.

[argv-minus-one]

Gist demonstrating the issue