return-undefined false positive with async function

[nicolashenry]

Bug Report

• TSLint version: 5.7.0
• TypeScript version: 2.5.2
• Running TSLint via: CLI / VSCode

TypeScript code being linted

declare function test(cb: () => Promise<void> | void): void;

test(async () => {
	return;
});

with tslint.json configuration:

{
  "defaultSeverity": "warning",
  "rules": {
    "return-undefined": true
  }
}

Actual behavior

[tslint] Value-returning function should use `return undefined;`, not just `return;`. (return-undefined)

Expected behavior

No error