Type guard not working on object property

[Joncom]

TypeScript Version: 3.5.1

Search Terms: typescript type guard object property properties not working

Code

class MyAudio {}
let clips: { [path: string]: HTMLAudioElement | MyAudio } = {};

let path = 'path/to/audio.ogg';
if( clips[path] ) {
    if( clips[path] instanceof HTMLAudioElement ) {
        clips[path].currentTime = 0; // This line produces error:
        // Property 'currentTime' does not exist on type 'MyAudio'.
    }
}

Expected behavior:
TypeScript should realize that clips[path] is an instance of HTMLAudioElement and therefore currentTime is a perfectly valid property to access.

Actual behavior:
See error in example.

Playground Link: http://www.typescriptlang.org/play/#code/MYGwhgzhAECyCeBBArgEwJYHtoG8C+AUCAKYAu0o6ADhAFy7QDaVYpAFvRKQE7oB2AcwC69ABIAVWABkUGTAFESAW2J9yAHzhI0WaHmgBeXHgDcBImWgt2h6AHJrbAPSlMTsDswA6TAIF2zdAAzAAoKEGoIZlY2IWgASlwCaBToYLDKGmj2OP4uMD5gYkwg6AlpWSxFYhU1BKTUxvDI7NivYGRublVScXQVWwAGE2gnJ2hxNnQYCL5iK25MVGQimGIuzG5aZKaUsegABUWqddJ4ew6unr6VO2hUTGIYPkxyYgAPafJMPmgzk-sCEqmDsXh2KUIhCAA

[MartinJohns]

This is a duplicate of #11483. Workaround is to just store the value in a local variable.

[fatcerberus]

See also #31445 for some insight into why this can't work, but tl;dr: TS doesn't know clips[path] accesses the same property every time and therefore doesn't narrow it.