Property not found on index accessed type in a union

[sandersn]

interface I {
    [key: string]: I
    (...args: any[]): I
}

declare let y: string | I
y.toUpperCase() // Error

Expected: No error: both string and I separately allow property access of toUpperCase.

Actual: Error, "Property 'toUpperCase' does not exist on type 'I'.

[ahejlsberg]

Less confusing repro:

interface A {
    a: string;
}
interface I {
    [key: string]: number;
}

declare let y: A | I
y.a;  // Got error, expected string | number

[mhegazy]

Looks like a duplicate of #21141

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.