TS 3.5 new error when using keyof with assignment to copy property

[mjbvz]

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms:

• keyof
• property

Code

export interface I {
    x: string;
    y: number;
}

function copy(from: I, to: I, key: keyof I) {
    const value = from[key];
    if (typeof value === 'string') {
        to[key] = value
    }
} 

Expected behavior:
This compiles under TS 3.4.5 without any errors

Actual behavior:
In TS 3.5, this is a compile error:

Type 'string' is not assignable to type 'string & number'.
  Type 'string' is not assignable to type 'number'.

This is a reasonable error IMO, the change should just be noted somewhere (an issue may already exist but I wasn't sure which keywords to search for). We saw this in the VS Code codebase

Playground Link:

https://www.typescriptlang.org/play/#src=export%20interface%20I%20%7B%0D%0A%20%20%20%20x%3A%20string%3B%0D%0A%20%20%20%20y%3A%20number%3B%0D%0A%7D%0D%0A%0D%0Afunction%20copy(from%3A%20I%2C%20to%3A%20I%2C%20key%3A%20keyof%20I)%20%7B%0D%0A%20%20%20%20const%20value%20%3D%20from%5Bkey%5D%3B%0D%0A%20%20%20%20if%20(typeof%20value%20%3D%3D%3D%20'string')%20%7B%0D%0A%20%20%20%20%20%20%20%20to%5Bkey%5D%20%3D%20value%0D%0A%20%20%20%20%7D%0D%0A%7D%20

Related Issues:

• TypeScript 3.5 compile errors in the VS Code codebase #31380

[jack-williams]

The PR introducing the change was this one #30769