Signatures of intersection/union types properties

Consider next example:

interface A{
    f:string;
    set_f(val:string) : void;
    get_f() : string;
}

interface B{
    f:number;
    set_f(val:number) : void;
    get_f() : number;
}

let n:number;
let str:string

let first : A&B;
first.f = n; // error, as f has type number&string. Probably OK, but probably it should be number|string, to be symmetric with set_f function.
first.f = str; // error, as f has type number&string.
n = first.f;
str = first.f;

first.set_f(n);
first.set_f(str);
n = first.get_f(); // error, typescript prefer to use first signature () => string. Shouldn't it be string&number.
str = first.get_f();

let second : A|B;
second.f = n; // Ops, no error! But it should be at least string&number here
second.f = str;// Ops, no error! But it should be at least string&number here
n = second.f; // error, typescript prefer to use first signature string. It should be string|number
str = second.f;// Ops, no error, typescript prefer to use first signature string. It should be string|number

second.set_f(n); //error, lack a call signature. It's really good, but better if it would be (string&number) => void
second.set_f(str); //error, lack a call signature. 
n = second.get_f(); // error, cool, typescript was able to correctly infer result as string|number
str = second.get_f();// error, cool, typescript was able to correctly infer result as string|number

With #9167 additional to previous was added preserving readonly flag for intersection. Should’t readonly flag be reset if any of intersection member has no such flag? As intersection should extend possibilities of type, and never narrow down it.

[RyanCavanaugh]

We’re continuing to look for a good algorithm that can produce “correct” signatures from intersection types.

[TylorS]

Hey all - I just wanted to add a simplified example of where this is currently failing for me as well.

https://www.typescriptlang.org/play/index.html#code/C4TwDgpgBAYgdlAvFAFCghgLigZ2AJwEs4BzASiQD5cDjyoAyVDbOAVwFsAjCfCxau268KAelFQA8m3y4QeCByhs4AE15RgEPAChQkKAAkI6VQEF8JADxnqyM1AgAPLWpyp82YgDMNAJQAaKAA6UPxtYGx0OBAAbQBdfmpokCgAfig-KFYIADcNKHEoAHF8dC5NAAtob0J8PCh0S04IOGAdPXBoACEkIxNzSyt4aiLJOAAbVKEefCCuNmBcSoB7NgnVKB4oACI8IlIoAB8oGd4dzoMAYT6-CGAZOAAVLuG4UYlxqdPOWfnF5ZrDZbaB7WiHE5nfAXHRFboA6rhWAIQjuPCECYTRpcCbQYArEFQADG6ExEE2AHdCMBKlsVjSaAcSI01D9hPgdOoiRMmtAiSs4A1vNh4FAOt4UAByaqYlaSsg6CUARgVQA