After combining multiple types, how to access the properties as optional properties?

[JYC-99]

TypeScript Version: 3.9.2

Search Terms:

Expected behavior:

obj.doSomethingA?.();

obj.doSomethingA ? obj.doSomethingA() : null;

Should work.

Actual behavior:

End up with error:

Property 'doSomethingA' does not exist on type 'TMyInterface'.
  Property 'doSomethingA' does not exist on type 'IInterfaceB'.

I have to use type casting to work around.

Related Issues:

Code

interface IInterfaceA {
    doSomethingA(): void;
}

interface IInterfaceB {
    doSomethingB(): void;
}

type TMyInterface = IInterfaceA | IInterfaceB;

const gen = (): TMyInterface => {
    if (Math.random() > 0.5) {
        return {
            doSomethingA: () => {}
        };
    }

    return {
        doSomethingB: () => {}
    }
}

const obj = gen();

obj.doSomethingA?.();

obj.doSomethingA ? obj.doSomethingA() : null;

Output

"use strict";
var _a;
const gen = () => {
    if (Math.random() > 0.5) {
        return {
            doSomethingA: () => { }
        };
    }
    return {
        doSomethingB: () => { }
    };
};
const obj = gen();
(_a = obj.doSomethingA) === null || _a === void 0 ? void 0 : _a.call(obj);
obj.doSomethingA ? obj.doSomethingA() : null;

Compiler Options

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "alwaysStrict": true,
    "esModuleInterop": true,
    "declaration": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "moduleResolution": 2,
    "target": "ES2017",
    "jsx": "React",
    "module": "ESNext"
  }
}

Playground Link: Provided

[jcalz]

This is not a forum for asking TS questions; it's for reporting bugs or requesting features. And when you report a bug or request a feature, you are expected to search existing GitHub issues to see if it has been raised before.

This is a duplicate of #33736 and #1260