Why can’t I use optional chaining to test this?

function test2(value:{a:number}|{b:number}){
    // `.a` underlined with: "Property a does not exist on type {b:number}"
    if(value?.a != null){
        console.log('value of a:',value.a)
    }
}

test2({a:1})

test2({b:2})

It transpiles fine, works as designed, and throws no runtime errors.
The property check logically it narrows the scope to {a:number}.

So… What’s wrong with this?
Is there a setting I can change to make it allow and use this method of type narrowing?

Someone will probably tell me to use the in operator. So I suppose I’ll say now that I know I could use the in operator, but I don’t want to. If I union the type with string as well, then I can’t use in with string and I have to first check to make sure it’s not a string with if(typeof value != 'string'), but in regular JavasScript I could simply leave my property check with optional chaining and know that whatever I get will have the property and all will be well.

So, what’s the problem with typescript here? Is it just not smart enough to figure it out?