Andarist fix union intersection setters

[craigphicks]

This draft was created only to analyze a couple more tests files in the context of pull #56895

Fixes #

[craigphicks]

Was expecting pull #56895 to change this behavior, because therein it say it fixes #5689. But no change occurred as far as I can see.

[Andarist]

Getters are treated like properties so a union of getters returns the union of their return types. I think this just works as expected. Even if we treat them as functions - it still all checks out, return types are unionified:

declare const fn: (() => 1 | 2) | (() => 2 | 3)
const result = fn()
//    ^? const result: 1 | 2 | 3

[craigphicks]

My comment and the subject of #56894 is setters. The point of the result diff is that valid domain has not narrowed by your fix. For more clarity see #56922.

function f4(x: {set d(a:1|2); get r():1|2} | {set d(a:2|3); get r():2|3} ){
//           ^ (parameter) x: { d: 1 | 2; readonly r: 1 | 2; } | { d: 2 | 3; readonly r: 2 | 3; }
    x.d = id();
//        ^ const id: <1 | 2 | 3>() => 1 | 2 | 3      (5.3.2)
//        ^ const id: <2>() => 2      (expecting, if a setter was treated as a function)

[jakebailey]

#56895 is closed, so I'm going to close this draft too.