Switch inside for loop is converting to intersection

[apacheli]

Bug Report

When using a switch statement inside of a loop, it creates weird intersection issues, which should be union types. This causes issues where types are never, or just not correct in general.

🔎 Search Terms

switch case, loop, union intersection, assigning

🕗 Version & Regression Information

TypeScript 4.5

⏯ Playground Link

playground link

💻 Code

  update(data: Partial<Message>) {
    for (const key in data as Message) {
      switch (key) {
        case "content":
        case "tts": {
          const z = data[key]!;
          let g = this[key]!;
          g = z;
          this[key] = data[key]!;
          break;
        }

z is assignable to g, but data[key] is not assignable to this[key] despite being exactly the same type. key is already narrowed down to the correct keys "content" | "tts".

🙁 Actual behavior

When using intelisense, it seems that type is correctly using |. However, when TRULY assigning to this it seems to have created a union instead (& is not correct).

🙂 Expected behavior

It should stay as a union instead.

[jcalz]

#30769 is why this is happening, and this issue is probably a duplicate of #32693

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.