Mapped types cannot be reassigned to union type

[gutentag2012]

🔎 Search Terms

mapped types, union, union types, extract, narrow

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about mapped type

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.3.0-dev.20230903#code/JYOwLgpgTgZghgYwgAgPICMBWBGZBvAKGWQGsIBPALmQHJsaBuI5EOAWwmoGcwpQBzJgF8CoSLEQoMmAEz5mZKrRmNm7APYBXcNRCa26aMIJhyABylYICMF2QBeNFlwAfJ7KYnzlzNbABZODMAFW8HeWIAbQBpClCLZFB3Py5ImkUaAF1MgH5qAFEAD15EMAAeaRSAGnxSCmpY8niUIQA+YU8EdRAeZHUrG0CzakrBoObwwmJsainiOqU6GirmYlYOahp1iGXmIRWhJhhtG2Bu5DgAE0vRsGD1IYqsZAhiiBBLu1uuVoAKfswIywAEoIn0BgEgpEAQA6RSZcIA4RAA

💻 Code

interface Obj1 {
  key: '1';
  name: string;
}
interface Obj2 {
  key: '2';
  amount: number;
}
type Objects = Obj1 | Obj2;

type ObjectMapType = {
  [KeyType in Objects['key']]?: Extract<Objects, { key: KeyType }>;
};

const objectMap: ObjectMapType = {
  1: {
    key: '1',
    name: 'name',
  },
};
function addObjectToMap<Obj extends Objects>(obj: Obj) {
  objectMap[obj.key] = obj;
}

🙁 Actual behavior

The object map does not accept a new entry unless it is specifically typed as Obj1 or Obj2, therefore more broadly typed Objects are not accepted.

🙂 Expected behavior

Since the ObjectMapType uses a mapped type for Objects, all of its variations are included, so everything that is typed as Objects should be accepted.

Additional information about the issue

No response

[andrewbranch]

Duplicate of #31445 (the repro there looks a little different, but the explanation works perfectly, and many of the other issues linked to that one look more similar to this)

[typescript-bot]

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