Union type breaks type safety of dynamic object keys

[aelgn]

🔎 Search Terms

"union type as dynamic object key"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Union types

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.3#code/FAguE8AcFMAIEVYF5YG9i1gQwPwC5YBnUAJwEsA7AcwG5gBfOgMwFcKBjUMgewtgEcA7t0gAKUAQDkWSbAA+sSRRHRJASgKJ0mEtFAsSfbZlgBtUAF0CANlsAaWAHpHsUAAsyhWO24BbSGQANtCEGLCMDMCsHFy8AsKQAEziUjIaCGhhuvqGmSZmljb2Ti7unrBUZABuIa5QcNAkJNwk2F7QAB4wnNAAJmER9GD1CInIeWYA0rCUijLyigBGkhb4RKSUtAzMbJw8fEIiAMwpc+qaY8bZBkZhmOZWsLbWDs6uHl6VNV4QMLCNzVaWHaXWgPX6mEGUV2sQOCQALKdpLIFJJlul4JcsnobhN7oUnsU3mUvD5-EEQg5FiwqDgBnQhsBfnB4EdxsZcARiORqHRMIs1tzNgydjF9vERABWJHzVHozRsq443LGfGPZ6vUofbx+ALBQhUml0yEMoA

💻 Code

type Q = {
  a?: string;
};
function qwop(t: 'a' | 'nope'): Q {
  return {
    [t]: 666, // this compiles
  };
}
function qwop2(t: 'a'): Q {
  return {
    [t]: 666, // this gives type error as expected
  };
}

type Q2 = {
  [K in 'a' | 'b']?: string;
};
function qwop3(t: 'a'): Q2 {
  return {
    [t]: 666, // this gives type error as expected
  };
}
function qwop4(t: 'a' | 'b'): Q2 {
  return {
    [t]: 666, // this compiles, bug?
  };
}

type Q3 = {
  a?: string;
  b?: string;
};
function qwop5(t: 'a' | 'b'): Q3 {
  return {
    [t]: 666, // this compiles, bug?
  };
}

🙁 Actual behavior

Type check stops working when using a string union type as dynamic key to an object.

🙂 Expected behavior

First I expected 'a' | 'nope' to not be a valid type for key of { a?: string } but I guess extraneous keys are just fine in a duck eat duck world.

What seems to be a bug to me is that the type declaration is ignored when the key type is a union - even when the full union is properly declared in the type. Ie. the type of properties 'a' | 'b' is clearly string and I expected a number to produce a type error.

Additional information about the issue

No response

[jcalz]

Duplicate of #13948

[typescript-bot]

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