Union of classes with same structure collapsed, leading to missing type errors

[blickly]

🔎 Search Terms

collapsed union, instanceof, structural types, nominal types

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play/?downlevelIteration=true&importHelpers=true&target=99&module=1&ts=5.8.2#code/MYGwhgzhAEBiD28A8AVAfNA3gKGtY8AdhAC4BOArsCfGQBRkCmYAJkSAJ7QAeAXNCgCUWAL65oAdwCWJABYANVGjpgyAc35D+CZOizi8TEhTKFohRhLiIV6wQG5xYsdlCQYAIVVL9eAsXIqGnomVnYuPgFhTDE8aTlFdFsNKP4vMh8cPENGY1NzS2h05IcnbBcAMwpCaikiaArEAHkydKVkzUFtRCUAHza9LOgjEzMLK2LVNVLK6tr690YyEgBlcilCNTpI0jIN6d9oKQroOhIOAAdGeBPuaABCAF5H6AByXf3X4TkyeCtx6AAUTIv3or0YjAA1vcvo5ZjUSHUzBU6F1oAA3eBSFiHfykHjQF6NeAtYoAFgATKU8AB6GnZaAAPQA-OI8SRoFwXtwAHTxBR0ACM1OgdIZLPEx1OXA2pDANWuJx00QMw1yo0ceFi0EWyzWe02dA4DlF9JlMAs6KWABpoBB4NA5FIYOcrvhZIxgJCYE0ANLiMUMoNM1kiIA

💻 Code

class Foo<T> {
  constructor(readonly x: T) {}
  withX<T>(arg: T): Foo<T> {
    return new Foo(arg);
  }
}

class Bar<T> {
  constructor(readonly x: T) {}
  withX<T>(arg: T): Bar<T> {
    return new Bar(arg);
  }
}

function fooOrBar<T>(arg: T): Foo<T>|Bar<T> {
  return new Bar(arg);
}

function assertString(x: string) {
  if (typeof x !== 'string') throw new Error('eek!');
}

function f(): void {
  const x = fooOrBar(42);
  const y = x.withX(1);
  if (y instanceof Foo) {
    return;
  }
  assertString(y); // y is never, so this type checks OK
}

🙁 Actual behavior

assertString call gives no error

🙂 Expected behavior

assertString gives an error, since neither Foo nor Bar are assignable to string

Additional information about the issue

This appears somewhat related to #202 (and having a way to force classes to be treated nominally would have avoided this issue for us). However, this issue does seem distinct in that it has a missing error even when classes are interpreted structurally.

[MartinJohns]

Duplicate of #60033.

[typescript-bot]

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