Bug Report

Typescript is unable to correctly determine that a function’s return satisfies the specified type.

🔎 Search Terms

“Typescript expanding boolean into true/false” (https://github.com/microsoft/TypeScript/issues/30029)

🕗 Version & Regression Information

v. 4.7.4

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

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.9.0-dev.20221014#code/LAKALgngDgpgBAIQIYBMBiA7OBeOAKPAShwD44wAnAVxmIB98jS4AzJAGwGdaBuUAYwD2GTmHIxRALkSpMOfMWxkA3qDhwKMMFQpYAskjAALAHQUkGFIIC2TMgAY+IAL48gA

💻 Code

type SomeInferredFunctionType = (() => true) | (() => false); 

// Gives TS error!
const test: SomeInferredFunctionType = () => { 
  return Math.random() > 0.5;
};

🙁 Actual behavior

Typescript thinks test does not satisfy its given type when it does. Note, that it is possible (with much difficulty) to collapse SomeInferredFunctionType into the correct simplified type() => boolean when I have direct access to the type. However, in my codebase, the type is actually automatically inferred and I do not have access to it and I am just consuming it.

🙂 Expected behavior

Typescript should be able to determine that test satisfies the condition given by SomeInferredFunctionType