Union types do not match narrowed overloads #46778
Description
Bug Report
I am not sure if this is a bug or a design limitation, but when you have a function with narrowed overloads (see example) you cannot call the function with a broader union type although the union type does not go out of bounds of the overloads.
🔎 Search Terms
union types overloads
🕗 Version & Regression Information
- I was unable to test this on prior versions because of time restraints
⏯ Playground Link
💻 Code
function isOneOrTwo(it: unknown): it is (1 | 2) {
return it === 1 || it === 2;
}
function doSomething(it: unknown) {
if (isOneOrTwo(it)) {
return doSomethingElse(it);
} else {
throw new Error("out of range");
}
}
function doSomethingElse(it: 1): "one";
function doSomethingElse(it: 2): "two";
function doSomethingElse(it: number) {
switch (it) {
case 1:
return "one";
case 2:
return "two";
default:
throw new Error("out of range");
}
}🙁 Actual behavior
The compiler reports an error on the call to doSomethingElse because the union type is not assignable to either of the overloads.
🙂 Expected behavior
The function is callable and returns a union type of the relevant overload's return values.