Unions of assertion functions do not act as assertion functionsΒ #59707
Description
π Search Terms
narrowing, assertion function
π Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about everything
β― Playground Link
π» Code
class box<T> {
constructor(public value: T){}
check(): this is box<string> {
return typeof this.value == 'string';
}
assert(): asserts this is box<string> {
if (typeof this.value != 'string') throw new Error();
}
private test() {
this.assert();
// type correctly narrowed
this.value.substring(0);
}
}
function make() : box<string> | box<number> {
return new box('a');
}
function assert(b: box<string> | box<number>): asserts b is box<string> {
if (typeof b.value != 'string') throw new Error();
}
const b = make();
if (b.check()) {
// type correctly narrowed
b.value.substring(0);
}
b.assert();
// type not narrowed (substring does not exist on type 'string | number')
b.value.substring(0);
assert(b);
// type correctly narrowed
b.value.substring(0);π Actual behavior
Type is not narrowed after method call
π Expected behavior
I expected the type to be narrowed after method call since it is narrowed within other methods or when using a function external to the type.