The following code example illustrates an issue where TypeScript is unable to infer the generic type U in the fooBar function, leading to the return type of fooRef.doIt() being unknown. What is the reason for this behavior and what modifications are required to obtain the correct return type?
interface Foo<T> {
foo(): T;
}
class Bar implements Foo<string> {
foo(): string {
return 'Bar';
}
}
class Baz implements Foo<number> {
foo(): number {
return 43;
}
}
class FooRef<T extends Foo<U>, U> {
constructor(private instance: T) {}
doIt() {
return this.instance.foo();
}
}
function fooBar<T extends Foo<U>, U>(foo: new (...args: any[]) => T) {
return new FooRef(new foo());
}
const barRef = fooBar(Bar);
barRef.doIt(); // unknown, expected string
const bazRef = fooBar(Baz);
bazRef.doIt(); // unknown, expected number