After compiling the code below, it runs without any issues
interface A {
x: string;
}
interface B {
x: string;
}
type C<D extends A> = D | B;
declare function funcA<D extends A, E extends C<D> = C<D>>(): E | E[] | undefined;
declare function funcB<D extends A, E extends C<D> = C<D>>(param?: E | E[]): void;
const varX = funcA();
funcB(varX);
However, interesting things start happening when the function funcB is enclosed within a class:
export class MyClass<D extends A, E extends C<D>> {
functionF(param?: E | E[]): void {}
functionG() {
const varY = funcA();
this.functionF(funcA());
this.functionF(varY);
}
}
According to the code, the function MyClass.functionF should be the same as function funcB. Nonetheless, an error pops up at the line this.functionF(varY). Interestingly, the line this.functionF(funcA()) compiles without any issues.
Argument of type 'B | A | (B | A)[] | undefined' is not assignable to parameter of type 'E | E[] | undefined'.
Type 'B' is not assignable to type 'E | E[] | undefined'.
Type 'B' is missing properties like length, pop, push, concat, and more which are required by type 'E[]'.
This error is perplexing because type E should be equivalent to D | A, so how come A cannot be assigned to E?