output.txt

$> tsc src/foo.ts 

src/foo.ts:29:20 - error TS2339: Property 'length' does not exist on type 'string | {}'.
  Property 'length' does not exist on type '{}'.

18 new Foo("wot").x().length
                      ~~~~~~

test.ts

class Foo<V> {
    value: V
    
    constructor(val: V) {
        this.value = val
    }
    
    x<T>(x?: T): V | T {
        if (x === null || x === undefined) {
            return this.value
        } else {
            return x
        }
    }
}


new Foo("wot").x().length
new Foo("wot").x<any>().length