Generic types combined with & instead of |

[RoelVB]

🔎 Search Terms

generic, type, class, and, &, or, |, combined

⏯ Playground Link

Playground

💻 Code

class ExampleClass<T>
{
    test(value: T): T
    {
        return value;
    }
}

class Example1 extends ExampleClass<String> { }
class Example2 extends ExampleClass<Number> { }

function Example(example: 1|2) // returns `typeof Example1 | typeof Example2`
{
    return example===1?Example1:Example2;
}

const instance = new (Example(1))();
instance.test(''); // ERROR: Argument of type 'string' is not assignable to parameter of type 'Number & String'

🙁 Actual behavior

Combining classes with generic type results in generic types on arguments to be combined with &.
But get combined with | for return types.

🙂 Expected behavior

Expected the types to be combined with |

[whzx5byb]

This is a correct error. Here is a simple example in which your expected behavior is unsound:

class ExampleClass<T> {
    test(value: T): T {
        return value;
    }
}

class Example1 extends ExampleClass<string> { 
    test(value: string): string {
        return value.toLowerCase();
    }
}

class Example2 extends ExampleClass<number> { 
    test(value: number): number {
        return Number(value.toFixed(2));
    }
}

function Example(example: 1|2) // returns `typeof Example1 | typeof Example2`
{
    return Math.random() > 0.5 ? Example1 : Example2;
}

const instance = new (Example(1))();

console.log(instance.test('Foo'))
// You expect it return 'foo', but there's a 50% chance of runtime error

See: #30769 for more details.

[RoelVB]

I don't see how this would be correct.
In test(value: T): T the first T becomes String & Number, the second T becomes String | Number

[fatcerberus]

Yes, because the first T is in an input position (contravariant) and the other is in an output position (covariant). If you have a function and the only thing you know about it is that it accepts either an X or a Y, and will throw if you give it the wrong thing, the only thing you can safely pass to it is X & Y, i.e. a value that satisfies both types. That’s what the error you’re seeing reflects.

[RoelVB]

Thanks @fatcerberus, I understand now