Better examples for Optional Variance Annotations for Type Parameters

[versed stream]

Hi all, I was reading the docs for previous typescript release and saw this part: https://devblogs.microsoft.com/typescript/announcing-typescript-4-7/#optional-variance-annotations-for-type-parameters, however even after reading through the examples, I still didn't understand what it did so I googled it, but I can't find a single result about it, would anyone be able to help explain it?

[plush cosmos]

in allows subtypes to be passed in, out allows casting the result to a supertype

or in other words:

• for Foo<out T>, Foo<T> extends Foo<U> when T extends U
• for Foo<in T>, Foo<U> extends Foo<T> when T extends U

[plush cosmos]

i think this explanantion is wrong

[grave otter]

type Extends<T extends U, U> = T;
type Foo<in T> = {};
type Bar<out T> = {};
type T1 = Extends<Foo<"foo">, Foo<string>>;
//                ^^^^^^^^^^
// Type 'Foo<"foo">' does not satisfy the constraint 'Foo<string>'.
//   Type 'string' is not assignable to type '"foo"'.
type T2 = Extends<Foo<string>, Foo<"foo">>;
type T3 = Extends<Bar<"foo">, Bar<string>>;
type T4 = Extends<Bar<string>, Bar<"foo">>;
//                ^^^^^^^^^^^
// Type 'Bar<string>' does not satisfy the constraint 'Bar<"foo">'.
//   Type 'string' is not assignable to type '"foo"'.

type Extends<T extends U, U> = T;
type Foo<T> = (arg: T) => void;
type Bar<T> = { value: T; };
type T1 = Extends<Foo<"foo">, Foo<string>>;
//                ^^^^^^^^^^
// Type 'Foo<"foo">' does not satisfy the constraint 'Foo<string>'.
//   Type 'string' is not assignable to type '"foo"'.
type T2 = Extends<Foo<string>, Foo<"foo">>;
type T3 = Extends<Bar<"foo">, Bar<string>>;
type T4 = Extends<Bar<string>, Bar<"foo">>;
//                ^^^^^^^^^^^
// Type 'Bar<string>' does not satisfy the constraint 'Bar<"foo">'.
//   Type 'string' is not assignable to type '"foo"'.

[plush cosmos]

here Foo and Bar are real types for comparison

a function that accepts all strings naturally also can accept "foo"

and conversely, a value that is "foo" is also a string

[versed stream]

thanks, this makes sense

[plush cosmos]

!close