Add Extends

[danieldietrich]

Solves #13

[danieldietrich]

Hi @garronej, this PR is complete, however, I marked it as draft because of the following questions:

• is the prettier formatting of src/Extends.ts ok?
• is the handling of never ok? (see this discussion)

TODO: I will add the missing docs https://docs.tsafe.dev/main/extends

[danieldietrich]

I aligned the behavior of the type Extends to the one of the built-in extends TS functionality (see my second commit). Introducing another semantics seems odd to me and would confuse more than being helpful.

[danieldietrich]

Oh boy, I've found some kind of unexpected behavior of TypeScript:

type Extends<T1, T2> = T1 extends T2 ? true : false;

type A1 = never extends any ? true : false    // = true
type B1 = Extends<never, any>                 // = never ❌

type A2 = any extends never ? true : false    // = boolean
type B2 = Extends<any, never>                 // = boolean ✅

type A3 = never extends 1 ? true : false      // = true
type B3 = Extends<never, 1>                   // = never ❌

type A4 = 1 extends never ? true : false      // = false
type B4 = Extends<1, never>                   // = false ✅

type A5 = never extends never ? true : false; // = true
type B5 = Extends<never, never>               // = never ❌

The following does not solve the problem:

type Extends<T1, T2> = T1 extends never ? true : T1 extends T2 ? true : false;

However, the following does solve the problem:

type Extends<T1, T2> = StrictEquals<T1, never> extends true ? true : T1 extends T2 ? true : false;

I will provide a fix with another commit...

[garronej]

Hi @danieldietrich,

is the prettier formatting of src/Extends.ts ok?

LGTM

is the handling of never ok? (see #13 (comment) discussion)

It's probably a bug but let's leave it since I have no time to investigate right now. It's maybe on purpose.

TODO: I will add the missing docs https://docs.tsafe.dev/main/extends

<3

I will provide a fix with another commit...

Thanks you for taking this so seriously!

[danieldietrich]

Ok, now I consider the type logic to be complete. I will mark it as ready for review.

[danieldietrich]

Just a little remark regarding the following tests - I intentionally added duplications because it is important in the respective context to understand the whole picture. Overlapping tests do not really harm.

[garronej]

Got you!