Support `satisfies` in type declaration

Suggestion

satisfies should work in type declarations, similar to how it works currently.

i.e. similar to

const x = y satisfies Z;

we could have

type X = Y satisfies Z;

🔍 Search Terms

satisfies, type

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

📃 Motivating Example

Say there is some library, 'to-upper', that deals with lower case letters, both Roman and Greek.

// external library 'to-upper@1.0.0'

export type LowercaseChar = 
  | 'a' | 'b' | 'c' | 'd' | 'e' | 'f' | 'g' | 'h' | 'i' | 'j' | 'k' | 'l' | 'm' | 'n' | 'o' | 'p' | 'q' | 'r' | 's' 
  | 't'  | 'u' | 'v' | 'w' | 'x' | 'y' | 'z' |  'α' | 'β' | 'γ' | 'δ' | 'ε' | 'ζ' | 'η' | 'θ' | 'ι' | 'κ' | 'λ' 
  | 'μ' | 'ν' | 'ξ' | 'ο' | 'π' | 'ρ' | 'σ' | 'τ' | 'υ' | 'φ' | 'χ' | 'ψ' | 'ω';
export type UppercaseChar = ...;
export const toUpper = (lower: LowercaseChar): UppercaseChar => ...

And I want to use this library, but I actually only need to deal with a type that is narrower than LowercaseChar. I only need to deal with vowels. So I make my type,

// my-types.ts

export type LowercaseVowel = 'a' | 'e' | 'i' | 'o' | 'u' | 'α' | 'ε' | 'η' | 'ι' | 'ο' | 'ω' | 'υ';

and then I use it with 'to-upper'

// my-app.ts

import { lowerToUpper } from 'char.js';
import type { LowercaseVowel } from './my-types';

const myLowercaseVowel: LowercaseVowel = 'ω';
toUpper(myLowercaseVowel);

All good.

However, now the maintainer of "to-upper" decides they don't want to deal with Greek characters anymore, so they make a breaking change. Being diligent and considerate of their users, they update the LowercaseChar type definition as such:

// external library 'to-upper@2.0.0'
export type LowercaseChar = 
  | 'a' | 'b' | 'c' | 'd' | 'e' | 'f' | 'g' | 'h' | 'i' | 'j' | 'k' | 'l' | 'm' | 'n' | 'o' | 'p' | 'q' | 'r' | 's'
  | 't' | 'u' | 'v' | 'w' | 'x' | 'y' | 'z';
...

And I update my dependencies to to-upper@2.0.0.
It's true that my code will break on type checking, but it will fail where I've called toLower(), because my LowercaseVowel (which includes lowercase Greek characters) no longer satisfies the parameter of toLower(), LowercaseChar, which doesn't.

What would be preferable is if I had defined LowecaseVowel explicitly with the constraint that it satisfies LowecaseChar, i.e.

import type { LowercaseChar } from 'char.js';

type LowercaseVowel = 'a' | 'e' | 'i' | 'o' | 'u' | 'α' | 'ε' | 'η' | 'ι' | 'ο' | 'ω' | 'υ' satisfies LowercaseChar;

(Using the syntax suggested.)
If this were supported, I can see in the type declaration whether or not my narrow type satisfies the broader type.

💻 Use Cases

Similar to the above example, this could be used to narrow usages of overly broad types like any in third-party libraries, e.g.

// third-party library

export type Payload = { data: any };
export function handlePayload(payload: Payload) {
  ...
}

// my satisfying library

import { type Payload, handlePayload } from 'third-party-library';

export type SpecialPayload = { data: { foo: string } } satisfies Payload;
export function handleSpecialPayload(specialPayload: SpecialPayload) {
   handlePayload(specialPayload);
   ...
}

// consumer of my library
import { type SpecialPayload, handleSpecialPayload } from 'satisfying-library';

const mySpecialPayload: SpecialPayload = { data: { foo: 'bar' } };
handleSpecialPayload(mySpecialPayload);

In this particular contrived example, the same thing could be done with using interfaces interface SpecialPayload extends Payload { data: { foo: string; } }, but I'm sure you could think of more complex examples where interfaces cannot be used.

まず、既存の satisfies 式の動作と、この issue で提案されている type 宣言の例を確認します。動機となる例とユースケースを比較し、そのうえで、この機能に必要な型システムのセマンティクスと検証を決定します。完了条件には、承認された設計と、示されている narrowing およびライブラリ制約のシナリオに対するカバレッジが含まれる必要があります。