Custom syntax for distributive conditional

まだ誰も着手していません。

### 🔍 Search Terms

Distributive conditional, large type with numerous(10,20+) union types limitation, union type aggregation

### ✅ Viability Checklist

- [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
- [x] This wouldn't change the runtime behavior of existing JavaScript code
- [x] This could be implemented without emitting different JS based on the types of the expressions
- [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
- [x] This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
- [x] This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

### ⭐ Suggestion

My suggestion is to improve the current syntax of the awesome 'distributive conditional' feature.
This feature is pretty much hidden due to the verbosity required to use it and the inability to reason with the syntax.

My example may show an idea for how a syntax may look, it isn't covering everything (like for example the scope of distribution like T extends {SCOPE}, typically 'any')

### 📃 Motivating Example

```typescript
type A = {
A1: {
Type: "type.1";
Sub: {
Type: "type.1.sub.1" | "type.1.sub.2"
};
};
A2: {
Type: "type.2";
Sub: {
Type: "type.2.sub.1" | "type.2.sub.2"
};
}
}

// With generics it is simple
export namespace generic_example {
type B = {
MainType: MainType;
SubType: (A[keyof A] & {Type: MainType})["Sub"]["Type"]
};

// Results in a safe type when given a single type
const b:B<"type.1"> = {
MainType: "type.1",
SubType: "type.1.sub.1"
}
}

// Just a demonstration of a generally invalid example
export namespace invalid_example {
type B = {
MainType: A[keyof A]["Type"];
SubType: A[keyof A]["Sub"]["Type"]
};

// As expected, there are no type error, even though type.1.sub.1 is not part of type.2
const b:B = {
MainType: "type.2",
SubType: "type.1.sub.1"
}
}

// With distributive conditional
export namespace wild_syntax_example {
type theUnion = A[keyof A];

type B = theUnion extends infer U ? U extends {Type: any; Sub: {
Type: any
}} ? {
MainType: U["Type"];
SubType: U["Sub"]["Type"]
} : never : never;

const b:B = {
MainType: "type.1",
SubType: "type.1.sub.1"
}

const b_invalid:B = { //<-- Correctly guards the type, but can you really work like this?
MainType: "type.2",
SubType: "type.1.sub.1"
}
}

// Some ideas for syntax that can simplify it
export namespace suggested_syntax {
// You wont have access to the main A anymore but adding optional alias can solve it like A.{}
type Syntax1 = A.{
MainType: A["Type"];
SubType: A["Sub"]["Type"];
}

// Similar to destructure which could fit here nicely
type Syntax2 = {
...A {
Type,
Sub: {
Type
}
}
}
}

```

### 💻 Use Cases

1. What do you want to use this for?
I think this is a missing/hidden piece in typescript and should be added as soon as possible. Please let me know if you think otherwise.
3. What shortcomings exist with current approaches?
Long, easy to forget, super messy syntax for very "simple" things ( at least in most of my use cases )

### 🍭 Bonus addition
1. Optionally, add the option to specify something like proprietaryDistributiveConditional=true in tsconfig to allow to disable the current distributive conditional syntax so it is possible to use statements like T extends Union without distributing it.

まず、issue に記載されている動機となる例と既存の distributive conditional 構文を確認します。提案されている Syntax1 と Syntax2 の形式を比較し、分配のスコープの扱いと、オプションの設定という考え方も含めて検討します。具体的な構文と、それに対応する TypeScript の型チェック動作について合意できれば完了です。

索引モデルが issue の本文から書いたものです。