Proposal: Allow isolated declarations to infer results of constructor calls

🔍 Search Terms

isolated declarations constructor generic infer

✅ Viability Checklist

• 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 isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

Add a new flag or change the behavior of --isolatedDeclarations so that in the following code which is currently disallowed by isolated declarations:

export const foo = new Foo(a);

if the type of foo is not Foo, there is a typecheck error. In return, the type of foo will be emitted as Foo.

Similarly, if somehow in

export const foo: new Foo<A>(a);

foo is not a Foo<A>, there is a typecheck error.

📃 Motivating Example

Currently under isolated declarations, code must redundantly declare the type of a variable which is the result of a constructor call.

export const foo: Foo = new Foo(a);

This setting would also eliminate https://github.com/microsoft/TypeScript/issues/59768

💻 Use Cases

^

この issue ではファイル、テスト、エントリポイントが指定されていないため、まず既存の --isolatedDeclarations チェックと宣言推論の挙動を追跡してください。提案された設定でコンストラクター呼び出しの結果を推論でき、型チェックによって推論された型または注釈付きの型の不一致が拒否され、出力される JavaScript が変更されなければ完了です。