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

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El issue no menciona archivos, pruebas ni puntos de entrada; empieza rastreando las comprobaciones existentes de --isolatedDeclarations y el comportamiento de inferencia de declaraciones. Se considera completado cuando los resultados de las llamadas a constructores puedan inferirse con la configuración propuesta, y la comprobación de tipos rechace tipos inferidos o anotados que no coincidan sin cambiar el JavaScript emitido.