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

^

The issue names no files, tests, or entry points; start by tracing the existing --isolatedDeclarations checks and declaration inference behavior. Done means constructor-call results can be inferred under the proposed setting, with typechecking rejecting mismatched inferred or annotated types without changing emitted JavaScript.