Type inference should consider all function parameters

[jeengbe]

Suggestion

🔍 Search Terms

Inference, Function, Generic, Parameters

✅ 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 behaviour 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.

⭐ Suggestion

When inferring type parameters of a function based on parameters, TypeScript should consider all parameters that use the generic type.

// @errors: 2345
interface A {
  x: string;
  y?: string;
};

declare function fn<opts extends A>(
  opts1: Partial<opts>,
  opts2: Partial<opts>
): void;


  fn({ x: "x" }, { y: "y" });
//^?
  fn<{ x: string, y: string }>({ x: "x" }, { y: "y" });
//^?

Workbench Repro

In this example, in the first function call narrows opts down to { x: string } based on the first parameter opts1 only. However, as opts is used for both opts1 and opts2, TypeScript should consider both parameters when narrowing a type for opts.
As seen in the second call, a solution does exist in which the constraint is met and neither parameter errors.

In the meantime, this can either be completely avoided by a different solution, or the generic type can be passed explicitly.

[MartinJohns]

See #32596 (comment)

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.