Improve type inference by object parameters types

[jozefchutka]

Suggestion

🔍 Search Terms

inference

✅ 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 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 feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

It would be nice if typescript could recognize/infer and apply safe guard for the following scenario:

type T1 = {foo:number | undefined}
type T2 = {foo:number}

const t1:T1 = {foo:123};
if(t1.foo !== undefined) {
    const t2:T2 = t1;
    console.log(t2);
}

Currently the compiler fails with the following (playground)

Type 'T1' is not assignable to type 'T2'.
  Types of property 'foo' are incompatible.
    Type 'number | undefined' is not assignable to type 'number'.
      Type 'undefined' is not assignable to type 'number'.

[MartinJohns]

Essentially the same issue as #50651.

But this would easily lead to unsoundness. You could store t2 somewhere, and set the property to undefined via t1.

[typescript-bot]

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

[ericbf]

But this would easily lead to unsoundness. You could store t2 somewhere, and set the property to undefined via t1.

The same can be said for almost any valid typescript.