Search Terms

conditional assignable both

Suggestion

A type T should be assignable to A extends B ? C : D if it is assignable to both C and D. Previously mentioned here and here, but it doesn't appear that anyone ever filed an issue to track this.

It's unclear what to do if C references infer type parameters, but even an assignability rule that works only when C does not reference infer type parameters would be useful.

If the conditional type is distributive, then the rule only applies if A is known not to be never.

Use Cases

See this comment for one.

Examples

TBD

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. new expression-level syntax)