Ability to raise type checking error using something like a typeerror type.

[vilicvane]

Search Terms

typeerror type, user-defined type checking

Suggestion

Conditional type brings a useful method for composing types, but there are many scenarios that conditional type works while losing type safety. It would be nice to have a special type like typeerror to notify the compiler of incorrect types:

let foo: typeerror;
    ^^^ error

let bar: {yo: string; ha: typeerror};
    ^^^ error

Examples

interface Foo {
  yo: string;
  ha: number;
  pia: boolean;
};

type Bar = {
  [K in keyof Foo]:
    Foo[K] extends string ? string[] :
    Foo[K] extends number ? number :
    typeerror;
};

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)

[DanielRosenwasser]

This has come up, but the biggest question is really just a matter of where and how to give the error when the type appears in an arbitrarily complex type.

[mattmccutchen]

Looks like a duplicate of #23689.

[vilicvane]

@mattmccutchen Thanks for referring.