Strict version of Extract for literal union types

Search Terms

extract, pick, union, literal, string

Suggestion

Currently we have Pick<T, K> to select a type-safe subset of a given type T using a union of keys K. Unfortunately this does not work for unions of literals and requires Extract which subjectively isn't strict enough:

type ShirtSizes = "xxxl" | "xxl" | "xl" | "l" | "m" | "s" | "xs" | "xxs";
type Subset = Extract<ShirtSizes, "l" | "m" | "s" | "this-is-allowed-as-well">;

One caveat is that Extract does not support code completion when writing the literal union for the keys and it allows keys not in the first type as well. The former is error-prone while the latter is not causing a different behavior since the illegal keys are ignored but it can't be strictly checked in the long run. Of course I understand there are use cases wherein you'd like to use Extract more liberally so the definition

type Extract<T, U> = T extends U ? T : never;

is reasonable enough. Nevertheless, I'd like to propose an additional strict variant as follows

// short-hand form
type ExtractStrict<T, U extends T> = Extract<T, U>;
// equivalent full form
type ExtractStrict<T, U extends T> = T extends U ? T : never;

Use Cases

type ShirtSizes = "xxxl" | "xxl" | "xl" | "l" | "m" | "s" | "xs" | "xxs";
// this allows for code completion
type Subset = ExtractStrict<ShirtSizes, "l" | "m" | "s">;
// this is causing an error
type Subset2 = ExtractStrict<ShirtSizes, "l" | "m" | "s" | "illegal-literal">;

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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Start by reviewing the existing Extract utility type and the issue's ShirtSizes examples. Determine how a strict variant would fit TypeScript's type-system design and test behavior, including rejection of illegal literals and support for completion. Done means the proposed type is specified, implemented, and covered by compiler tests.