Generic Type Alias Overloading

Search Terms

• variadic type alias
• Generic variadic Type Alias
• Generic Overload
• Generic Type Alias Overloading

Suggestion

Ability to either:

• overload generic type definitions
  or
• define variadic type parameters for generic type aliases. (not sure how this would work)

Use Cases

My specific, immediate need is to create a generic variable number of input type alias for Intersection. I could see other use cases though, and imagine others would find it useful.

The closest work around is if I define default type parameters, like:

type Intersection<A, B, C = {} , D = {}, E = {}, F = {}> = A & B & C & D & E & F;

// and use like
let foo: Intersection<{ prop1: number}, {prop2: string}>;

// it evaluates to
{ prop1: number } &
{ prop2: number } &
{ } &
{ } &
{ } &
{ }

Examples

With overloads, the above type could be declared like:

type Intersection<A , B> = A & B;
type Intersection<A , B , C> = A & B & C;
type Intersection<A , B , C , D> = A & B & C & D;
type Intersection<A , B , C , D , E> = A & B & C & D & E;
type Intersection<A , B , C , D , E , F > = A & B & C & D & E & F;

I am not sure of a good syntax for the variadic type, it also seems like it would have a more limited use.

Checklist

My suggestion meets these guidelines: ( I think )

• 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.

Review the proposed generic type alias overloads and the alternative variadic type parameter idea in the issue examples. Determine the intended type-checking behavior, syntax, compatibility implications, and scope before identifying implementation and test entry points in the TypeScript compiler.