Feature Request: Instantiate Generic Functions within Conditional Types

🔍 Search Terms

Instantiation Expressions, Conditional Types

✅ Viability Checklist

• 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 isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

I am looking for a way to be able to instantiate a generic function / return type via conditional type.

// some method

declare function method<T>(value: T): T

// ... this is possible

type R0 = ReturnType<typeof method<number>> 

// ... but this isn't 

type Infer<Func extends <T>(value: T) => unknown, U> = ReturnType<Func<U>>

type R1 = Infer<typeof method, number>

Related Issue: https://github.com/microsoft/TypeScript/issues/22617

📃 Motivating Example

I have written a small type-level parser for one of the libraries I manage. I have support for type parameterization.

Parameterized Type

const T = Syntax('number') 

const Vector = Syntax({ T }, `{ x: T, y: T, z: T }`)  // const T: TObject<{
                                                      //   x: TNumber,
                                                      //   y: TNumber,
                                                      //   z: TNumber
                                                      // }>

... but would also like to support generic functions as parameters (and resolve exterior types via function call / instantiation expression)

const Vector = <T extends TSchema>(T: T) => Type.Object({ x: T, y: T, z: T })

const T = Syntax({ Vector }, `Vector<number>`)        // const T: TObject<{
                                                      //    x: TNumber,
                                                      //    y: TNumber,
                                                      //    z: TNumber
                                                      // }>
                                      

When implementing the second case, I noticed it's not actually possible to instantiate the Vector signature within the context of a conditional type expression (i.e. the parser). In this case, I have a parsed TNumber type ready to supply the generic Vector function, but no way to instantiate it.

Essentially I would need the following but in a conditional type (Reference)

const Vector = <T extends TSchema>(T: T) => Type.Object({ x: T, y: T, z: T })

// ... internal to the parser

type ParsedNumber = TNumber 

type Instanced = ReturnType<typeof Vector<ParsedNumber>> // need this via conditional type

💻 Use Cases

1. What do you want to use this for?

Implement generic types in a type level syntax parser.

2. What shortcomings exist with current approaches?

Cannot instantiate generic signatures within a conditional type.

3. What workarounds are you using in the meantime?

I'm not sure there are workarounds.

Comienza con el comportamiento de los tipos condicionales y las expresiones de instanciación descrito en los ejemplos y, después, revisa el issue relacionado #22617. Se considera terminado cuando las firmas de funciones genéricas puedan instanciarse con argumentos de tipo proporcionados desde un tipo condicional, incluidos los casos mostrados de Infer y Vector, sin cambiar el JavaScript emitido.