How do I specify generic type parameters for a function invocation in Closure?

Question

I am working on reducing the use of accidentally-unknown generic type parameters in a code base. I am using a Closure TTL incantation like the following to coerce unknown generic types to the undefined type, hopefully provoking errors that let me find these cases (see this related question).

 * @template T
 * @template TYPE_I_ACTUALLY_USE := cond(isUnknown(T), 'undefined', T) =:

That has brought me to a case like this:

/**
 * @template T
 * @template R := cond(isUnknown(T), 'undefined', T) =: 
 * @param {function():R} factory
 * @return {{prop: R}}
 */
function factoryWrapper(factory) {
  return {prop: factory()};
}

var instance = factoryWrapper(() => 2);

This factoryWrapper function takes a factory function that produces a value of type R, calls it, and wraps the result in {prop: ...}, for a return type of {prop: R}. But because I don't specify T or R when calling factoryWrapper, it doesn't know what type they're supposed to be, and can't tell whether the provided factory function returns the right type, so it produces an error or untyped output:

input0:11: WARNING - inconsistent return type
found   : number
required: undefined
var instance = factoryWrapper(() => 2);

In a language with a native type annotation syntax, like TypeScript, I might fix this by including the generic type parameter in the invocation:

var instance = factoryWrapper<number>(() => 2);

However, I haven't been able to figure out any equivalent syntax for Closure. I took a few guesses, like the following, but none worked.

var instance = factoryWrapper/** <number> */(() => 2);

How do I specify generic type parameters for a function invocation in Closure JavaScript?

Answer 1

Closure Compiler doesn't allow you to specify type parameters when calling a generic function. But I think casting the callback will do what you want. Something like: var instance = factoryWrapper(/** @type {function(): number} */ (() => 2));