Assertion functions don't work when defined as methods

TypeScript Version:

3.9.0-dev.20200220

Search Terms:

assertion signature

Code

type Constructor<T> = new (...args: any[]) => T

let str: any = 'foo'

str.toUpperCase() // str is any

function assert(condition: unknown, message: string = 'Assertion failure', ErrorConstructor: Constructor<Error> = AssertionError): asserts condition {
    if(!condition) {
        throw new ErrorConstructor(message);
    }
}

assert(typeof str == 'string')

str.toUpperCase() // str is string

class AssertionError extends Error { }

class Assertion {
    assert(condition: unknown, message: string = 'Assertion failure', ErrorConstructor: Constructor<Error> = AssertionError): asserts condition {
        if(condition) {
            throw new ErrorConstructor(message);
        }
    }
}

let azzert = new Assertion().assert

let str2: any = 'bar'

azzert(typeof str2 == 'string')  //error 2775

str2.toUpperCase() // str2 is any

Expected behavior:

Assertion functions are usable as methods of a class

Actual behavior:

Assertions require every name in the call target to be declared with an explicit type annotation.(2775) ‘azzert’ needs an explicit type annotation.

Playground Link:

https://www.typescriptlang.org/play/?ts=3.9.0-dev.20200220&ssl=1&ssc=1&pln=33&pc=34#code/C4TwDgpgBAwg9gOwM7AE4FcDGw6oDwAqAfFALxQIQDuUAFAHSMCGqA5kgFxRMIgDaAXQCUZEgQCwAKCkAbCMCgpUXHiDJQA5ADM4cDVKlL6OAKphIqGEyQRaIgPT3FaKAEsk3Xgclb0CbK6I3Eg2qMC0mIgAJq7AgQhcfgDWCHBUCAA0UAC2ECFMrBBcSq4IrOoaAIIhEGHxUFpMrjLoqBAaWQCiqKi48MhoWDjKsIhKQ7h43b2oJOTVoXGI07hCKjVhHpEIMUsIUADeUlAnblq0AITbu-EiR5Knj1DAABa9NJQ0K5Zjg9i4tFy+UKQgA3MdTgBfKTQ6SSayLWigSBwLTOVBkcgaEplDRCbxGUzmWpWGx2KCOdFuDw41jeTAyBFQBa1PbfKAQAAewAgOw87IOUFhUgZTJZdSC90eCNZEWisXiiQQKTSmRyeSQBSK6NK5Sx4r2DSaLTaHSg33643+I0tf2GUx6uDmzI2bMdqDWwUWW3lhqlTxOrnO1wViDuEIDj1e7wo1HN7ttGGtgI1WrBEaesMesOFkjkCiYAC9C6z1J8XYt4nZ6DKwt58+iAEwqXgVABGLH0cKLJbCSPAEFRTcxmlpeIp9lqMygjYA7LOAKwEtCN4xwMwWUm2BxOJSN6meEBAA

Related Issues:

Perhaps this a design limitation per the following PR?

https://github.com/microsoft/TypeScript/pull/33622 https://github.com/microsoft/TypeScript/pull/33622#issuecomment-575301357

[RyanCavanaugh]

Assertion functions always need explicit annotations. You can write this:

let azzert: Assertion['assert'] = new Assertion().assert;

[btmorex]

Without being too repetitive, the following works for me:

type Assert = (condition: unknown, message?: string) => asserts condition;

const assert: Assert = (condition, message) => {
  if (!condition) {
    throw new Error(message);
  }
};

export default assert;