Warn/error when testing a non-nullable type for null

TypeScript Version: 2.0.0

TL;DR:

If a project is configured for strict null checks, checking a variable which is typed as non-null is nonsense and could indicate a type mismatch.

Code:

Here's a motivating example I experienced:

async function someFunc1(): Promise<string | null> {
  // do some stuff and return the appropriately typed promise
}

async function someFunc2(): Promise<boolean> {
  // Negating a non-null Promise is crazy talk, but here I go!
  return !!someFunc1()
}

Of course what I really meant was:

async function someFunc2(): Promise<boolean> {
  return !!(await someFunc1())
}

Expected behavior:

TypeScript warns that I'm effectively testing my non-null Promise for null, and points out that that's nonsense.

Actual behavior:

TypeScript happily accepted the code, which led to a bug.

L’issue ne nomme aucun fichier, test ni point d’entrée. Commencez par examiner le comportement de strict-null-checking décrit dans les exemples et suivez la manière dont TypeScript traite les tests de null sur des expressions non nullables ; le travail est terminé lorsque la vérification incorrecte produit un diagnostic sans rejeter les vérifications nullable valides.