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.

Nell’issue non sono indicati file, test o punti di ingresso. Inizia esaminando il comportamento di strict-null-checking descritto negli esempi e segui il modo in cui TypeScript gestisce i test di null sulle espressioni non nullable; il lavoro è completo quando il controllo errato produce una diagnostica senza rifiutare i controlli nullable validi.