Either understand const arrow assertions or give better errors on them

As mentioned by @threehams in https://github.com/microsoft/TypeScript/issues/35838#issuecomment-569832017, people are pretty confused about the rules for assertion functions, and I don't blame them. For example

const assert = (blah: unknown): asserts blah => { throw "hai"; }

let x: string | undefined;;
assert(x);

In our nightly releases, we give an elaboration on the declaration of assert like

'assert' needs an explicit type annotation.

For all intents and purposes, it looks like assert does have an annotation! The problem is that assert itself needs a : (x: unknown) => asserts x, because it's not automatically inferred from the arrow function.

We have two options:

• Remove this restriction when the declaration of an assertion function is trivially detectable.

• Make assertion function errors more clear for arrow functions:

  'assert' needs an explicit type annotation. While the function it has been assigned to is fully annotated, the variable declaration for 'assert' does not.
  

Aucun fichier du dépôt ni aucun test n’est indiqué. Reproduisez l’exemple d’assertion de fonction fléchée const de l’issue dans un build nightly de TypeScript, puis examinez le chemin de diagnostic existant pour les fonctions d’assertion. C’est terminé si les assertions de fonction fléchée triviales à détecter sont acceptées ou si le diagnostic explique que la déclaration de variable elle-même ne comporte pas l’annotation de type explicite requise.