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.
  

リポジトリのファイルもテストも指定されていません。issue にある const arrow assertion の例を nightly TypeScript build で再現し、その後、既存の assertion-function の診断経路を調べてください。自明に検出できる arrow assertion が受け入れられるか、診断によって変数宣言自体に必要な明示的な型注釈がないことが説明されれば完了です。