AllowJs + ES6 lib result in compiler crash

[JimmyPruitt]

TypeScript Version: 4.0.2

Search Terms:
typescript Cannot read property 'name' of undefined

Code
For reasons beyond my control, my team has a few node modules checked into our code that we subsequently run through the Typescript compiler as a part of building our front end through the ts-loader Webpack loader. We can probably work around this on our end because this case is special, but I'm reporting this because I have a hunch that there could be other legitimate cases that fall to the same error. Here's a minimal package.json and tsconfig.json that I can use to recreate the problem:

{
  "name": "broken-typescript",
  "version": "1.0.0",
  "scripts": {
    "build": "tsc -p tsconfig.json"
  },
  "dependencies": {
    "@okta/okta-signin-widget": "4.3.3",
    "typescript": "4.0.2"
  }
}

{
  "compilerOptions": {
    "rootDir": "./",
    "outDir": "out",
    "allowJs": true,
    "strict": true,
    "lib": ["es6"]
  },
  "include": [
    "./node_modules/@okta/okta-signin-widget/dist/js/okta-sign-in.js"
  ]
}

Just npm i && npm run build and you'll see the error

Expected behavior:
The compiler finishes without failure

Actual behavior:
An error gets thrown:

TypeError: Cannot read property 'name' of undefined

[frank-dspeed]

That is because https://unpkg.com/browse/@okta/okta-signin-widget@4.3.3/dist/js/okta-sign-in.js is not a JS Module its a webpack module TS does not support importing webpack modules at present.

[JimmyPruitt]

The same module works in version 3.9 of Typescript. If you change the TS version in that package ☝️ to 3.9.7 and run npm i && npm build, there are no errors and a file is emitted to the correct directory. Are you saying that this was an undocumented breaking change of version 4?

[ahejlsberg]

The root issue is that this produces an error:

declare let s: (...items: never[]) => never[];
let t: () => unknown[] = s;  // Error, but should be ok

To make matters worse, when attempting to report the erroneous error, we crash if there's a rest parameter:

declare let s: (...items: never[]) => never[];
let t: (...args: []) => unknown[] = s;  // Crash

The issue only occurs with a rest parameter of type never[] and it's caused by compareSignaturesRelated assuming that any (the type used for an unmatched parameter position) is assignable to anything. It is--except for never. We should change the logic to not even attempt to relate unmatched positions. I will put up a PR to that effect.