[no-unnecessary-type-assertion] Exhaustiveness check in TS requires ! to "reset" the never type

[Skalman]

Repro

{
  "rules": {
    "@typescript-eslint/no-unnecessary-type-assertion": "error"
  }
}

type A = { kind: "abc" } | { kind: "def" };

function f(a: A) {
  switch (a.kind) {
    case "abc":
      return 1;
    case "def":
      return 2;
    default:
      // This `!` is necessary:
      throw new Error("Bad kind: " + a!?.kind);
  }
}

Expected Result
No error, since the ! is necessary.

Actual Result

"This assertion is unnecessary since it does not change the type of the expression @typescript-eslint/no-unnecessary-type-assertion"

Versions

package	version
@typescript-eslint/eslint-plugin	2.9.0
@typescript-eslint/parser	2.9.0
TypeScript	3.7.2
ESLint	6.7.1
node	12.10.0
yarn	1.19.2

[bradzacher]

Hmm. This is a really really weird behaviour within the typescript typechecker.
I'm not sure if we can detect this.

Without the non-null assertion, the type of a is never, however, with the operator, typescript instead reports the type of a as A.

Better described by the on-hover information:

The reason the rule reports on the usage of the !, is because typescript tells us the type is A, which is non-nullable.

This probably deserves a bug report of some kind on the typescript side..

[Skalman]

Huh, weird. Would you like me to report the TypeScript bug?

Edit: Done, see ref below.

[bradzacher]

microsoft/TypeScript#35431 has been closed and the fix will be released with the next TS version