[no-unnecessary-type-assertion] False positive with value that can have the value null

[mdouglass]

Ok, I know I already tried to open this as #1195, but I totally borked that report. Hopefully, I'm not doing it again 🤞, the issue was with no-unnecessary-type-assertion all along.

Repro

{
  "parser": "@typescript-eslint/parser",
  "parserOptions": { "project": "./tsconfig.json", "sourceType": "module" },
  "plugins": ["@typescript-eslint"],
  "rules": {
    "@typescript-eslint/no-unnecessary-type-assertion": "error"
  }
}

function testFunction(_param: string | undefined): void { /* noop */ }
const value = 'test' as string | null | undefined
testFunction(value!)

Expected Result
No errors

Actual Result
3:14 error This assertion is unnecessary since the receiver accepts the original type of the expression @typescript-eslint/no-unnecessary-type-assertion

Additional Info
With the ! this compiles, but if the ! is removed, then you get a compiler error:

Argument of type 'string | null | undefined' is not assignable to parameter of type 'string | undefined'.
  Type 'null' is not assignable to type 'string | undefined'.ts(2345)

Versions

package	version
@typescript-eslint/eslint-plugin	2.6.1
@typescript-eslint/parser	2.6.1
TypeScript	3.7.2
ESLint	6.6.0
node	13.1.0
npm	6.12.1

[bradzacher]

This report makes more sense :)

This is actually something I found a while ago but never got around to fixing.
I'll see if I can put up a PR today.

[mdouglass]

Thank goodness :)
And thanks for taking a look at it!