Bug: no-unnecessary-condition false negative inside string and number conditionals

[guilhermesimoes]

Before You File a Bug Report Please Confirm You Have Done The Following...

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have searched for related issues and found none that matched my issue.
• I have read the FAQ and my problem is not listed.

Playground Link

https://typescript-eslint.io/play/#ts=5.2.2&fileType=.tsx&code=CYUwxgNghgTiAEEQBd4CMD2GIC51aSgDsBuAWACgBLAM3gApNsBKeAb0vnloaYlY4UuXMBiIBnbCAB0EDAHNGBeAB8V8AOQ0sG5uSHwAvpWMVKoSLARJURAK4BbPPYdoQMfT3ouBnbnW9HAXgAehD4cQALDDsIYHhQZHBUdxgMGHhI9xA-ETFJJFkFQIdVdS0dPVDwqJi4hJRk%2BFT0zOy-U1NzcGg4RBQI5Bg8cSGqInlPANGYXwMvGeCwiOjY%2BMSmloysuFz4UQkpIsUZss1tDF0SapW69cawFJg07faDTqA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Jge1tiacTJTIAhtEK0yHJgBNK%2BSpPRRE0aB2iRwYAL4gtQA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA&tokens=false

Repro Code

declare let str: string;
if (str) {
  if (str) {
    console.log(str || 'foo');
  }
}

ESLint Config

module.exports = {
  parser: "@typescript-eslint/parser",
  rules: {
    "@typescript-eslint/no-unnecessary-condition": "error",
  },
};

tsconfig

{
  "compilerOptions": {
    "strictNullChecks": true
  }
}

Expected Result

The error "Unnecessary conditional, value is always truthy" should be reported on lines 3 and 4

Actual Result

No errors are detected

Additional Info

I discovered this because I was trying out eslint-plugin-sonarjs, and that plugin has a similar rule called no-gratuitous-expression that does detect these cases. The code for that rule is available here.

[bradzacher]

This is working as expected.
The no-unnecessary-condition rule is powered solely by types.

Under-the-hood within TS the boolean type is actually defined as a union type (true | false) so it follows logically that TS can refine boolean to true or false based on a conditional check.

However other primitives are not and cannot be defined like that. Which means that the result of refining string via a conditional is always going to be string.
There's talk of "negated types" (eg microsoft/TypeScript#4196) which would allow TS to refine this case to eg string & not '' but currently that feature doesn't exist.
TS also doesn't narrow the false branch to values like '' or 0 | NaN - presumably for simplicity's sake.

---

The rule is already complicated enough going through all of the logic required to evaluate types - and I don't think there's much value in adding a lot of additional complexity to catch a relatively rare case - especially when there are other rules that already exist and catch the case.