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fix(eslint-plugin): [no-unnecessary-type-assertion] false positive on non-null assertion after an implicitly-any variable gets initialised inside conditional block #11082

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fix(eslint-plugin): [no-unnecessary-type-assertion] false positive on non-null assertion after an implicitly-any variable gets initialised inside conditional block #11082

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6 changes: 4 additions & 2 deletions packages/eslint-plugin/src/rules/no-unnecessary-type-assertion.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -118,9 +118,11 @@ export default createRule<Options, MessageIds>({
if (
// is it `const x!: number`
declaration.initializer == null &&
declaration.exclamationToken == null &&
declaration.type != null
declaration.exclamationToken == null
) {
if (declaration.type == null) {
return true;
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We're definitely in the right ballpark, but I notice that this introduces a regression with the following case:

function foo() {
  let x
  if (Math.random() > 0.5) {
    x = 3;
  } else {
    x = 4;
  }
  // should be flagged as unnecessary
  x!;
}

I wonder if it's possible to get that case correct still? (it may or may not be feasible to prevent some false positives or negatives)

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actually - I think you can ignore this regression... This is most likely a symptom of #10334 / microsoft/TypeScript#60514

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@mdm317 mdm317 Apr 18, 2025 •
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@kirkwaiblinger
Thanks for review! 🙏

The regression also occurs in this simplified case:

function foo() {
  let x;
  x =4;
  // should be flagged as unnecessary
  x!;
}

This is most likely a symptom of #10334 / microsoft/TypeScript#60514

As you mentioned, it's not possible to fix that case for now.

Should I close this PR until that issue is resolved?

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Hmmm. I'm not sure which choice is the lesser of two evils here... fixing the bug at the cost of that rather undesirable-looking regression, or marking the bug as blocked. I'm gently leaning towards making this work as blocked unless we can find away to proceed while mitigating this regression 🤔 But then again if it was that important of a case, you'd think we might have a test case that caught it?

Looking for thoughts from @typescript-eslint/triage-team too.

}
// check if the defined variable type has changed since assignment
const declarationType = checker.getTypeFromTypeNode(declaration.type);
const type = getConstrainedTypeAtLocation(services, node);
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7 changes: 7 additions & 0 deletions packages/eslint-plugin/tests/rules/no-unnecessary-type-assertion.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -429,6 +429,13 @@ declare function foo<T extends unknown>(bar: T): T;
const baz: unknown = {};
foo(baz!);
`,
`
let x;
if (Math.random() > -1) {
x = 2;
}
x!;
`,
],

invalid: [
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