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[no-unnecessary-type-assertion] false positive on uninitialized variables #453

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mysticatea opened this issue Apr 22, 2019 · 2 comments · Fixed by #478
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[no-unnecessary-type-assertion] false positive on uninitialized variables #453

mysticatea opened this issue Apr 22, 2019 · 2 comments · Fixed by #478
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bug Something isn't working has pr there is a PR raised to close this package: eslint-plugin Issues related to @typescript-eslint/eslint-plugin

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@mysticatea
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mysticatea commented Apr 22, 2019 •
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Repro

{
  "rules": {
    "@typescript-eslint/no-unnecessary-type-assertion": "error"
  }
}
function test() {
    let resolve: () => void
    new Promise(resolve0 => {
        resolve = resolve0
    })
    return resolve!
}

Expected Result

No errors because it's a syntax error if I removed the !.

Actual Result

6:12  error  This assertion is unnecessary since it does not change the type of the expression  @typescript-eslint/no-unnecessary-type-assertion

Additional Info

Versions

package version
@typescript-eslint/eslint-plugin 1.7.0
@typescript-eslint/parser 1.7.0
TypeScript 3.4.3
ESLint 5.16.0
node 10.15.3
npm 6.9.0
@mysticatea mysticatea added package: eslint-plugin Issues related to @typescript-eslint/eslint-plugin triage Waiting for team members to take a look labels Apr 22, 2019
@bradzacher
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bradzacher commented Apr 22, 2019 •
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I'm not sure that we can do anything about this.
The syntax error is because TS has decided the variable is uninitialised.
However "uninitialised" is not a type.
At the return statement, the variable has the type () => void, which is why there is an error - you're non-nulling a non-null type.
The operator is just telling ts to ignore the uninitialised state.

I haven't tested it, but I'd assume that the following code works:

function test() {
    let resolve: null | () => void = null
    new Promise(resolve0 => {
        resolve = resolve0
    })
    return resolve!
}

@bradzacher bradzacher added awaiting response Issues waiting for a reply from the OP or another party bug Something isn't working and removed triage Waiting for team members to take a look labels Apr 22, 2019
@mysticatea
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mysticatea commented Apr 22, 2019 •
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That rule has autofix and the autofix broke the code. ESLint must not fix code if it changed semantics. Generating syntax error is definitely a bug. Also, to tell the variable was actually initialized is a intentional feature of !. It's definitely not "unnecessary assertion" and the rule should not warn it.

@bradzacher bradzacher removed the awaiting response Issues waiting for a reply from the OP or another party label Apr 22, 2019
This was referenced Apr 26, 2019
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@bradzacher bradzacher added the has pr there is a PR raised to close this label Apr 26, 2019
@bradzacher bradzacher mentioned this issue May 15, 2019
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bug Something isn't working has pr there is a PR raised to close this package: eslint-plugin Issues related to @typescript-eslint/eslint-plugin
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