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fix(eslint-plugin): [no-unnecessary-type-parameters] descend into all parts of mapped types in no-unnecessary-type-parameters #9530

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merged 6 commits into from
Jul 14, 2024
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fix(eslint-plugin): [no-unnecessary-type-parameters] descend into all parts of mapped types in no-unnecessary-type-parameters #9530

merged 6 commits into from
Jul 14, 2024

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danvk
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@danvk danvk commented Jul 9, 2024 •
edited by kirkwaiblinger
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Overview

Mapped types have three parts: the type parameter, the template type and the constraint type:

> type.checker.typeToString(type)
'{ [key in K]?: V | undefined; }'
> type.checker.typeToString(type.typeParameter)
'key'
> type.checker.typeToString(type.templateType)
'V | undefined'
> type.checker.typeToString(type.constraintType)
'K'

The rule previously descended into the typeParameter. It doesn't need to descend into the constraintType because we get that through another code path. It does need to descend into the templateType, though.

One wrinkle is that if the mapped type is something like {[k in 'a']: T}, then TypeScript will treat it like {a: T} and we'll process it as an object type. In this case, we need to avoid also processing it as a mapped type, even though isMappedType returns true.

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Thanks for the PR, @danvk!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.37%. Comparing base (0d4b098) to head (9af9db6).
Report is 1 commits behind head on main.

Additional details and impacted files 
@@           Coverage Diff           @@
##             main    #9530   +/-   ##
=======================================
  Coverage   88.37%   88.37%           
=======================================
  Files         419      419           
  Lines       14618    14621    +3     
  Branches     4279     4280    +1     
=======================================
+ Hits        12919    12922    +3     
  Misses       1375     1375           
  Partials      324      324
Flag Coverage Δ
unittest 88.37% <100.00%> (+<0.01%) ⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Coverage Δ
...plugin/src/rules/no-unnecessary-type-parameters.ts 96.21% <100.00%> (+0.08%) ⬆️

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danvk commented Jul 9, 2024

Incidentally @JoshuaKGoldberg I'm not sure why the new test case doesn't trigger the "quick path:"

typescript-eslint/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts

Lines 59 to 63 in bebbd0c

// Quick path: if the type parameter is used multiple times in the AST,
// we don't need to dip into types to know it's repeated.
if (isTypeParameterRepeatedInAST(esTypeParameter, scope.references)) {
continue;
}

scope.references is empty for both K and V in:

declare function mapObj<
  K extends string,
  V,  // Type parameter V is used only once
>(
  obj: { [key in K]: V },
  fn: (key: K, val: V) => number,
): number[];

I would have expected it to contain the references.

@JoshuaKGoldberg JoshuaKGoldberg mentioned this pull request Jul 10, 2024
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LGTM, thanks for the fast follow! ⚡

Just requesting changes on a bit of testing. This basically an approval otherwise.

packages/eslint-plugin/tests/rules/no-unnecessary-type-parameters.test.ts Outdated Show resolved Hide resolved
packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts Show resolved Hide resolved
@JoshuaKGoldberg JoshuaKGoldberg added the awaiting response Issues waiting for a reply from the OP or another party label Jul 10, 2024
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danvk commented Jul 10, 2024

This should be good to go. I have no idea what the "Semantic Breaking Change PR / Validate Breaking Change PR (pull_request_target)" is about, and the error logs aren't illuminating:

Error: Cannot find module '@actions/core'

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oop #9533 @auvred - looks like they were necessary after all?

We can ignore them in this PR.

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Beautiful, thanks! 🙌

@auvred auvred mentioned this pull request Jul 10, 2024
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auvred commented Jul 10, 2024

oop #9533 @auvred - looks like they were necessary after all?

My bad, sorry 😬

#9535

@JoshuaKGoldberg JoshuaKGoldberg added 1 approval >=1 team member has approved this PR; we're now leaving it open for more reviews before we merge and removed awaiting response Issues waiting for a reply from the OP or another party labels Jul 10, 2024
@bradzacher bradzacher changed the title fix: descend into all parts of mapped types in no-unnecessary-type-parameters fix(eslint-plugin): [no-unnecessary-type-parameters] descend into all parts of mapped types in no-unnecessary-type-parameters Jul 14, 2024
@bradzacher bradzacher merged commit 10f8a4d into typescript-eslint:main Jul 14, 2024
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@bradzacher bradzacher bradzacher approved these changes

@JoshuaKGoldberg JoshuaKGoldberg JoshuaKGoldberg approved these changes

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Bug: [no-unnecessary-type-parameters] double false positive
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@danvk @JoshuaKGoldberg @auvred @jakebailey @bradzacher