Fix "never nullish" diagnostic missing expressions wrapped in parentheses #62789

Copilot · 2025-11-21T05:07:33Z

The "never nullish" diagnostic (TS2881) was not reported when the left operand of a ?? operator was wrapped in parentheses, causing valid errors to be silently ignored.

const x: { y: string | undefined } | undefined = undefined as any;

const foo = x?.y ?? `oops` ?? "";      // Error reported ✓
const bar = (x?.y ?? `oops`) ?? "";    // Error NOT reported (bug)

Changes

checker.ts: Modified isNotWithinNullishCoalesceExpression to use walkUpOuterExpressions() instead of directly checking node.parent, allowing it to traverse through parentheses and other outer expressions to find the actual parent binary expression
Tests: Added neverNullishThroughParentheses.ts test case and updated predicateSemantics.ts baseline to include additional errors now correctly detected

Impact

The fix catches previously missed errors in cases like:

(expr ?? value) ?? fallback - now detects never-nullish value
opt ?? (null ?? 1) - now detects never-nullish 1
Multiple levels of parenthesis nesting

Original prompt

This section details on the original issue you should resolve

<issue_title>"Never nullish" checks miss "never nullish" through pareans</issue_title>
<issue_description>### 🔎 Search Terms

never nullish parenthesis

🕗 Version & Regression Information

This changed between versions 5.8 and 5.9

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYewdgzgLgBAHgLhgbxgTydATgSzAcxgB8YBXMAEwFMAzPKimAX2LMtvsYF43q6wGMAIYRhYNAG4AsAChZs0JFg0QIGDzgB+AHRoYmzTAAGqgA4Qj+wwCJr0mYugwARkKzqYACi26rxsxYAlH629grgECAANlTaUSD4nioggfaO0bHxia5YqUA

💻 Code
const x: { y: string | undefined } | undefined = undefined as any;


const foo = x?.y ?? `oops` ?? "";
const bar = (x?.y ?? `oops`) ?? "";

console.log(foo);
console.log(bar);
🙁 Actual behavior

Error on foo's expression, no error on bar's.

🙂 Expected behavior

Error on both; both expressions are guaranteed to be "oops".

Additional information about the issue

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes "Never nullish" checks miss "never nullish" through parens #62787

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

Co-authored-by: jakebailey <[email protected]>

RyanCavanaugh · 2025-11-24T22:37:38Z

@typescript-bot test this
@typescript-bot pack this

typescript-bot · 2025-11-24T22:37:42Z

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
`pack this`	✅ Started	✅ Results
`test top400`	✅ Started	✅ Results
`user test this`	✅ Started	✅ Results
`run dt`	✅ Started	✅ Results
`perf test this faster`	✅ Started	👀 Results

typescript-bot · 2025-11-24T22:41:33Z

Hey @RyanCavanaugh, I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/166605/artifacts?artifactName=tgz&fileId=A2C1143A3223567266A61A051AE7DFAA71E2A7FAC36CCA19230F756A7766160902&fileName=/typescript-6.0.0-insiders.20251124.tgz"
    }
}

and then running npm install.

There is also a playground for this build and an npm module you can use via "typescript": "npm:@typescript-deploys/[email protected]".;

typescript-bot · 2025-11-24T22:59:31Z

Hey @RyanCavanaugh, the results of running the DT tests are ready.

Everything looks the same!

You can check the log here.

typescript-bot · 2025-11-24T23:06:00Z

@RyanCavanaugh Here are the results of running the user tests with tsc comparing main and refs/pull/62789/merge:

There were infrastructure failures potentially unrelated to your change:

1 instance of "Git clone failed"

Otherwise...

Everything looks good!

typescript-bot · 2025-11-24T23:31:04Z

@RyanCavanaugh
The results of the perf run you requested are in!

Here they are:

tsc

Comparison Report - baseline..pr

_Metric	_baseline	_pr	_Delta	_Best	_Worst	_p-value
_{Compiler-Unions - node (v18.15.0, x64)}
_Errors	₁	₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_62,370	_62,370	_~	_~	_~	_{p=1.000 n=6}
_Types	_50,386	_50,386	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{193,841k (± 0.81%)}	_{194,569k (± 0.96%)}	_~	_192,634k	_196,301k	_{p=0.378 n=6}
_{Parse Time}	_{1.30s (± 0.76%)}	_{1.30s (± 0.58%)}	_~	_1.29s	_1.31s	_{p=1.000 n=6}
_{Bind Time}	_0.75s	_{0.75s (± 0.54%)}	_~	_0.74s	_0.75s	_{p=0.405 n=6}
_{Check Time}	_{9.90s (± 0.46%)}	_{9.89s (± 0.33%)}	_~	_9.84s	_9.94s	_{p=0.935 n=6}
_{Emit Time}	_{2.73s (± 0.79%)}	_{2.74s (± 1.12%)}	_~	_2.70s	_2.78s	_{p=0.573 n=6}
_{Total Time}	_{14.68s (± 0.35%)}	_{14.68s (± 0.15%)}	_~	_14.66s	_14.72s	_{p=0.810 n=6}
_{angular-1 - node (v18.15.0, x64)}
_Errors	₂	₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_956,047	_956,047	_~	_~	_~	_{p=1.000 n=6}
_Types	_415,881	_415,881	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{1,255,006k (± 0.00%)}	_{1,255,010k (± 0.00%)}	_~	_1,254,967k	_1,255,062k	_{p=0.936 n=6}
_{Parse Time}	_{6.52s (± 0.76%)}	_{6.56s (± 0.73%)}	_~	_6.47s	_6.61s	_{p=0.332 n=6}
_{Bind Time}	_{1.96s (± 0.21%)}	_{1.96s (± 0.32%)}	_~	_1.95s	_1.97s	_{p=0.673 n=6}
_{Check Time}	_{32.40s (± 0.25%)}	_{32.39s (± 0.32%)}	_~	_32.26s	_32.52s	_{p=0.873 n=6}
_{Emit Time}	_{14.87s (± 0.27%)}	_{14.90s (± 0.28%)}	_~	_14.83s	_14.95s	_{p=0.261 n=6}
_{Total Time}	_{55.75s (± 0.17%)}	_{55.80s (± 0.15%)}	_~	_55.69s	_55.95s	_{p=0.173 n=6}
_{mui-docs - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_2,717,214	_2,717,214	_~	_~	_~	_{p=1.000 n=6}
_Types	_937,297	_937,297	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,044,183k (± 0.00%)}	_{3,044,181k (± 0.00%)}	_~	_3,044,113k	_3,044,274k	_{p=0.689 n=6}
_{Parse Time}	_{8.54s (± 0.29%)}	_{8.54s (± 0.40%)}	_~	_8.51s	_8.60s	_{p=0.936 n=6}
_{Bind Time}	_{2.32s (± 0.35%)}	_{2.32s (± 0.54%)}	_~	_2.30s	_2.33s	_{p=0.558 n=6}
_{Check Time}	_{93.14s (± 0.31%)}	_{93.14s (± 0.23%)}	_~	_92.80s	_93.35s	_{p=1.000 n=6}
_{Emit Time}	_{0.31s (± 2.44%)}	_{0.31s (± 1.68%)}	_~	_0.30s	_0.31s	_{p=0.784 n=6}
_{Total Time}	_{104.30s (± 0.29%)}	_{104.31s (± 0.19%)}	_~	_104.02s	_104.52s	_{p=1.000 n=6}
_{self-build-src - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,251,545	_1,251,546	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_259,756	_259,756	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{2,514,295k (± 7.47%)}	_{2,514,152k (±11.83%)}	_~	_2,392,166k	_3,121,774k	_{p=0.298 n=6}
_{Parse Time}	_{5.17s (± 1.25%)}	_{5.19s (± 1.07%)}	_~	_5.11s	_5.25s	_{p=0.689 n=6}
_{Bind Time}	_{1.83s (± 0.98%)}	_{1.81s (± 0.81%)}	_~	_1.79s	_1.82s	_{p=0.067 n=6}
_{Check Time}	_{35.31s (± 0.77%)}	_{35.47s (± 0.47%)}	_~	_35.33s	_35.80s	_{p=0.872 n=6}
_{Emit Time}	_{3.03s (± 0.74%)}	_{2.99s (± 2.16%)}	_~	_2.92s	_3.11s	_{p=0.065 n=6}
_{Total Time}	_{45.34s (± 0.47%)}	_{45.48s (± 0.40%)}	_~	_45.32s	_45.78s	_{p=0.689 n=6}
_{self-build-src-public-api - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,251,545	_1,251,546	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_259,756	_259,756	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,187,568k (± 0.03%)}	_{3,066,916k (± 9.64%)}	_~	_2,462,855k	_3,187,984k	_{p=0.936 n=6}
_{Parse Time}	_{6.81s (± 0.77%)}	_{6.77s (± 1.77%)}	_~	_6.58s	_6.91s	_{p=0.575 n=6}
_{Bind Time}	_{2.26s (± 2.19%)}	_{2.26s (± 1.39%)}	_~	_2.21s	_2.30s	_{p=0.748 n=6}
_{Check Time}	_{43.13s (± 0.29%)}	_{43.08s (± 0.27%)}	_~	_42.91s	_43.24s	_{p=0.575 n=6}
_{Emit Time}	_{3.52s (± 3.15%)}	_{3.48s (± 2.15%)}	_~	_3.39s	_3.58s	_{p=0.630 n=6}
_{Total Time}	_{55.73s (± 0.47%)}	_{55.59s (± 0.32%)}	_~	_55.34s	_55.83s	_{p=0.471 n=6}
_{self-compiler - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_264,177	_264,178	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_103,979	_103,979	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{443,228k (± 0.01%)}	_{443,189k (± 0.01%)}	_~	_443,124k	_443,279k	_{p=0.230 n=6}
_{Parse Time}	_{3.52s (± 0.93%)}	_{3.52s (± 0.84%)}	_~	_3.48s	_3.55s	_{p=1.000 n=6}
_{Bind Time}	_{1.38s (± 1.64%)}	_{1.38s (± 0.76%)}	_~	_1.36s	_1.39s	_{p=0.451 n=6}
_{Check Time}	_{19.21s (± 0.23%)}	_{19.16s (± 0.37%)}	_~	_19.10s	_19.28s	_{p=0.171 n=6}
_{Emit Time}	_{1.54s (± 0.86%)}	_{1.53s (± 1.04%)}	_~	_1.51s	_1.55s	_{p=0.498 n=6}
_{Total Time}	_{25.64s (± 0.24%)}	_{25.59s (± 0.22%)}	_~	_25.52s	_25.67s	_{p=0.172 n=6}
_{ts-pre-modules - node (v18.15.0, x64)}
_Errors	₇₂	₇₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_225,386	_225,386	_~	_~	_~	_{p=1.000 n=6}
_Types	_94,304	_94,304	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{370,073k (± 0.02%)}	_{370,054k (± 0.02%)}	_~	_369,950k	_370,210k	_{p=0.689 n=6}
_{Parse Time}	_{2.85s (± 0.92%)}	_{2.84s (± 1.13%)}	_~	_2.81s	_2.88s	_{p=0.627 n=6}
_{Bind Time}	_{1.65s (± 1.37%)}	_{1.64s (± 1.48%)}	_~	_1.60s	_1.67s	_{p=0.566 n=6}
_{Check Time}	_{16.65s (± 0.13%)}	_{16.64s (± 0.13%)}	_~	_16.61s	_16.67s	_{p=0.373 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{21.15s (± 0.20%)}	_{21.12s (± 0.14%)}	_~	_21.08s	_21.16s	_{p=0.199 n=6}
_{vscode - node (v18.15.0, x64)}
_Errors	₁₁	₁₂	_{🔻+1 (+ 9.09%)}	_~	_~	_{p=0.001 n=6}
_Symbols	_4,051,172	_4,051,172	_~	_~	_~	_{p=1.000 n=6}
_Types	_1,275,269	_1,275,269	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,837,189k (± 0.01%)}	_{3,837,075k (± 0.00%)}	_~	_3,836,965k	_3,837,318k	_{p=0.575 n=6}
_{Parse Time}	_{15.60s (± 0.60%)}	_{15.55s (± 0.40%)}	_~	_15.46s	_15.63s	_{p=0.377 n=6}
_{Bind Time}	_{5.24s (± 0.45%)}	_{5.29s (± 0.85%)}	_{+0.05s (+ 0.89%)}	_5.25s	_5.37s	_{p=0.043 n=6}
_{Check Time}	_{113.01s (± 3.12%)}	_{111.71s (± 2.99%)}	_~	_107.29s	_117.31s	_{p=0.575 n=6}
_{Emit Time}	_{40.74s (±15.04%)}	_{41.24s (±25.40%)}	_~	_32.25s	_60.13s	_{p=0.810 n=6}
_{Total Time}	_{174.60s (± 5.18%)}	_{173.79s (± 5.90%)}	_~	_164.16s	_188.25s	_{p=0.936 n=6}
_{webpack - node (v18.15.0, x64)}
_Errors	₄₀	₄₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_380,027	_380,027	_~	_~	_~	_{p=1.000 n=6}
_Types	_166,673	_166,673	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{539,669k (± 0.03%)}	_{539,599k (± 0.02%)}	_~	_539,407k	_539,726k	_{p=0.173 n=6}
_{Parse Time}	_{4.62s (± 0.50%)}	_{4.62s (± 0.47%)}	_~	_4.59s	_4.65s	_{p=0.747 n=6}
_{Bind Time}	_{2.00s (± 0.61%)}	_{2.01s (± 1.02%)}	_~	_1.98s	_2.04s	_{p=0.254 n=6}
_{Check Time}	_{22.98s (± 1.04%)}	_{23.01s (± 1.62%)}	_~	_22.66s	_23.51s	_{p=0.873 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{29.60s (± 0.79%)}	_{29.63s (± 1.27%)}	_~	_29.31s	_30.17s	_{p=0.810 n=6}
_{xstate-main - node (v18.15.0, x64)}
_Errors	₃₀	₃₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_690,397	_690,397	_~	_~	_~	_{p=1.000 n=6}
_Types	_208,790	_208,790	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{586,175k (± 0.02%)}	_{586,122k (± 0.02%)}	_~	_585,995k	_586,283k	_{p=0.689 n=6}
_{Parse Time}	_{4.16s (± 0.87%)}	_{4.17s (± 0.78%)}	_~	_4.13s	_4.21s	_{p=1.000 n=6}
_{Bind Time}	_{1.41s (± 0.73%)}	_{1.41s (± 1.07%)}	_~	_1.39s	_1.42s	_{p=0.564 n=6}
_{Check Time}	_{20.77s (± 2.10%)}	_{20.84s (± 1.78%)}	_~	_20.27s	_21.15s	_{p=0.575 n=6}
_{Emit Time}	_{0.00s (±154.76%)}	_{0.00s (±154.76%)}	_~	_0.00s	_0.01s	_{p=1.000 n=6}
_{Total Time}	_{26.35s (± 1.66%)}	_{26.42s (± 1.44%)}	_~	_25.88s	_26.75s	_{p=0.936 n=6}

System info unknown

Hosts

node (v18.15.0, x64)

Scenarios

Compiler-Unions - node (v18.15.0, x64)
angular-1 - node (v18.15.0, x64)
mui-docs - node (v18.15.0, x64)
self-build-src - node (v18.15.0, x64)
self-build-src-public-api - node (v18.15.0, x64)
self-compiler - node (v18.15.0, x64)
ts-pre-modules - node (v18.15.0, x64)
vscode - node (v18.15.0, x64)
webpack - node (v18.15.0, x64)
xstate-main - node (v18.15.0, x64)

_Benchmark	_Name	_Iterations
_Current	_pr	₆
_Baseline	_baseline	₆

Developer Information:

Download Benchmarks

typescript-bot · 2025-11-24T23:58:38Z

@RyanCavanaugh Here are the results of running the top 400 repos with tsc comparing main and refs/pull/62789/merge:

Everything looks good!

RyanCavanaugh

LGTM. For clarity, this code does go through casts, which it sounds like we're at least tentatively OK with

Copilot

Pull request overview

This PR fixes a bug where the "never nullish" diagnostic (TS2881) was not reported when the left operand of a nullish coalescing operator (??) was wrapped in parentheses, causing valid type errors to be silently ignored.

Key Changes:

Modified isNotWithinNullishCoalesceExpression in checker.ts to use walkUpOuterExpressions() instead of directly checking node.parent, allowing it to traverse through parentheses and other outer expressions
Added comprehensive test case neverNullishThroughParentheses.ts covering various levels of parenthesis nesting
Updated predicateSemantics.ts baseline to reflect additional errors now correctly detected in existing test cases

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
`src/compiler/checker.ts`	Fixed `isNotWithinNullishCoalesceExpression` to walk up through outer expressions (like parentheses) to find the actual parent binary expression
`tests/cases/compiler/neverNullishThroughParentheses.ts`	Added new test case demonstrating the bug fix with various levels of parenthesis nesting
`tests/baselines/reference/neverNullishThroughParentheses.errors.txt`	Baseline showing 8 errors correctly detected in the new test
`tests/baselines/reference/neverNullishThroughParentheses.types`	Type baseline for new test
`tests/baselines/reference/neverNullishThroughParentheses.symbols`	Symbol baseline for new test
`tests/baselines/reference/neverNullishThroughParentheses.js`	JS output baseline for new test
`tests/baselines/reference/predicateSemantics.errors.txt`	Updated baseline showing 3 additional errors now correctly caught in existing tests (lines with parenthesized expressions)

Initial plan

6585b21

Copilot AI assigned Copilot and jakebailey Nov 21, 2025

typescript-bot added For Uncommitted Bug PR for untriaged, rejected, closed or missing bug labels Nov 21, 2025

Copilot started work on behalf of jakebailey November 21, 2025 05:08 View session

Copilot AI and others added 2 commits November 21, 2025 05:17

Fix never nullish checks to work through parentheses

dc0d4ea

Co-authored-by: jakebailey <[email protected]>

Update predicateSemantics baseline with new errors caught by fix

1961197

Co-authored-by: jakebailey <[email protected]>

Copilot AI changed the title ~~[WIP] Fix never nullish checks to include parenthesis cases~~ Fix "never nullish" diagnostic missing expressions wrapped in parentheses Nov 21, 2025

Copilot AI requested a review from jakebailey November 21, 2025 05:42

Copilot finished work on behalf of jakebailey November 21, 2025 05:42

RyanCavanaugh approved these changes Dec 2, 2025

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jakebailey marked this pull request as ready for review December 2, 2025 20:59

Copilot AI review requested due to automatic review settings December 2, 2025 20:59

Copilot started reviewing on behalf of jakebailey December 2, 2025 21:00 View session

Copilot finished reviewing on behalf of jakebailey December 2, 2025 21:02

Copilot AI reviewed Dec 2, 2025

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Fix "never nullish" diagnostic missing expressions wrapped in parentheses #62789

Fix "never nullish" diagnostic missing expressions wrapped in parentheses #62789

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Fix "never nullish" diagnostic missing expressions wrapped in parentheses #62789

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