Only check properties with multiple declarations for never-reduction by ahejlsberg · Pull Request #64499 · microsoft/TypeScript

Anders Hejlsberg (ahejlsberg) · 2026-09-28T15:34:57Z

Inspired by #64476, this PR optimizes getReducedType by checking only properties with declarations in multiple intersection constituents for reduction to never. The implementation is simpler than #64476 but should have the same effect.

Fixes #64474.

Anders Hejlsberg (ahejlsberg) · 2026-09-28T15:35:18Z

TypeScript Bot (@typescript-bot) test it

typescript-automation · 2026-09-28T15:35:25Z

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

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

Copilot

Copilot review overview

🟡 Changes recommended

The new pruning behavior lacks regression coverage for reduction edge cases.

Review effort: Balanced
Findings: 1

Open (1)

Add regression tests for intersection pruning reduction behavior · New

What changed in this PR

Optimizes intersection reduction by avoiding unnecessary combined-property creation.

Changes:

Counts property names across intersection constituents.
Checks only names appearing in multiple constituents for never reduction.

File	Description
`tsc/internal/checker/checker.go`	Adds targeted intersection-property reduction logic.

Anders Hejlsberg (ahejlsberg) · 2026-09-28T17:41:09Z

 	return t
 }

+func (c *Checker) somePropertyReducesToNever(t *Type) bool {


There are plenty of tests that already prove the existing reduction behavior.

typescript-automation · 2026-09-28T16:02:04Z

Anders Hejlsberg (@ahejlsberg)
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 - native}
_Errors	₄₁	₄₁	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_115,480	_{115,332 (± 0.00%)}	_{-148 (- 0.13%)}	_115,327	_115,337	_{p=0.000 n=12}
_Types	_96,949	_{90,333 (± 1.67%)}	_{🟩-6,616 (- 6.82%)}	_85,923	_93,354	_{p=0.000 n=12}
_{Memory Used}	_{148,590k (± 0.42%)}	_{145,270k (± 0.54%)}	_{-3,320k (- 2.23%)}	_144,002k	_147,132k	_{p=0.000 n=12}
_{Memory Allocs}	_{2,271,613 (± 0.01%)}	_{2,233,426 (± 0.30%)}	_{-38,187 (- 1.68%)}	_2,215,958	_2,249,382	_{p=0.000 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.041s (± 3.62%)}	_{0.040s (± 3.41%)}	_~	_0.036s	_0.043s	_{p=0.866 n=12}
_{Bind Time}	_{0.013s (±13.21%)}	_{0.012s (± 9.93%)}	_~	_0.010s	_0.016s	_{p=0.483 n=12}
_{Check Time}	_{0.497s (± 0.67%)}	_{0.498s (± 0.93%)}	_~	_0.488s	_0.514s	_{p=0.559 n=12}
_{Emit Time}	_{0.269s (± 2.04%)}	_{0.268s (± 1.93%)}	_~	_0.252s	_0.282s	_{p=0.681 n=12}
_{Total Time}	_{0.827s (± 0.93%)}	_{0.825s (± 0.65%)}	_~	_0.814s	_0.840s	_{p=0.619 n=12}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_{847,449 (± 0.08%)}	_{846,851 (± 0.07%)}	_~	_845,103	_847,992	_{p=0.101 n=12}
_Types	_{250,003 (± 0.00%)}	_{249,974 (± 0.01%)}	_{-29 (- 0.01%)}	_249,938	_250,012	_{p=0.003 n=12}
_{Memory Used}	_{793,750k (± 0.08%)}	_{790,591k (± 0.07%)}	_{-3,159k (- 0.40%)}	_789,112k	_792,107k	_{p=0.000 n=12}
_{Memory Allocs}	_{12,970,052 (± 0.12%)}	_{12,937,786 (± 0.07%)}	_{-32,266 (- 0.25%)}	_12,931,035	_12,980,389	_{p=0.000 n=12}
_{Config Time}	_0.016s	_0.016s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.271s (± 2.63%)}	_{0.273s (± 2.23%)}	_~	_0.255s	_0.285s	_{p=0.723 n=12}
_{Bind Time}	_{0.068s (±14.69%)}	_{0.065s (±16.57%)}	_~	_0.057s	_0.117s	_{p=0.210 n=12}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=12}
_{Emit Time}	_{1.633s (± 1.64%)}	_{1.614s (± 1.33%)}	_~	_1.584s	_1.671s	_{p=0.259 n=12}
_{Total Time}	_{2.008s (± 1.87%)}	_{1.986s (± 1.47%)}	_~	_1.939s	_2.077s	_{p=0.326 n=12}
_{mui-docs - native}
_Errors	_{11,399 (± 0.03%)}	_{11,400 (± 0.04%)}	_~	_11,382	_11,405	_{p=0.364 n=12}
_Symbols	_4,303,968	_{4,092,318 (± 0.02%)}	_{🟩-211,651 (- 4.92%)}	_4,090,308	_4,094,053	_{p=0.000 n=12}
_Types	_1,511,034	_{1,502,553 (± 0.00%)}	_{-8,481 (- 0.56%)}	_1,502,488	_1,502,588	_{p=0.000 n=12}
_{Memory Used}	_{4,766,018k (± 0.03%)}	_{2,953,064k (± 0.05%)}	_{🟩-1,812,954k (-38.04%)}	_2,948,725k	_2,955,670k	_{p=0.000 n=12}
_{Memory Allocs}	_{42,541,906 (± 0.04%)}	_{37,281,228 (± 0.07%)}	_{🟩-5,260,678 (-12.37%)}	_37,217,276	_37,349,519	_{p=0.000 n=12}
_{Config Time}	_{0.016s (± 1.98%)}	_{0.015s (± 2.10%)}	_~	_0.015s	_0.016s	_{p=0.414 n=12}
_{Parse Time}	_{0.583s (± 1.69%)}	_{0.582s (± 1.52%)}	_~	_0.548s	_0.598s	_{p=0.580 n=12}
_{Bind Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{14.830s (± 0.54%)}	_{8.733s (± 0.53%)}	_{🟩-6.097s (-41.11%)}	_8.619s	_8.890s	_{p=0.000 n=12}
_{Emit Time}	_{0.464s (± 2.32%)}	_{0.468s (± 9.57%)}	_~	_0.425s	_0.677s	_{p=0.504 n=12}
_{Total Time}	_{16.647s (± 0.46%)}	_{10.579s (± 0.46%)}	_{🟩-6.068s (-36.45%)}	_10.467s	_10.720s	_{p=0.000 n=12}
_{strada-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,394,252	_{1,391,780 (± 0.00%)}	_{-2,473 (- 0.18%)}	_1,391,695	_1,391,852	_{p=0.000 n=12}
_Types	_443,566	_{442,846 (± 0.01%)}	_{-721 (- 0.16%)}	_442,717	_443,084	_{p=0.000 n=12}
_{Memory Used}	_{1,685,197k (± 0.75%)}	_{1,675,091k (± 0.62%)}	_~	_1,655,442k	_1,721,368k	_{p=0.128 n=12}
_{Memory Allocs}	_{93,459,680 (± 0.14%)}	_{92,666,861 (± 0.12%)}	_{-792,819 (- 0.85%)}	_92,462,909	_92,958,778	_{p=0.000 n=12}
_{Config Time}	_{0.004s (±11.74%)}	_{0.004s (±13.36%)}	_~	_0.002s	_0.005s	_{p=0.677 n=12}
_{Parse Time}	_{0.201s (± 3.70%)}	_{0.204s (± 4.76%)}	_~	_0.184s	_0.234s	_{p=0.943 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.841s (± 0.45%)}	_{1.814s (± 0.60%)}	_{-0.028s (- 1.51%)}	_1.783s	_1.835s	_{p=0.000 n=12}
_{Emit Time}	_{0.302s (± 3.75%)}	_{0.294s (± 5.71%)}	_~	_0.263s	_0.340s	_{p=0.523 n=12}
_{Total Time}	_{23.791s (± 0.61%)}	_{23.425s (± 0.59%)}	_{-0.366s (- 1.54%)}	_23.142s	_23.820s	_{p=0.001 n=12}
_{strada-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_337,531	_{335,926 (± 0.01%)}	_{-1,605 (- 0.48%)}	_335,863	_335,965	_{p=0.000 n=12}
_Types	_198,885	_{198,347 (± 0.03%)}	_{-538 (- 0.27%)}	_198,206	_198,480	_{p=0.000 n=12}
_{Memory Used}	_{319,674k (± 0.02%)}	_{314,338k (± 0.03%)}	_{-5,336k (- 1.67%)}	_314,117k	_314,537k	_{p=0.000 n=12}
_{Memory Allocs}	_{4,673,007 (± 0.01%)}	_{4,624,240 (± 0.02%)}	_{-48,767 (- 1.04%)}	_4,622,144	_4,626,234	_{p=0.000 n=12}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.117s (± 5.61%)}	_{0.114s (± 4.80%)}	_~	_0.100s	_0.126s	_{p=0.503 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.064s (± 0.53%)}	_{1.047s (± 0.36%)}	_{-0.017s (- 1.60%)}	_1.036s	_1.059s	_{p=0.000 n=12}
_{Emit Time}	_{0.122s (±12.73%)}	_{0.135s (±11.76%)}	_~	_0.102s	_0.164s	_{p=0.053 n=12}
_{Total Time}	_{1.358s (± 1.08%)}	_{1.348s (± 1.01%)}	_~	_1.313s	_1.379s	_{p=0.401 n=12}
_{ts-pre-modules - native}
_Errors	₈₇	₈₇	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_305,179	_{304,312 (± 0.00%)}	_{-867 (- 0.28%)}	_304,287	_304,343	_{p=0.000 n=12}
_Types	_181,926	_{181,813 (± 0.01%)}	_{-113 (- 0.06%)}	_181,789	_181,837	_{p=0.000 n=12}
_{Memory Used}	_{277,162k (± 0.03%)}	_{274,552k (± 0.02%)}	_{-2,610k (- 0.94%)}	_274,398k	_274,669k	_{p=0.000 n=12}
_{Memory Allocs}	_{1,642,366 (± 0.02%)}	_{1,620,750 (± 0.02%)}	_{-21,616 (- 1.32%)}	_1,620,009	_1,621,571	_{p=0.000 n=12}
_{Config Time}	_{0.000s (±146.91%)}	_{0.000s (±146.91%)}	_~	_0.000s	_0.001s	_{p=1.000 n=12}
_{Parse Time}	_{0.101s (± 4.12%)}	_{0.102s (± 4.42%)}	_~	_0.091s	_0.113s	_{p=0.764 n=12}
_{Bind Time}	_{0.041s (±10.79%)}	_{0.038s (±12.48%)}	_~	_0.027s	_0.052s	_{p=0.433 n=12}
_{Check Time}	_{0.843s (± 0.44%)}	_{0.829s (± 0.64%)}	_{-0.013s (- 1.57%)}	_0.816s	_0.841s	_{p=0.000 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{0.998s (± 0.82%)}	_{0.983s (± 0.58%)}	_{-0.015s (- 1.49%)}	_0.965s	_0.997s	_{p=0.001 n=12}
_{vscode - native}
_Errors	₃₇₆	₃₇₆	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_9,966,435	_{9,965,808 (± 0.00%)}	_{-627 (- 0.01%)}	_9,965,784	_9,965,833	_{p=0.000 n=12}
_Types	_3,360,533	_{3,360,469 (± 0.00%)}	_{-64 (- 0.00%)}	_3,360,428	_3,360,493	_{p=0.000 n=12}
_{Memory Used}	_{6,541,432k (± 0.02%)}	_{6,522,506k (± 0.01%)}	_{-18,925k (- 0.29%)}	_6,520,107k	_6,524,394k	_{p=0.000 n=12}
_{Memory Allocs}	_{50,314,406 (± 0.02%)}	_{50,173,949 (± 0.01%)}	_{-140,457 (- 0.28%)}	_50,167,561	_50,189,146	_{p=0.000 n=12}
_{Config Time}	_{0.067s (± 0.46%)}	_{0.066s (± 0.49%)}	_~	_0.066s	_0.067s	_{p=0.414 n=12}
_{Parse Time}	_{1.835s (± 3.95%)}	_{1.811s (± 3.32%)}	_~	_1.673s	_1.948s	_{p=0.370 n=12}
_{Bind Time}	_{0.524s (±20.47%)}	_{0.479s (±22.06%)}	_~	_0.400s	_0.848s	_{p=0.171 n=12}
_{Check Time}	_{13.702s (± 1.47%)}	_{13.805s (± 1.21%)}	_~	_13.247s	_13.977s	_{p=0.876 n=12}
_{Emit Time}	_{4.528s (±11.78%)}	_{4.047s (±10.93%)}	_~	_3.638s	_5.498s	_{p=0.178 n=12}
_{Total Time}	_{20.766s (± 1.80%)}	_{20.312s (± 1.73%)}	_{-0.454s (- 2.18%)}	_19.857s	_21.451s	_{p=0.017 n=12}
_{webpack - native}
_Errors	₈₄₈	₈₄₈	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,376,073	_{1,374,324 (± 0.00%)}	_{-1,749 (- 0.13%)}	_1,374,265	_1,374,409	_{p=0.000 n=12}
_Types	_603,821	_{603,141 (± 0.03%)}	_{-680 (- 0.11%)}	_602,503	_603,532	_{p=0.000 n=12}
_{Memory Used}	_{968,858k (± 0.02%)}	_{953,554k (± 0.01%)}	_{-15,304k (- 1.58%)}	_953,303k	_953,798k	_{p=0.000 n=12}
_{Memory Allocs}	_{6,270,549 (± 0.02%)}	_{6,157,161 (± 0.02%)}	_{-113,388 (- 1.81%)}	_6,154,230	_6,160,036	_{p=0.000 n=12}
_{Config Time}	_0.009s	_{0.009s (± 5.25%)}	_~	_0.009s	_0.011s	_{p=0.478 n=12}
_{Parse Time}	_{0.263s (± 2.55%)}	_{0.268s (± 3.03%)}	_~	_0.242s	_0.284s	_{p=0.297 n=12}
_{Bind Time}	_{0.072s (±19.38%)}	_{0.067s (±17.85%)}	_~	_0.053s	_0.106s	_{p=0.619 n=12}
_{Check Time}	_{2.007s (± 0.49%)}	_{2.038s (± 2.48%)}	_~	_1.950s	_2.223s	_{p=0.640 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{2.388s (± 0.25%)}	_{2.419s (± 2.29%)}	_~	_2.347s	_2.613s	_{p=0.921 n=12}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,045,695	_{1,045,060 (± 0.00%)}	_{-635 (- 0.06%)}	_1,045,059	_1,045,061	_{p=0.000 n=12}
_Types	_383,049	_{382,957 (± 0.00%)}	_{-92 (- 0.02%)}	_382,956	_382,957	_{p=0.000 n=12}
_{Memory Used}	_{611,284k (± 0.01%)}	_{605,191k (± 0.01%)}	_{-6,093k (- 1.00%)}	_605,010k	_605,410k	_{p=0.000 n=12}
_{Memory Allocs}	_{4,828,314 (± 0.03%)}	_{4,792,231 (± 0.04%)}	_{-36,083 (- 0.75%)}	_4,787,988	_4,796,294	_{p=0.000 n=12}
_{Config Time}	_{0.003s (±12.54%)}	_{0.003s (± 8.64%)}	_~	_0.002s	_0.003s	_{p=0.371 n=12}
_{Parse Time}	_{0.145s (± 3.19%)}	_{0.151s (± 3.20%)}	_~	_0.138s	_0.161s	_{p=0.080 n=12}
_{Bind Time}	_{0.040s (±19.98%)}	_{0.043s (±18.49%)}	_~	_0.033s	_0.071s	_{p=0.302 n=12}
_{Check Time}	_{1.194s (± 0.40%)}	_{1.206s (± 1.75%)}	_~	_1.170s	_1.283s	_{p=0.744 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.390s (± 0.61%)}	_{1.414s (± 1.57%)}	_{+0.024s (+ 1.71%)}	_1.373s	_1.485s	_{p=0.037 n=12}

System info unknown

Hosts

native

Scenarios

Compiler-Unions - native
angular-1 - native
mui-docs - native
strada-build-src - native
strada-compiler - native
ts-pre-modules - native
vscode - native
webpack - native
xstate-main - native

_Benchmark	_Name	_Iterations
_Current	_pr	₁₂
_Baseline	_baseline	₁₂

Developer Information:

Download Benchmarks

typescript-automation · 2026-09-28T16:08:34Z

Anders Hejlsberg (@ahejlsberg) Here are the results of running the user tests with tsc comparing main and refs/pull/64499/merge:

There were infrastructure failures potentially unrelated to your change:

1 instance of "Git clone failed"
1 instance of "Package install failed"

Otherwise...

Everything looks good!

typescript-automation · 2026-09-28T16:18:06Z

Hey Anders Hejlsberg (@ahejlsberg), it looks like the DT test run failed. Please check the log for more details.

You can check the log here.

typescript-automation · 2026-09-28T16:56:21Z

Anders Hejlsberg (@ahejlsberg) Here are the results of running the top 400 repos with tsc comparing main and refs/pull/64499/merge:

Everything looks good!

Jake Bailey (jakebailey)

Wowza

Only check properties with multiple declarations for never-reduction

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Copilot AI balanced review requested due to automatic review settings September 28, 2026 15:34

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only build intersection props that can reduce it to never #64476

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build member tables of instantiated classes/interfaces lazily #64475

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Anders Hejlsberg (ahejlsberg) requested review from Daniel Rosenwasser (DanielRosenwasser), Ryan Cavanaugh (RyanCavanaugh) and Jake Bailey (jakebailey) September 28, 2026 17:41

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Merged via the queue into main with commit 04228e2 Sep 28, 2026
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Anders Hejlsberg (ahejlsberg) deleted the fix-64474 branch September 28, 2026 18:58

Max Schwenk (maschwenk) mentioned this pull request Sep 29, 2026

check intersection props for never-reduction in a stable order #64521

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Only check properties with multiple declarations for never-reduction - #64499

Only check properties with multiple declarations for never-reduction#64499
Anders Hejlsberg (ahejlsberg) merged 1 commit into
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fix-64474

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