Don't reduce intersections of mappings of the same object type by ahejlsberg · Pull Request #64481 · microsoft/TypeScript

Anders Hejlsberg (ahejlsberg) · 2026-09-27T15:54:38Z

With this PR, when a mapped type is applied to an intersection of homomorphic mappings of the same object type, we longer attempt to reduce the intersection. This in turn means that we don't eagerly resolve the members of the target object type, which helps enable creation of recursive schemas in Zod and similar libraries.

Fixes #64415.

Anders Hejlsberg (ahejlsberg) · 2026-09-27T15:55:03Z

TypeScript Bot (@typescript-bot) test it

typescript-automation · 2026-09-27T15:55:08Z

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 broad exemption prevents valid conflicting mapped intersections from reducing to never.

Review effort: Balanced
Findings: 1

Open (1)

Narrow exemption to preserve conflicting mapped-property reduction · New

What changed in this PR

Prevents eager intersection reduction during recursive Zod-style schema inference.

Changes:

Detects mapped intersections sharing an object source.
Adds regression coverage and updates inferred-type baselines.

File	Description
`tsc/internal/checker/checker.go`	Skips reduction for same-source mapped intersections.
`tsc/testdata/tests/cases/compiler/recursiveTypeInference2.ts`	Adds the Zod regression case.
`tsc/testdata/baselines/reference/compiler/recursiveTypeInference2.types`	Records inferred types.
`tsc/testdata/baselines/reference/compiler/recursiveTypeInference2.symbols`	Records resolved symbols.
`tsc/testdata/baselines/reference/compiler/recursiveTypeInference.types`	Updates existing recursive inference results.

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Anders Hejlsberg (ahejlsberg) · 2026-09-27T16:44:13Z

 		if t.objectFlags&ObjectFlagsIsNeverIntersectionComputed == 0 {
 			t.objectFlags |= ObjectFlagsIsNeverIntersectionComputed
-			if core.Some(c.getPropertiesOfUnionOrIntersectionType(t), c.isNeverReducedProperty) {
+			if !c.isMappingOfSameObjectType(t.Types()) && core.Some(c.getPropertiesOfUnionOrIntersectionType(t), c.isNeverReducedProperty) {


True, but the goal here is to implement a heuristic that defers intersection reduction in cases where it is highly unlikely that impossible intersections will occur. An intersection consisting solely of homomorphic mapped types applied to the same object type is typically a conditional mapping that applies different templates to non-overlapping subsets of properties, and intersection reduction doesn't affect such types--except it forces eager member resolution for the constituent object type. Which is what we want to avoid.

typescript-automation · 2026-09-27T16:21:49Z

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	_115,480	_~	_~	_~	_{p=1.000 n=12}
_Types	_96,949	_96,949	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{148,754k (± 0.29%)}	_{148,218k (± 0.37%)}	_~	_146,933k	_150,121k	_{p=0.052 n=12}
_{Memory Allocs}	_{2,271,759 (± 0.00%)}	_{2,271,626 (± 0.00%)}	_{-134 (- 0.01%)}	_2,271,421	_2,271,882	_{p=0.028 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.041s (± 3.14%)}	_{0.041s (± 4.25%)}	_~	_0.037s	_0.047s	_{p=0.745 n=12}
_{Bind Time}	_{0.012s (±12.99%)}	_{0.012s (± 7.91%)}	_~	_0.010s	_0.015s	_{p=0.480 n=12}
_{Check Time}	_{0.501s (± 0.82%)}	_{0.500s (± 1.03%)}	_~	_0.491s	_0.517s	_{p=0.560 n=12}
_{Emit Time}	_{0.270s (± 1.46%)}	_{0.270s (± 2.23%)}	_~	_0.245s	_0.279s	_{p=0.599 n=12}
_{Total Time}	_{0.830s (± 0.67%)}	_{0.829s (± 0.92%)}	_~	_0.799s	_0.846s	_{p=0.766 n=12}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_{847,045 (± 0.08%)}	_{846,835 (± 0.11%)}	_~	_843,932	_849,017	_{p=0.920 n=12}
_Types	_{250,003 (± 0.00%)}	_{250,004 (± 0.00%)}	_~	_250,001	_250,014	_{p=0.917 n=12}
_{Memory Used}	_{793,187k (± 0.03%)}	_{793,831k (± 0.08%)}	_{+644k (+ 0.08%)}	_792,084k	_795,801k	_{p=0.028 n=12}
_{Memory Allocs}	_{12,960,004 (± 0.04%)}	_{12,957,291 (± 0.02%)}	_~	_12,954,238	_12,966,176	_{p=0.443 n=12}
_{Config Time}	_0.016s	_0.016s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.280s (± 1.71%)}	_{0.275s (± 2.53%)}	_~	_0.250s	_0.291s	_{p=0.182 n=12}
_{Bind Time}	_{0.058s (± 0.86%)}	_{0.062s (± 8.94%)}	_~	_0.058s	_0.088s	_{p=0.665 n=12}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=12}
_{Emit Time}	_{1.599s (± 0.46%)}	_{1.619s (± 1.09%)}	_{+0.020s (+ 1.24%)}	_1.578s	_1.687s	_{p=0.029 n=12}
_{Total Time}	_{1.967s (± 0.35%)}	_{1.987s (± 1.05%)}	_~	_1.950s	_2.069s	_{p=0.091 n=12}
_{mui-docs - native}
_Errors	_{11,400 (± 0.04%)}	_{11,400 (± 0.05%)}	_~	_11,376	_11,408	_{p=0.763 n=12}
_Symbols	_4,603,369	_4,601,618	_{-1,751 (- 0.04%)}	_~	_~	_{p=0.000 n=12}
_Types	_1,643,150	_1,415,629	_{🟩-227,521 (-13.85%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{4,956,058k (± 0.04%)}	_{4,877,999k (± 0.03%)}	_{-78,059k (- 1.58%)}	_4,873,417k	_4,880,612k	_{p=0.000 n=12}
_{Memory Allocs}	_{48,662,811 (± 0.04%)}	_{38,569,188 (± 0.04%)}	_{🟩-10,093,624 (-20.74%)}	_38,525,522	_38,614,810	_{p=0.000 n=12}
_{Config Time}	_{0.016s (± 1.55%)}	_{0.016s (± 2.12%)}	_~	_0.015s	_0.016s	_{p=0.193 n=12}
_{Parse Time}	_{0.573s (± 2.20%)}	_{0.585s (± 1.42%)}	_~	_0.563s	_0.605s	_{p=0.164 n=12}
_{Bind Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{15.152s (± 0.81%)}	_{14.931s (± 0.53%)}	_{-0.221s (- 1.46%)}	_14.738s	_15.061s	_{p=0.007 n=12}
_{Emit Time}	_{0.440s (± 2.21%)}	_{0.436s (± 0.33%)}	_~	_0.432s	_0.440s	_{p=0.348 n=12}
_{Total Time}	_{16.968s (± 0.71%)}	_{16.740s (± 0.56%)}	_{-0.229s (- 1.35%)}	_16.524s	_16.866s	_{p=0.004 n=12}
_{strada-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,394,252	_1,394,252	_~	_~	_~	_{p=1.000 n=12}
_Types	_443,566	_443,566	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{1,711,016k (± 0.79%)}	_{1,698,151k (± 1.04%)}	_~	_1,662,222k	_1,747,684k	_{p=0.347 n=12}
_{Memory Allocs}	_{93,674,821 (± 0.07%)}	_{93,539,875 (± 0.15%)}	_~	_93,115,707	_93,839,799	_{p=0.219 n=12}
_{Config Time}	_{0.004s (±10.30%)}	_{0.004s (±17.94%)}	_~	_0.003s	_0.007s	_{p=0.718 n=12}
_{Parse Time}	_{0.214s (± 3.41%)}	_{0.213s (± 4.13%)}	_~	_0.190s	_0.232s	_{p=0.965 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.830s (± 0.40%)}	_{1.835s (± 0.78%)}	_~	_1.796s	_1.882s	_{p=0.660 n=12}
_{Emit Time}	_{0.291s (± 5.84%)}	_{0.293s (± 4.99%)}	_~	_0.256s	_0.329s	_{p=0.832 n=12}
_{Total Time}	_{23.727s (± 0.56%)}	_{23.697s (± 0.64%)}	_~	_23.392s	_24.103s	_{p=0.630 n=12}
_{strada-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_337,531	_337,531	_~	_~	_~	_{p=1.000 n=12}
_Types	_198,885	_198,885	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{319,776k (± 0.07%)}	_{319,687k (± 0.02%)}	_~	_319,521k	_319,855k	_{p=0.932 n=12}
_{Memory Allocs}	_{4,673,215 (± 0.01%)}	_{4,673,156 (± 0.01%)}	_~	_4,672,047	_4,674,125	_{p=0.811 n=12}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.110s (± 4.71%)}	_{0.112s (± 4.86%)}	_~	_0.098s	_0.127s	_{p=0.619 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.068s (± 0.42%)}	_{1.067s (± 0.41%)}	_~	_1.058s	_1.079s	_{p=0.701 n=12}
_{Emit Time}	_{0.146s (± 6.30%)}	_{0.136s (±11.14%)}	_~	_0.093s	_0.157s	_{p=0.259 n=12}
_{Total Time}	_{1.371s (± 0.56%)}	_{1.365s (± 1.21%)}	_~	_1.325s	_1.404s	_{p=0.702 n=12}
_{ts-pre-modules - native}
_Errors	₈₇	₈₇	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_305,179	_305,179	_~	_~	_~	_{p=1.000 n=12}
_Types	_181,926	_181,926	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{277,160k (± 0.01%)}	_{277,163k (± 0.03%)}	_~	_276,949k	_277,302k	_{p=0.701 n=12}
_{Memory Allocs}	_{1,642,312 (± 0.01%)}	_{1,642,114 (± 0.01%)}	_~	_1,641,502	_1,642,531	_{p=0.178 n=12}
_{Config Time}	_{0.000s (±217.90%)}	_{0.000s (±217.90%)}	_~	_0.000s	_0.001s	_{p=1.000 n=12}
_{Parse Time}	_{0.102s (± 3.46%)}	_{0.099s (± 3.48%)}	_~	_0.088s	_0.106s	_{p=0.224 n=12}
_{Bind Time}	_{0.036s (±11.37%)}	_{0.039s (±10.35%)}	_~	_0.027s	_0.049s	_{p=0.283 n=12}
_{Check Time}	_{0.837s (± 0.56%)}	_{0.838s (± 0.65%)}	_~	_0.825s	_0.851s	_{p=0.831 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{0.989s (± 0.88%)}	_{0.989s (± 0.81%)}	_~	_0.966s	_1.007s	_{p=0.943 n=12}
_{vscode - native}
_Errors	₃₇₁	₃₇₁	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_9,894,472	_9,889,352	_{-5,120 (- 0.05%)}	_~	_~	_{p=0.000 n=12}
_Types	_3,335,673	_3,334,351	_{-1,322 (- 0.04%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{6,500,296k (± 0.02%)}	_{6,498,464k (± 0.01%)}	_{-1,833k (- 0.03%)}	_6,496,309k	_6,501,517k	_{p=0.010 n=12}
_{Memory Allocs}	_{49,995,466 (± 0.01%)}	_{49,995,323 (± 0.03%)}	_~	_49,975,509	_50,060,230	_{p=0.160 n=12}
_{Config Time}	_{0.066s (± 0.27%)}	_{0.066s (± 0.41%)}	_~	_0.065s	_0.067s	_{p=1.000 n=12}
_{Parse Time}	_{1.795s (± 2.53%)}	_{1.854s (± 3.00%)}	_~	_1.729s	_1.967s	_{p=0.124 n=12}
_{Bind Time}	_{0.517s (±17.62%)}	_{0.466s (±16.44%)}	_~	_0.403s	_0.802s	_{p=0.876 n=12}
_{Check Time}	_{13.477s (± 1.07%)}	_{13.515s (± 1.12%)}	_~	_13.053s	_13.775s	_{p=0.478 n=12}
_{Emit Time}	_{4.152s (±11.86%)}	_{4.230s (±12.71%)}	_~	_3.703s	_5.683s	_{p=0.128 n=12}
_{Total Time}	_{20.117s (± 1.96%)}	_{20.241s (± 2.15%)}	_~	_19.691s	_21.418s	_{p=0.284 n=12}
_{webpack - native}
_Errors	₈₄₈	₈₄₈	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,447,477	_1,440,949	_{-6,528 (- 0.45%)}	_~	_~	_{p=0.000 n=12}
_Types	_618,246	_618,246	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{996,583k (± 0.02%)}	_{994,411k (± 0.02%)}	_{-2,171k (- 0.22%)}	_993,991k	_994,727k	_{p=0.000 n=12}
_{Memory Allocs}	_{6,364,068 (± 0.02%)}	_{6,340,957 (± 0.04%)}	_{-23,111 (- 0.36%)}	_6,337,797	_6,351,721	_{p=0.000 n=12}
_{Config Time}	_0.009s	_0.009s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.262s (± 2.92%)}	_{0.263s (± 3.18%)}	_~	_0.244s	_0.288s	_{p=0.810 n=12}
_{Bind Time}	_{0.075s (±19.09%)}	_{0.076s (±15.26%)}	_~	_0.055s	_0.111s	_{p=0.943 n=12}
_{Check Time}	_{1.998s (± 0.48%)}	_{1.986s (± 0.45%)}	_~	_1.970s	_2.019s	_{p=0.091 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{2.381s (± 0.44%)}	_{2.373s (± 0.43%)}	_~	_2.354s	_2.415s	_{p=0.202 n=12}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,045,695	_1,045,648	_{-47 (- 0.00%)}	_~	_~	_{p=0.000 n=12}
_Types	_383,049	_383,035	_{-14 (- 0.00%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{611,355k (± 0.01%)}	_{611,251k (± 0.02%)}	_~	_611,047k	_611,478k	_{p=0.101 n=12}
_{Memory Allocs}	_{4,830,373 (± 0.06%)}	_{4,829,547 (± 0.05%)}	_~	_4,823,863	_4,837,108	_{p=0.671 n=12}
_{Config Time}	_{0.003s (±12.54%)}	_{0.003s (±10.34%)}	_~	_0.002s	_0.003s	_{p=0.667 n=12}
_{Parse Time}	_{0.145s (± 3.13%)}	_{0.154s (± 4.74%)}	_~	_0.141s	_0.179s	_{p=0.155 n=12}
_{Bind Time}	_{0.045s (±21.22%)}	_{0.043s (±19.58%)}	_~	_0.033s	_0.075s	_{p=0.595 n=12}
_{Check Time}	_{1.190s (± 0.60%)}	_{1.230s (± 3.28%)}	_~	_1.176s	_1.347s	_{p=0.117 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.392s (± 0.49%)}	_{1.440s (± 3.48%)}	_~	_1.367s	_1.585s	_{p=0.066 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 · 2026-09-27T16:24:26Z

Anders Hejlsberg (@ahejlsberg) Here are the results of running the user tests with tsc comparing main and refs/pull/64481/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 · 2026-09-27T16:32:47Z

Hey Anders Hejlsberg (@ahejlsberg), the results of running the DT tests are ready.

Everything looks the same!

You can check the log here.

Anders Hejlsberg (ahejlsberg) · 2026-09-27T16:47:13Z

Apparently this pattern accounts for a substantial number of types in mui-docs, so nice savings there.

typescript-automation · 2026-09-27T17:13:16Z

Anders Hejlsberg (@ahejlsberg) Here are the results of running the top 400 repos with tsc comparing main and refs/pull/64481/merge:

Everything looks good!

Anders Hejlsberg (ahejlsberg) added 3 commits September 27, 2026 08:35

Don't reduce intersections of mappings of the same object type

6dd4013

Accept new baselines

e065127

Add regression test

daf10b8

Copilot AI balanced review requested due to automatic review settings September 27, 2026 15:54

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github-project-automation Bot moved this to Not started in PR Backlog Sep 27, 2026

typescript-automation Bot assigned Anders Hejlsberg (ahejlsberg) Sep 27, 2026

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Don't reduce intersections of mappings of the same object type - #64481

Don't reduce intersections of mappings of the same object type#64481
Anders Hejlsberg (ahejlsberg) wants to merge 3 commits into
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fix-64415

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