Fix stack overflow crash in base constraint resolution by ahejlsberg · Pull Request #4242 · microsoft/typescript-go

ahejlsberg · 2026-06-08T16:02:54Z

This PR modifies base constraint resolution for conditional types to ensure the logic is protected by the recursion limiter in getResolvedBaseConstraint.

Fixes microsoft/TypeScript#63269.
Fixes microsoft/TypeScript#62966.

Copilot

Pull request overview

This PR adjusts base-constraint resolution for conditional types in the Go checker to avoid stack overflows by ensuring distributive-conditional base-constraint computation stays within the getResolvedBaseConstraint recursion limiter. It also adds a regression test and updates affected baselines.

Changes:

Update computeBaseConstraint to inline distributive-conditional constraint computation for TypeFlagsConditional.
Add a new compiler regression test infiniteConstraints2.ts.
Update submodule baseline diagnostics for infiniteConstraints.ts and add new baselines for infiniteConstraints2.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
`internal/checker/checker.go`	Changes conditional-type base constraint resolution to avoid unbounded recursion/stack overflow.
`testdata/tests/cases/compiler/infiniteConstraints2.ts`	Adds a repro-style regression test for the conditional-type constraint recursion scenario.
`testdata/baselines/reference/compiler/infiniteConstraints2.types`	New baseline for the added regression test.
`testdata/baselines/reference/compiler/infiniteConstraints2.symbols`	New baseline for the added regression test.
`testdata/baselines/reference/submodule/compiler/infiniteConstraints.errors.txt`	Updates expected diagnostics (now includes recursion limiter diagnostics instead of crashing).
`testdata/baselines/reference/submodule/compiler/infiniteConstraints.errors.txt.diff`	Corresponding submodule baseline diff update.

+		d := t.AsConditionalType()
+		if d.root.isDistributive && c.cachedTypes[CachedTypeKey{kind: CachedTypeKindRestrictiveInstantiation, typeId: t.id}] != t {


+
+export type Prepend<Elm, T extends unknown[]> =
+ T extends unknown ?
+ ((arg: Elm, ...rest: T) => void) extends ((...args: infer T2) => void) ? T2 : 


ahejlsberg · 2026-06-08T16:12:28Z


    type T1<B extends { [K in keyof B]: Extract<B[Exclude<keyof B, K>], { val: string }>["val"] }> = B;
+                                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2321: Excessive stack depth comparing types '"val"' and 'keyof Extract<B[Exclude<keyof B, K>], { val: string; }>'.


This excessive stack depth error occurs because we recursively build up a relation stack of 100 keyof XXX types, where XXX is an inifinitely recurring conditional type. We could silence this error (and instead produce a "maybe" result) by introducing distinct recursion identities for keyof types, but the error seems reasonable.

ahejlsberg · 2026-06-12T13:52:25Z

@typescript-bot test it

typescript-automation · 2026-06-12T13:52:30Z

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

Command	Status	Results
`test top400`	✅ Started	✅ Results
`perf test this faster`	✅ Started	👀 Results

typescript-automation · 2026-06-12T14:14:49Z

@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	₄	₄	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_{81,791 (± 0.02%)}	_{81,786 (± 0.06%)}	_~	_81,715	_81,842	_{p=1.000 n=6}
_Types	_98,821	_98,821	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{180,175k (± 0.26%)}	_{179,009k (± 0.69%)}	_~	_177,061k	_180,425k	_{p=0.128 n=6}
_{Memory Allocs}	_{1,564,803 (± 0.01%)}	_{1,564,646 (± 0.02%)}	_~	_1,564,166	_1,564,892	_{p=0.298 n=6}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=6}
_{Parse Time}	_{0.065s (± 8.47%)}	_{0.059s (± 9.34%)}	_~	_0.052s	_0.068s	_{p=0.092 n=6}
_{Bind Time}	_{0.018s (±22.92%)}	_{0.018s (±16.46%)}	_~	_0.015s	_0.023s	_{p=1.000 n=6}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_{0.854s (± 0.86%)}	_{0.858s (± 2.04%)}	_~	_0.833s	_0.875s	_{p=0.378 n=6}
_{Total Time}	_{0.939s (± 1.58%)}	_{0.937s (± 2.12%)}	_~	_0.909s	_0.963s	_{p=1.000 n=6}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_{885,094 (± 0.04%)}	_{884,675 (± 0.08%)}	_~	_883,806	_885,856	_{p=0.230 n=6}
_Types	_{263,972 (± 0.00%)}	_{263,767 (± 0.00%)}	_{-205 (- 0.08%)}	_263,766	_263,770	_{p=0.004 n=6}
_{Memory Used}	_{831,232k (± 0.10%)}	_{830,727k (± 0.08%)}	_~	_830,131k	_831,900k	_{p=0.173 n=6}
_{Memory Allocs}	_{6,808,760 (± 0.24%)}	_{6,818,620 (± 0.27%)}	_~	_6,797,777	_6,839,578	_{p=0.575 n=6}
_{Config Time}	_{0.017s (± 2.38%)}	_{0.017s (± 2.38%)}	_~	_0.017s	_0.018s	_{p=1.000 n=6}
_{Parse Time}	_{0.255s (± 2.81%)}	_{0.253s (± 6.41%)}	_~	_0.229s	_0.271s	_{p=0.936 n=6}
_{Bind Time}	_{0.049s (±31.34%)}	_{0.051s (±40.94%)}	_~	_0.034s	_0.083s	_{p=0.936 n=6}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_{1.899s (± 0.80%)}	_{1.879s (± 1.28%)}	_~	_1.857s	_1.910s	_{p=0.228 n=6}
_{Total Time}	_{2.235s (± 1.05%)}	_{2.212s (± 1.07%)}	_~	_2.184s	_2.241s	_{p=0.149 n=6}
_{mui-docs - native}
_Errors	_{11,242 (± 0.04%)}	_{11,244 (± 0.02%)}	_~	_11,240	_11,246	_{p=0.511 n=6}
_Symbols	_4,186,698	_4,212,918	_{+26,220 (+ 0.63%)}	_~	_~	_{p=0.001 n=6}
_Types	_1,531,646	_1,532,172	_{+526 (+ 0.03%)}	_~	_~	_{p=0.001 n=6}
_{Memory Used}	_{4,874,153k (± 0.05%)}	_{4,963,426k (± 0.06%)}	_{+89,273k (+ 1.83%)}	_4,960,225k	_4,968,391k	_{p=0.005 n=6}
_{Memory Allocs}	_{79,969,052 (±17.24%)}	_{71,409,049 (±16.14%)}	_~	_56,733,224	_90,153,019	_{p=0.298 n=6}
_{Config Time}	_{0.017s (± 6.94%)}	_{0.017s (± 7.42%)}	_~	_0.016s	_0.019s	_{p=0.389 n=6}
_{Parse Time}	_{0.992s (±19.46%)}	_{0.889s (±18.39%)}	_~	_0.700s	_1.144s	_{p=0.378 n=6}
_{Bind Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{16.830s (± 0.53%)}	_{17.010s (± 0.49%)}	_{+0.180s (+ 1.07%)}	_16.905s	_17.110s	_{p=0.013 n=6}
_{Emit Time}	_{0.440s (± 3.41%)}	_{0.443s (± 3.42%)}	_~	_0.435s	_0.474s	_{p=0.226 n=6}
_{Total Time}	_{18.918s (± 1.22%)}	_{18.986s (± 1.42%)}	_~	_18.681s	_19.357s	_{p=0.936 n=6}
_{self-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,396,711	_1,396,711	_~	_~	_~	_{p=1.000 n=6}
_Types	_442,164	_442,161	_{-3 (- 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory Used}	_{1,646,420k (± 0.20%)}	_{1,642,125k (± 0.47%)}	_~	_1,634,028k	_1,653,499k	_{p=0.298 n=6}
_{Memory Allocs}	_{54,845,041 (± 0.03%)}	_{54,811,560 (± 0.14%)}	_~	_54,737,849	_54,940,221	_{p=0.298 n=6}
_{Config Time}	_{0.013s (±32.96%)}	_{0.010s (±74.22%)}	_~	_0.005s	_0.024s	_{p=0.199 n=6}
_{Parse Time}	_{0.276s (± 3.49%)}	_{0.271s (± 4.08%)}	_~	_0.260s	_0.285s	_{p=0.378 n=6}
_{Bind Time}	_0.000s	_{0.000s (±154.76%)}	_~	_0.000s	_0.001s	_{p=0.174 n=6}
_{Check Time}	_{2.351s (± 0.88%)}	_{2.326s (± 1.08%)}	_~	_2.311s	_2.376s	_{p=0.065 n=6}
_{Emit Time}	_{0.226s (± 2.65%)}	_{0.231s (± 5.80%)}	_~	_0.216s	_0.253s	_{p=0.470 n=6}
_{Total Time}	_{29.872s (± 0.61%)}	_{29.675s (± 1.06%)}	_~	_29.171s	_29.994s	_{p=0.298 n=6}
_{self-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_338,176	_338,176	_~	_~	_~	_{p=1.000 n=6}
_Types	_199,522	_199,522	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{332,077k (± 0.03%)}	_{332,047k (± 0.03%)}	_~	_331,945k	_332,230k	_{p=0.575 n=6}
_{Memory Allocs}	_{2,368,983 (± 0.02%)}	_{2,370,172 (± 0.05%)}	_{+1,190 (+ 0.05%)}	_2,368,743	_2,372,446	_{p=0.045 n=6}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=6}
_{Parse Time}	_{0.128s (± 4.47%)}	_{0.129s (± 2.98%)}	_~	_0.125s	_0.136s	_{p=0.747 n=6}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{1.384s (± 0.51%)}	_{1.407s (± 1.33%)}	_{+0.024s (+ 1.70%)}	_1.387s	_1.440s	_{p=0.020 n=6}
_{Emit Time}	_{0.089s (± 6.88%)}	_{0.084s (±11.10%)}	_~	_0.076s	_0.101s	_{p=0.261 n=6}
_{Total Time}	_{1.655s (± 0.88%)}	_{1.673s (± 0.88%)}	_~	_1.660s	_1.699s	_{p=0.109 n=6}
_{ts-pre-modules - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_97,488	_97,488	_~	_~	_~	_{p=1.000 n=6}
_Types	₃₅₆	₃₅₆	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{133,414k (± 0.03%)}	_{133,395k (± 0.01%)}	_~	_133,364k	_133,415k	_{p=0.298 n=6}
_{Memory Allocs}	_{182,912 (± 0.08%)}	_{182,625 (± 0.15%)}	_~	_182,398	_183,138	_{p=0.066 n=6}
_{Config Time}	_{0.001s (±48.94%)}	_{0.001s (±48.94%)}	_~	_0.000s	_0.001s	_{p=1.000 n=6}
_{Parse Time}	_{0.114s (± 4.29%)}	_{0.114s (± 3.93%)}	_~	_0.107s	_0.118s	_{p=1.000 n=6}
_{Bind Time}	_{0.043s (± 4.84%)}	_{0.037s (±16.34%)}	_~	_0.029s	_0.045s	_{p=0.091 n=6}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{0.160s (± 3.55%)}	_{0.154s (± 3.96%)}	_~	_0.147s	_0.162s	_{p=0.170 n=6}
_{vscode - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_6,635,454	_6,635,454	_~	_~	_~	_{p=1.000 n=6}
_Types	_2,331,878	_2,331,859	_{-19 (- 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory Used}	_{4,486,100k (± 0.02%)}	_{4,485,554k (± 0.02%)}	_~	_4,484,823k	_4,486,848k	_{p=0.298 n=6}
_{Memory Allocs}	_{31,585,247 (± 0.04%)}	_{31,593,261 (± 0.04%)}	_~	_31,577,761	_31,613,962	_{p=0.230 n=6}
_{Config Time}	_{0.068s (± 8.10%)}	_{0.068s (±10.04%)}	_~	_0.061s	_0.079s	_{p=0.748 n=6}
_{Parse Time}	_{0.884s (± 1.93%)}	_{0.876s (± 3.25%)}	_~	_0.835s	_0.906s	_{p=0.471 n=6}
_{Bind Time}	_{0.133s (± 2.20%)}	_{0.157s (±21.98%)}	_~	_0.128s	_0.217s	_{p=0.567 n=6}
_{Check Time}	_{8.737s (± 1.09%)}	_{8.781s (± 0.93%)}	_~	_8.669s	_8.866s	_{p=0.378 n=6}
_{Emit Time}	_{2.177s (± 8.20%)}	_{2.119s (±10.66%)}	_~	_1.886s	_2.449s	_{p=0.810 n=6}
_{Total Time}	_{12.014s (± 0.90%)}	_{12.026s (± 1.24%)}	_~	_11.919s	_12.295s	_{p=0.810 n=6}
_{webpack - native}
_Errors	₂	₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_181,636	_181,636	_~	_~	_~	_{p=1.000 n=6}
_Types	₃₄₀	₃₄₀	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{219,185k (± 0.19%)}	_{219,336k (± 0.03%)}	_~	_219,254k	_219,419k	_{p=0.378 n=6}
_{Memory Allocs}	_{1,062,464 (± 1.08%)}	_{1,073,884 (± 1.62%)}	_~	_1,058,839	_1,103,149	_{p=0.230 n=6}
_{Config Time}	_{0.009s (±20.00%)}	_{0.010s (±11.88%)}	_~	_0.008s	_0.011s	_{p=0.621 n=6}
_{Parse Time}	_{0.143s (± 3.63%)}	_{0.149s (± 5.04%)}	_~	_0.140s	_0.159s	_{p=0.090 n=6}
_{Bind Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_{0.048s (±19.58%)}	_{0.044s (±21.73%)}	_~	_0.033s	_0.060s	_{p=0.521 n=6}
_{Total Time}	_{0.200s (± 3.83%)}	_{0.203s (± 2.99%)}	_~	_0.199s	_0.211s	_{p=0.808 n=6}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,060,815	_1,060,884	_{+69 (+ 0.01%)}	_~	_~	_{p=0.001 n=6}
_Types	_384,533	_382,962	_{-1,571 (- 0.41%)}	_~	_~	_{p=0.001 n=6}
_{Memory Used}	_{638,767k (± 0.01%)}	_{638,505k (± 0.01%)}	_{-262k (- 0.04%)}	_638,390k	_638,568k	_{p=0.005 n=6}
_{Memory Allocs}	_{5,019,589 (± 0.03%)}	_{5,003,094 (± 0.07%)}	_{-16,495 (- 0.33%)}	_4,997,700	_5,006,494	_{p=0.005 n=6}
_{Config Time}	_{0.005s (±11.05%)}	_{0.005s (±11.05%)}	_~	_0.004s	_0.005s	_{p=1.000 n=6}
_{Parse Time}	_{0.133s (± 1.41%)}	_{0.131s (± 5.11%)}	_~	_0.123s	_0.141s	_{p=0.748 n=6}
_{Bind Time}	_{0.028s (±14.02%)}	_{0.034s (±24.15%)}	_~	_0.024s	_0.044s	_{p=0.226 n=6}
_{Check Time}	_{1.286s (± 0.32%)}	_{1.277s (± 1.11%)}	_~	_1.259s	_1.297s	_{p=0.295 n=6}
_{Emit Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{1.456s (± 0.54%)}	_{1.451s (± 1.03%)}	_~	_1.435s	_1.477s	_{p=0.471 n=6}

System info unknown

Hosts

native

Scenarios

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

_Benchmark	_Name	_Iterations
_Current	_pr	₆
_Baseline	_baseline	₆

Developer Information:

Download Benchmarks

typescript-automation · 2026-06-12T14:49:48Z

@ahejlsberg Here are the results of running the top 400 repos with tsc comparing main and refs/pull/4242/merge:

Everything looks good!

jakebailey

I feel like a comment would be worth it, but not blocking I suppose

ahejlsberg · 2026-06-12T18:13:30Z

I feel like a comment would be worth it, but not blocking I suppose

The whole purpose of getResolvedBaseConstraint is to resolve base constraints with an infinite recursion limiter, so I don't think a comment would add anything new.

* origin/main: Make LS checkers time out after being idle, segment based on use (take 2) (microsoft#3435) Fix external helper crash after module-status updates (microsoft#3994) Allow LSP server to use builtin watcher on Windows and macOS (microsoft#4288) Rebuilt CLI watcher around new `fswatch` package (microsoft#4026) Fix stack overflow crash in base constraint resolution (microsoft#4242) # Conflicts: # internal/project/session.go

ahejlsberg added 3 commits June 8, 2026 08:56

Ensure base constraint resolution has recursion limiter

505dcba

Accept new baselines

7e491c3

Add regression test

2086611

Copilot AI review requested due to automatic review settings June 8, 2026 16:02

Copilot started reviewing on behalf of ahejlsberg June 8, 2026 16:03 View session

Copilot AI reviewed Jun 8, 2026

View reviewed changes

ahejlsberg commented Jun 8, 2026

View reviewed changes

ahejlsberg requested review from RyanCavanaugh and jakebailey June 8, 2026 16:12

RyanCavanaugh added this to the Post-7.0 milestone Jun 8, 2026

RyanCavanaugh modified the milestones: Post-7.0, TypeScript 7.0 RC Jun 12, 2026

jakebailey approved these changes Jun 12, 2026

View reviewed changes

ahejlsberg added this pull request to the merge queue Jun 12, 2026

Merged via the queue into main with commit 4cf361f Jun 12, 2026
22 checks passed

ahejlsberg deleted the fix-ts-63269 branch June 12, 2026 18:36

jakebailey mentioned this pull request Jun 22, 2026

const type parameter doesn't capture string-literal property when the parameter type is wrapped in Omit<...> #4391

Closed

ashvinsrivatsa mentioned this pull request Jun 28, 2026

Regression in 20260613.1 pertaining to conditional types #4463

Closed

ahejlsberg mentioned this pull request Jun 29, 2026

Add conditional type base constraint limiter #4468

Merged

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Fix stack overflow crash in base constraint resolution#4242

Fix stack overflow crash in base constraint resolution#4242
ahejlsberg merged 3 commits into
mainfrom
fix-ts-63269

ahejlsberg commented Jun 8, 2026 •

edited

Loading

Uh oh!

Copilot AI left a comment

Uh oh!

ahejlsberg Jun 8, 2026 •

edited

Loading

Uh oh!

ahejlsberg commented Jun 12, 2026

Uh oh!

typescript-automation Bot commented Jun 12, 2026 •

edited

Loading

Uh oh!

typescript-automation Bot commented Jun 12, 2026

tsc

Uh oh!

typescript-automation Bot commented Jun 12, 2026

Uh oh!

jakebailey left a comment

Uh oh!

ahejlsberg commented Jun 12, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

		d := t.AsConditionalType()
		if d.root.isDistributive && c.cachedTypes[CachedTypeKey{kind: CachedTypeKindRestrictiveInstantiation, typeId: t.id}] != t {

Uh oh!

Conversation

ahejlsberg commented Jun 8, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Reviewed changes

Uh oh!

ahejlsberg Jun 8, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

ahejlsberg commented Jun 12, 2026

Uh oh!

typescript-automation Bot commented Jun 12, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

typescript-automation Bot commented Jun 12, 2026

tsc

Uh oh!

typescript-automation Bot commented Jun 12, 2026

Uh oh!

jakebailey left a comment

Choose a reason for hiding this comment

Uh oh!

ahejlsberg commented Jun 12, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

ahejlsberg commented Jun 8, 2026 •

edited

Loading

ahejlsberg Jun 8, 2026 •

edited

Loading

typescript-automation Bot commented Jun 12, 2026 •

edited

Loading