check the target of a type reference in isWeakType by maschwenk · Pull Request #64654 · microsoft/TypeScript

Max Schwenk (maschwenk) · 2026-10-06T13:39:11Z

isWeakType resolves the members of the type it's given. for an instantiation of a generic class or interface that instantiates every declared member and copies in every inherited one, only to check that there are no signatures or index infos and every property is optional. instantiating doesn't change any of that unless a base type is a type parameter (a class declared in a generic function can extend base<T>()), so when none is, this asks the target instead, whose members are resolved once and never instantiated. weakTypeWithTypeParameterBase.ts covers the type parameter case

isRelatedToEx calls isWeakType on the target before it gets to structuredTypeRelatedTo, where two references to the same generic type are compared by variance without looking at members. so an instantiation that's only ever a relation target, and only ever related through its type arguments, still had its whole member table built here

tuple targets are left out. doing the same for them made vscode slightly worse, and tuples are never weak anyway (length is required)

heap after check (Memory used from --extendedDiagnostics), median of 3, same diagnostics with and without this:

	4 checkers	`--singleThreaded`
t3code `apps/server`	4.78 → 3.86 GB (−19%)	2.58 → 2.33 GB (−10%)
webpack	0.93 → 0.87 GB (−6%)	0.71 → 0.67 GB (−5%)
xstate-main	0.58 → 0.56 GB (−3%)	0.46 → 0.45 GB (−1%)
vscode	6.09 → 6.04 GB (−0.8%)	5.18 → 5.15 GB (−0.6%)
mui-docs, Compiler, Compiler-Unions	within ±0.3%	within ±0.3%
a private 38k-file monorepo project	19.83 → 17.55 GB (−11.5%)	13.07 → 11.85 GB (−9.3%)

on t3code peak rss goes from 5.49 to 4.62 GB with 4 checkers and the symbol count from 11.1M to 7.5M. on the monorepo project peak rss goes from 23.1 to 21.0 GB. check time doesn't get worse anywhere (it's within noise except the monorepo project single threaded, 42.9 → 38.7s)

my guess at why t3code gains the most: effect's api is built from generic interfaces with a lot of members, often through long extends chains (Rpc, the schema Bottom* interfaces, Effect), and every rpc or schema definition is its own instantiation. most checks between them are reference to reference with the same target, which variance answers from the type arguments, so before this the member tables were mostly built just for isWeakType. on t3code's server, 1.02M of the 1.03M instantiated members of Rpc came from this check

t3code is pingdotgg/t3code at 9bd1d800 (effect 4), tsc -p apps/server --noEmit. webpack, xstate-main, vscode, mui-docs and the Compiler cases are the typescript-benchmarking projects

related, avoids these resolutions (and more) by building member tables lazily:

build member tables of instantiated classes/interfaces lazily #64475

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

typescript-automation · 2026-10-06T13:41:52Z

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.

Copilot

Copilot review overview

🟡 Changes recommended

Captured generic base types can change inherited signatures, causing incorrect weak-type diagnostics.

Review effort: Balanced
Findings: 1

Open (1)

Preserve instantiation-specific checking for captured generic base shapes · New

What changed in this PR

Optimizes weak-type detection to avoid resolving members for generic class/interface instantiations.

Changes:

Checks the generic target instead of instantiated members.
Excludes tuples from the optimization.

File	Description
`tsc/internal/checker/relater.go`	Optimizes `isWeakType` for type references.

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…eter Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Jake Bailey (jakebailey) · 2026-10-06T16:49:21Z

TypeScript Bot (@typescript-bot) test it

typescript-automation · 2026-10-06T16:49:27Z

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

typescript-automation · 2026-10-06T17:04:45Z

Hey Jake Bailey (@jakebailey), it looks like the DT test run failed. Please check the log for more details.

You can check the log here.

typescript-automation · 2026-10-06T17:16:55Z

Jake Bailey (@jakebailey)
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	_115,276	_114,129	_{-1,147 (- 1.00%)}	_~	_~	_{p=0.000 n=12}
_Types	_79,613	_79,277	_{-336 (- 0.42%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{141,849k (± 0.33%)}	_{141,284k (± 0.30%)}	_~	_140,204k	_142,336k	_{p=0.052 n=12}
_{Memory Allocs}	_{2,193,620 (± 0.01%)}	_{2,191,308 (± 0.01%)}	_{-2,312 (- 0.11%)}	_2,190,868	_2,191,763	_{p=0.000 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.050s (± 3.12%)}	_{0.052s (± 4.15%)}	_~	_0.046s	_0.056s	_{p=0.134 n=12}
_{Bind Time}	_{0.015s (±13.12%)}	_{0.015s (± 8.11%)}	_~	_0.013s	_0.018s	_{p=0.893 n=12}
_{Check Time}	_{0.592s (± 0.65%)}	_{0.609s (± 0.86%)}	_{+0.017s (+ 2.81%)}	_0.599s	_0.624s	_{p=0.000 n=12}
_{Emit Time}	_{0.324s (± 2.30%)}	_{0.331s (± 2.30%)}	_~	_0.304s	_0.346s	_{p=0.117 n=12}
_{Total Time}	_{0.990s (± 0.77%)}	_{1.014s (± 1.06%)}	_{+0.025s (+ 2.52%)}	_0.981s	_1.046s	_{p=0.001 n=12}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_{846,902 (± 0.08%)}	_{837,259 (± 0.08%)}	_{-9,642 (- 1.14%)}	_836,007	_838,813	_{p=0.000 n=12}
_Types	_{249,941 (± 0.00%)}	_{249,860 (± 0.00%)}	_{-81 (- 0.03%)}	_249,856	_249,868	_{p=0.000 n=12}
_{Memory Used}	_{789,309k (± 0.04%)}	_{786,897k (± 0.03%)}	_{-2,412k (- 0.31%)}	_786,290k	_787,654k	_{p=0.000 n=12}
_{Memory Allocs}	_{12,864,708 (± 0.04%)}	_{12,858,412 (± 0.01%)}	_{-6,295 (- 0.05%)}	_12,854,892	_12,865,521	_{p=0.008 n=12}
_{Config Time}	_0.020s	_{0.020s (± 1.21%)}	_~	_0.020s	_0.021s	_{p=0.478 n=12}
_{Parse Time}	_{0.320s (± 2.77%)}	_{0.326s (± 2.80%)}	_~	_0.297s	_0.346s	_{p=0.131 n=12}
_{Bind Time}	_{0.084s (±18.20%)}	_{0.077s (± 7.34%)}	_~	_0.069s	_0.100s	_{p=0.286 n=12}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=12}
_{Emit Time}	_{1.917s (± 1.06%)}	_{1.928s (± 1.21%)}	_~	_1.897s	_2.030s	_{p=0.271 n=12}
_{Total Time}	_{2.355s (± 1.26%)}	_{2.368s (± 1.15%)}	_~	_2.324s	_2.488s	_{p=0.139 n=12}
_{mui-docs - native}
_Errors	_11,405	_11,405	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_4,351,480	_4,356,767	_{+5,287 (+ 0.12%)}	_~	_~	_{p=0.000 n=12}
_Types	_1,380,970	_1,379,976	_{-994 (- 0.07%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{2,974,898k (± 0.05%)}	_{2,982,342k (± 0.04%)}	_{+7,444k (+ 0.25%)}	_2,979,300k	_2,987,455k	_{p=0.000 n=12}
_{Memory Allocs}	_{32,827,685 (± 0.06%)}	_{32,822,610 (± 0.04%)}	_~	_32,781,895	_32,862,134	_{p=0.799 n=12}
_{Config Time}	_{0.020s (± 1.21%)}	_0.020s	_~	_~	_~	_{p=0.478 n=12}
_{Parse Time}	_{0.611s (± 2.56%)}	_{0.610s (± 2.69%)}	_~	_0.556s	_0.667s	_{p=0.766 n=12}
_{Bind Time}	_{0.003s (±12.54%)}	_{0.003s (±13.14%)}	_~	_0.002s	_0.003s	_{p=1.000 n=12}
_{Check Time}	_{10.164s (± 0.66%)}	_{10.239s (± 0.87%)}	_~	_9.918s	_10.400s	_{p=0.060 n=12}
_{Emit Time}	_{0.525s (± 6.39%)}	_{0.517s (± 5.00%)}	_{-0.008s (- 1.54%)}	_0.499s	_0.647s	_{p=0.023 n=12}
_{Total Time}	_{12.269s (± 0.44%)}	_{12.327s (± 0.47%)}	_~	_12.218s	_12.475s	_{p=0.089 n=12}
_{strada-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,390,229	_1,388,249	_{-1,980 (- 0.14%)}	_~	_~	_{p=0.000 n=12}
_Types	_441,498	_440,506	_{-992 (- 0.22%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{1,681,724k (± 1.10%)}	_{1,671,732k (± 0.76%)}	_~	_1,645,572k	_1,718,296k	_{p=0.671 n=12}
_{Memory Allocs}	_{92,114,503 (± 0.19%)}	_{92,011,194 (± 0.12%)}	_~	_91,667,286	_92,264,795	_{p=0.291 n=12}
_{Config Time}	_{0.004s (±11.30%)}	_{0.004s (±12.60%)}	_~	_0.003s	_0.005s	_{p=0.800 n=12}
_{Parse Time}	_{0.244s (± 4.85%)}	_{0.229s (± 3.00%)}	_{🟩-0.015s (- 6.04%)}	_0.213s	_0.249s	_{p=0.037 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{2.167s (± 0.58%)}	_{2.206s (± 0.76%)}	_{+0.038s (+ 1.78%)}	_2.155s	_2.243s	_{p=0.001 n=12}
_{Emit Time}	_{0.351s (± 5.10%)}	_{0.352s (± 4.35%)}	_~	_0.324s	_0.392s	_{p=0.943 n=12}
_{Total Time}	_{28.350s (± 0.67%)}	_{28.595s (± 0.61%)}	_~	_28.132s	_29.024s	_{p=0.052 n=12}
_{strada-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_334,953	_332,790	_{-2,163 (- 0.65%)}	_~	_~	_{p=0.000 n=12}
_Types	_197,616	_197,112	_{-504 (- 0.26%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{313,275k (± 0.04%)}	_{312,414k (± 0.03%)}	_{-861k (- 0.27%)}	_312,227k	_312,710k	_{p=0.000 n=12}
_{Memory Allocs}	_{4,622,964 (± 0.01%)}	_{4,619,290 (± 0.01%)}	_{-3,675 (- 0.08%)}	_4,617,640	_4,620,791	_{p=0.000 n=12}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.111s (± 5.99%)}	_{0.114s (± 4.42%)}	_~	_0.103s	_0.127s	_{p=0.385 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.048s (± 0.60%)}	_{1.062s (± 0.52%)}	_{+0.014s (+ 1.32%)}	_1.050s	_1.080s	_{p=0.004 n=12}
_{Emit Time}	_{0.119s (±13.13%)}	_{0.134s (±12.11%)}	_~	_0.098s	_0.160s	_{p=0.075 n=12}
_{Total Time}	_{1.336s (± 0.95%)}	_{1.362s (± 0.99%)}	_{+0.026s (+ 1.95%)}	_1.331s	_1.398s	_{p=0.015 n=12}
_{ts-pre-modules - native}
_Errors	₈₇	₈₇	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_303,834	_301,677	_{-2,157 (- 0.71%)}	_~	_~	_{p=0.000 n=12}
_Types	_181,663	_181,129	_{-534 (- 0.29%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{273,883k (± 0.02%)}	_{273,172k (± 0.02%)}	_{-712k (- 0.26%)}	_272,966k	_273,327k	_{p=0.000 n=12}
_{Memory Allocs}	_{1,615,976 (± 0.01%)}	_{1,612,516 (± 0.01%)}	_{-3,460 (- 0.21%)}	_1,612,077	_1,613,329	_{p=0.000 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.096s (± 2.91%)}	_{0.101s (± 5.62%)}	_~	_0.087s	_0.111s	_{p=0.139 n=12}
_{Bind Time}	_{0.038s (±10.20%)}	_{0.041s (±10.75%)}	_~	_0.029s	_0.051s	_{p=0.223 n=12}
_{Check Time}	_{0.823s (± 0.45%)}	_{0.832s (± 0.58%)}	_{+0.010s (+ 1.16%)}	_0.822s	_0.845s	_{p=0.004 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{0.971s (± 0.41%)}	_{0.990s (± 0.67%)}	_{+0.019s (+ 2.01%)}	_0.974s	_1.009s	_{p=0.000 n=12}
_{vscode - native}
_Errors	₃₈₀	₃₈₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_10,815,602	_10,650,969	_{-164,633 (- 1.52%)}	_~	_~	_{p=0.000 n=12}
_Types	_3,599,368	_3,587,540	_{-11,828 (- 0.33%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{6,988,889k (± 0.02%)}	_{6,936,545k (± 0.02%)}	_{-52,343k (- 0.75%)}	_6,934,153k	_6,940,196k	_{p=0.000 n=12}
_{Memory Allocs}	_{53,528,400 (± 0.02%)}	_{53,386,874 (± 0.01%)}	_{-141,527 (- 0.26%)}	_53,374,098	_53,406,640	_{p=0.000 n=12}
_{Config Time}	_{0.074s (± 0.53%)}	_{0.074s (± 0.49%)}	_~	_0.073s	_0.075s	_{p=0.870 n=12}
_{Parse Time}	_{1.807s (± 3.86%)}	_{1.795s (± 4.41%)}	_~	_1.641s	_1.960s	_{p=0.855 n=12}
_{Bind Time}	_{0.583s (±18.42%)}	_{0.600s (±20.19%)}	_~	_0.434s	_0.894s	_{p=0.932 n=12}
_{Check Time}	_{13.623s (± 1.29%)}	_{13.494s (± 1.35%)}	_~	_13.154s	_13.842s	_{p=0.410 n=12}
_{Emit Time}	_{4.628s (±11.07%)}	_{5.198s (±11.18%)}	_~	_4.130s	_6.117s	_{p=0.089 n=12}
_{Total Time}	_{20.797s (± 1.88%)}	_{21.248s (± 2.05%)}	_~	_20.392s	_21.985s	_{p=0.052 n=12}
_{webpack - native}
_Errors	₅₈₉	₅₈₉	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,298,698	_1,218,100	_{🟩-80,598 (- 6.21%)}	_~	_~	_{p=0.000 n=12}
_Types	_614,057	_604,703	_{-9,354 (- 1.52%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{1,003,869k (± 0.02%)}	_{976,904k (± 0.02%)}	_{-26,965k (- 2.69%)}	_976,309k	_977,470k	_{p=0.000 n=12}
_{Memory Allocs}	_{6,537,134 (± 0.03%)}	_{6,461,263 (± 0.04%)}	_{-75,871 (- 1.16%)}	_6,455,801	_6,468,841	_{p=0.000 n=12}
_{Config Time}	_0.009s	_0.009s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.283s (± 3.97%)}	_{0.287s (± 3.62%)}	_~	_0.257s	_0.313s	_{p=0.451 n=12}
_{Bind Time}	_{0.083s (±17.84%)}	_{0.082s (±16.81%)}	_~	_0.062s	_0.127s	_{p=0.831 n=12}
_{Check Time}	_{2.061s (± 1.02%)}	_{2.054s (± 1.38%)}	_~	_2.008s	_2.162s	_{p=0.355 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{2.472s (± 0.82%)}	_{2.467s (± 1.43%)}	_~	_2.407s	_2.588s	_{p=0.434 n=12}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,055,167	_987,563	_{🟩-67,604 (- 6.41%)}	_~	_~	_{p=0.000 n=12}
_Types	_387,939	_385,710	_{-2,229 (- 0.57%)}	_~	_~	_{p=0.000 n=12}
_{Memory Used}	_{610,243k (± 0.03%)}	_{591,449k (± 0.03%)}	_{🟩-18,793k (- 3.08%)}	_591,150k	_592,082k	_{p=0.000 n=12}
_{Memory Allocs}	_{4,880,457 (± 0.07%)}	_{4,833,642 (± 0.08%)}	_{-46,814 (- 0.96%)}	_4,824,907	_4,845,610	_{p=0.000 n=12}
_{Config Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.116s (± 2.59%)}	_{0.115s (± 3.41%)}	_~	_0.105s	_0.124s	_{p=0.560 n=12}
_{Bind Time}	_{0.031s (±12.91%)}	_{0.034s (±19.93%)}	_~	_0.025s	_0.055s	_{p=0.674 n=12}
_{Check Time}	_{1.062s (± 0.54%)}	_{1.050s (± 0.73%)}	_{-0.012s (- 1.16%)}	_1.038s	_1.081s	_{p=0.005 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.220s (± 0.66%)}	_{1.207s (± 0.69%)}	_~	_1.184s	_1.233s	_{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:

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typescript-automation · 2026-10-06T17:21:16Z

Jake Bailey (@jakebailey) Here are the results of running the user tests with tsc comparing baseline and pr:

Everything looks good!

typescript-automation · 2026-10-06T18:10:34Z

Jake Bailey (@jakebailey) Here are the results of running the top 400 repos with tsc comparing baseline and pr:

Everything looks good!

Check the target of a type reference in isWeakType

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Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

typescript-automation Bot added the For Uncommitted Bug PR for untriaged, rejected, closed or missing bug label Oct 6, 2026

Max Schwenk (maschwenk) marked this pull request as ready for review October 6, 2026 13:41

Copilot AI balanced review requested due to automatic review settings October 6, 2026 13:41

Copilot started reviewing on behalf of Max Schwenk (maschwenk) October 6, 2026 13:43 View session

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Comment thread tsc/internal/checker/relater.go Outdated

Don't check the target in isWeakType when a base type is a type param…

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…eter Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Jake Bailey (jakebailey) assigned Anders Hejlsberg (ahejlsberg) Oct 6, 2026

Jake Bailey (jakebailey) requested a review from Anders Hejlsberg (ahejlsberg) October 6, 2026 16:49

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check the target of a type reference in isWeakType - #64654

check the target of a type reference in isWeakType#64654
Max Schwenk (maschwenk) wants to merge 2 commits into
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