Fix narrowing of generic this parameters by Andarist · Pull Request #64187 · microsoft/TypeScript

Mateusz Burzyński (Andarist) · 2026-09-07T08:18:41Z

Copilot

🟢 Approval recommended

The focused checker change consistently matches parameter behavior and is covered by comprehensive regression baselines.

Pull request overview

Aligns control-flow narrowing of generic this parameters with ordinary parameters, fixing #64186.

Changes:

Applies narrowable constraint types to this references while preserving check modes.
Adds regression tests for direct, aliased, contextual, indexed, and inferential usage.
Adds diagnostic, type, and symbol baselines.

File summaries

File	Description
`tsc/internal/checker/checker.go`	Enables consistent narrowing for generic `this`.
`tsc/testdata/tests/cases/compiler/controlFlowGenericThis.ts`	Tests core narrowing behavior.
`tsc/testdata/tests/cases/compiler/controlFlowGenericThisContextual.ts`	Tests contextual and inferential behavior.
`tsc/testdata/baselines/reference/compiler/controlFlowGenericThis.errors.txt`	Records expected diagnostics.
`tsc/testdata/baselines/reference/compiler/controlFlowGenericThis.types`	Records inferred types.
`tsc/testdata/baselines/reference/compiler/controlFlowGenericThis.symbols`	Records resolved symbols.
`tsc/testdata/baselines/reference/compiler/controlFlowGenericThisContextual.types`	Records contextual inferred types.
`tsc/testdata/baselines/reference/compiler/controlFlowGenericThisContextual.symbols`	Records contextual symbols.

Review details

Files reviewed: 8/8 changed files
Comments generated: 0
Review effort level: Balanced

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Jake Bailey (jakebailey) · 2026-09-08T17:50:32Z

TypeScript Bot (@typescript-bot) test it

typescript-automation · 2026-09-08T17:50:38Z

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
`perf test this faster`	✅ Started	👀 Results

typescript-automation · 2026-09-08T18:23:34Z

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	_115,480	_115,480	_~	_~	_~	_{p=1.000 n=12}
_Types	_96,945	_96,945	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{148,605k (± 0.65%)}	_{148,909k (± 0.43%)}	_~	_147,039k	_150,759k	_{p=0.590 n=12}
_{Memory Allocs}	_{2,271,289 (± 0.00%)}	_{2,271,319 (± 0.01%)}	_~	_2,271,089	_2,271,679	_{p=0.932 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.051s (± 3.99%)}	_{0.050s (± 2.89%)}	_~	_0.047s	_0.054s	_{p=0.762 n=12}
_{Bind Time}	_{0.016s (±10.77%)}	_{0.015s (±13.36%)}	_~	_0.013s	_0.021s	_{p=0.180 n=12}
_{Check Time}	_{0.594s (± 1.16%)}	_{0.590s (± 1.11%)}	_~	_0.580s	_0.614s	_{p=0.163 n=12}
_{Emit Time}	_{0.326s (± 1.72%)}	_{0.328s (± 1.63%)}	_~	_0.310s	_0.337s	_{p=0.541 n=12}
_{Total Time}	_{0.994s (± 0.95%)}	_{0.991s (± 1.01%)}	_~	_0.967s	_1.027s	_{p=0.385 n=12}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_{846,997 (± 0.07%)}	_{847,374 (± 0.09%)}	_~	_845,039	_849,084	_{p=0.410 n=12}
_Types	_{249,852 (± 0.00%)}	_{249,851 (± 0.00%)}	_~	_249,849	_249,854	_{p=0.779 n=12}
_{Memory Used}	_{793,767k (± 0.07%)}	_{793,839k (± 0.06%)}	_~	_793,072k	_795,601k	_{p=0.671 n=12}
_{Memory Allocs}	_{12,956,705 (± 0.07%)}	_{12,956,144 (± 0.04%)}	_~	_12,949,527	_12,974,151	_{p=0.843 n=12}
_{Config Time}	_{0.020s (± 0.91%)}	_{0.021s (± 5.50%)}	_~	_0.019s	_0.024s	_{p=0.456 n=12}
_{Parse Time}	_{0.340s (± 2.55%)}	_{0.355s (± 3.92%)}	_~	_0.314s	_0.392s	_{p=0.075 n=12}
_{Bind Time}	_{0.076s (± 6.47%)}	_{0.095s (±16.80%)}	_{🔻+0.019s (+24.32%)}	_0.073s	_0.138s	_{p=0.007 n=12}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=12}
_{Emit Time}	_{1.949s (± 0.59%)}	_{2.076s (± 3.52%)}	_{🔻+0.126s (+ 6.47%)}	_1.929s	_2.359s	_{p=0.002 n=12}
_{Total Time}	_{2.404s (± 0.58%)}	_{2.570s (± 3.59%)}	_{🔻+0.166s (+ 6.89%)}	_2.382s	_2.932s	_{p=0.003 n=12}
_{mui-docs - native}
_Errors	_{11,275 (± 0.04%)}	_{11,279 (± 0.02%)}	_~	_11,269	_11,281	_{p=0.167 n=12}
_Symbols	_4,267,465	_4,267,465	_~	_~	_~	_{p=1.000 n=12}
_Types	_1,475,860	_1,475,860	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{4,670,834k (± 0.03%)}	_{4,671,718k (± 0.04%)}	_~	_4,669,028k	_4,678,797k	_{p=0.799 n=12}
_{Memory Allocs}	_{42,721,268 (± 0.04%)}	_{42,728,042 (± 0.05%)}	_~	_42,687,199	_42,779,034	_{p=0.799 n=12}
_{Config Time}	_0.019s	_0.019s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.678s (± 1.73%)}	_{0.668s (± 2.24%)}	_~	_0.619s	_0.692s	_{p=0.385 n=12}
_{Bind Time}	_{0.002s (±13.40%)}	_{0.002s (±13.40%)}	_~	_0.002s	_0.003s	_{p=1.000 n=12}
_{Check Time}	_{20.030s (± 0.60%)}	_{20.129s (± 0.63%)}	_~	_19.842s	_20.365s	_{p=0.193 n=12}
_{Emit Time}	_{0.574s (±13.62%)}	_{0.595s (±17.06%)}	_~	_0.514s	_1.010s	_{p=0.259 n=12}
_{Total Time}	_{22.231s (± 0.67%)}	_{22.365s (± 0.75%)}	_~	_21.917s	_22.695s	_{p=0.219 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	_443,418	_443,418	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{1,643,523k (± 0.25%)}	_{1,649,166k (± 0.22%)}	_{+5,643k (+ 0.34%)}	_1,639,569k	_1,656,490k	_{p=0.039 n=12}
_{Memory Allocs}	_{97,038,306 (± 0.04%)}	_{97,069,537 (± 0.04%)}	_~	_96,971,021	_97,160,171	_{p=0.160 n=12}
_{Config Time}	_{0.006s (±13.36%)}	_{0.006s (± 5.47%)}	_~	_0.005s	_0.006s	_{p=0.503 n=12}
_{Parse Time}	_{0.236s (± 2.74%)}	_{0.241s (± 2.39%)}	_~	_0.219s	_0.252s	_{p=0.224 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{2.163s (± 0.37%)}	_{2.163s (± 0.41%)}	_~	_2.137s	_2.188s	_{p=0.810 n=12}
_{Emit Time}	_{0.325s (± 4.72%)}	_{0.351s (± 3.16%)}	_{🔻+0.026s (+ 8.11%)}	_0.318s	_0.370s	_{p=0.008 n=12}
_{Total Time}	_{29.690s (± 0.53%)}	_{29.954s (± 0.52%)}	_{+0.264s (+ 0.89%)}	_29.504s	_30.373s	_{p=0.021 n=12}
_{strada-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_337,531	_337,531	_~	_~	_~	_{p=1.000 n=12}
_Types	_198,830	_198,830	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{319,484k (± 0.05%)}	_{319,494k (± 0.05%)}	_~	_318,929k	_319,755k	_{p=0.319 n=12}
_{Memory Allocs}	_{4,672,435 (± 0.01%)}	_{4,672,896 (± 0.01%)}	_~	_4,671,185	_4,674,443	_{p=0.143 n=12}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.093s (± 3.47%)}	_{0.093s (± 5.67%)}	_~	_0.078s	_0.107s	_{p=0.876 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{0.871s (± 0.39%)}	_{0.868s (± 0.54%)}	_~	_0.861s	_0.889s	_{p=0.096 n=12}
_{Emit Time}	_{0.111s (±13.55%)}	_{0.108s (±12.87%)}	_~	_0.081s	_0.145s	_{p=0.681 n=12}
_{Total Time}	_{1.118s (± 0.92%)}	_{1.117s (± 1.27%)}	_~	_1.094s	_1.154s	_{p=0.723 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	_181,876	_181,876	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{277,178k (± 0.02%)}	_{277,214k (± 0.02%)}	_~	_277,079k	_277,363k	_{p=0.478 n=12}
_{Memory Allocs}	_{1,641,906 (± 0.02%)}	_{1,641,820 (± 0.01%)}	_~	_1,641,488	_1,642,206	_{p=0.713 n=12}
_{Config Time}	_{0.000s (±146.91%)}	_{0.000s (±113.79%)}	_~	_0.000s	_0.001s	_{p=1.000 n=12}
_{Parse Time}	_{0.103s (± 3.74%)}	_{0.097s (± 3.71%)}	_{🟩-0.006s (- 5.76%)}	_0.087s	_0.105s	_{p=0.029 n=12}
_{Bind Time}	_{0.037s (± 9.62%)}	_{0.041s (± 8.31%)}	_~	_0.034s	_0.050s	_{p=0.069 n=12}
_{Check Time}	_{0.831s (± 0.56%)}	_{0.832s (± 0.70%)}	_~	_0.820s	_0.845s	_{p=0.721 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{0.985s (± 0.94%)}	_{0.984s (± 0.82%)}	_~	_0.969s	_1.007s	_{p=0.944 n=12}
_{vscode - native}
_Errors	₃₄₉	₃₄₉	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_8,463,646	_8,463,646	_~	_~	_~	_{p=1.000 n=12}
_Types	_2,898,728	_2,898,728	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{5,713,814k (± 0.02%)}	_{5,713,051k (± 0.01%)}	_~	_5,710,544k	_5,715,798k	_{p=0.075 n=12}
_{Memory Allocs}	_{43,915,824 (± 0.02%)}	_{43,907,516 (± 0.01%)}	_~	_43,898,967	_43,928,628	_{p=0.128 n=12}
_{Config Time}	_{0.064s (± 0.42%)}	_{0.064s (± 0.42%)}	_~	_0.063s	_0.065s	_{p=1.000 n=12}
_{Parse Time}	_{1.672s (± 2.31%)}	_{1.704s (± 2.19%)}	_~	_1.544s	_1.763s	_{p=0.182 n=12}
_{Bind Time}	_{0.422s (±18.72%)}	_{0.381s (±12.32%)}	_{🟩-0.041s (- 9.77%)}	_0.357s	_0.618s	_{p=0.003 n=12}
_{Check Time}	_{10.297s (± 1.49%)}	_{10.444s (± 1.11%)}	_~	_9.894s	_10.536s	_{p=0.235 n=12}
_{Emit Time}	_{3.784s (±11.78%)}	_{3.402s (± 8.24%)}	_~	_3.202s	_4.771s	_{p=0.071 n=12}
_{Total Time}	_{16.338s (± 2.10%)}	_{16.087s (± 1.15%)}	_~	_15.929s	_17.007s	_{p=0.524 n=12}
_{webpack - native}
_Errors	₅₆₀	₅₆₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_929,346	_929,346	_~	_~	_~	_{p=1.000 n=12}
_Types	_426,678	_426,678	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{718,565k (± 0.02%)}	_{718,548k (± 0.02%)}	_~	_718,327k	_718,887k	_{p=0.755 n=12}
_{Memory Allocs}	_{4,699,829 (± 0.01%)}	_{4,700,180 (± 0.03%)}	_~	_4,698,231	_4,706,814	_{p=0.755 n=12}
_{Config Time}	_0.008s	_0.008s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.226s (± 3.52%)}	_{0.228s (± 3.04%)}	_~	_0.207s	_0.248s	_{p=0.788 n=12}
_{Bind Time}	_{0.053s (±18.45%)}	_{0.049s (±12.32%)}	_~	_0.044s	_0.075s	_{p=0.317 n=12}
_{Check Time}	_{1.241s (± 1.01%)}	_{1.244s (± 0.76%)}	_~	_1.214s	_1.257s	_{p=0.966 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.555s (± 0.74%)}	_{1.555s (± 0.58%)}	_~	_1.537s	_1.580s	_{p=0.899 n=12}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,022,532	_1,022,532	_~	_~	_~	_{p=1.000 n=12}
_Types	_376,962	_376,962	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{603,429k (± 0.02%)}	_{603,461k (± 0.01%)}	_~	_603,340k	_603,640k	_{p=0.713 n=12}
_{Memory Allocs}	_{4,753,643 (± 0.02%)}	_{4,753,670 (± 0.03%)}	_~	_4,749,060	_4,756,513	_{p=0.713 n=12}
_{Config Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.117s (± 3.26%)}	_{0.115s (± 4.76%)}	_~	_0.106s	_0.134s	_{p=0.339 n=12}
_{Bind Time}	_{0.031s (±20.22%)}	_{0.039s (±20.53%)}	_~	_0.026s	_0.057s	_{p=0.050 n=12}
_{Check Time}	_{0.860s (± 1.27%)}	_{0.858s (± 0.55%)}	_~	_0.847s	_0.871s	_{p=1.000 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.018s (± 1.20%)}	_{1.022s (± 0.90%)}	_~	_0.996s	_1.041s	_{p=0.619 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 · 2026-09-08T18:36:18Z

Jake Bailey (@jakebailey) Here are the results of running the user tests with tsc comparing main and refs/pull/64187/merge:

There were infrastructure failures potentially unrelated to your change:

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

Otherwise...

Everything looks good!

typescript-automation · 2026-09-08T19:17:29Z

Jake Bailey (@jakebailey) Here are the results of running the top 400 repos with tsc comparing main and refs/pull/64187/merge:

Everything looks good!

Fix narrowing of generic this parameters

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Copilot AI balanced review requested due to automatic review settings September 7, 2026 08:18

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Copilot started reviewing on behalf of Mateusz Burzyński (Andarist) September 7, 2026 08:19 View session

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Fix narrowing of generic this parameters - #64187

Fix narrowing of generic this parameters#64187
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