Don't ignore all generic self tail calls when collecting the return type of a function by Andarist · Pull Request #63999 · microsoft/TypeScript

Mateusz Burzyński (Andarist) · 2026-08-25T09:53:07Z

fixes #63990 (a bug introduced with #53995 )

This PR avoids ignoring recursive self calls when the arguments list is not assignable to the parameter list of the signature declaration.

So this errors:

const f = <T>(a: T, p: number) => {
    if (!p) return a;
    return f(p, p - 1);
};

But this continues to work:

function h<T>(value: T, count: number) {
    if (!count) return value;
    return h(value, count - 1);
}

It also ignores generic calls with type argument lists because that can lead to other instantiations of the signature declaration.

…ype of a function

Copilot

Pull request overview

Tightens generic recursive return-type inference so only same-instantiation tail calls are ignored.

Changes:

Checks recursive arguments against the current generic signature.
Adds regression coverage and generated baselines.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
`tsc/internal/checker/checker.go`	Refines recursive-call inference handling.
`tsc/testdata/tests/cases/compiler/genericRecursiveFunctionReturn.ts`	Adds regression scenarios.
`tsc/testdata/baselines/reference/compiler/genericRecursiveFunctionReturn.errors.txt`	Records expected diagnostics.
`tsc/testdata/baselines/reference/compiler/genericRecursiveFunctionReturn.types`	Records inferred types.
`tsc/testdata/baselines/reference/compiler/genericRecursiveFunctionReturn.symbols`	Records symbols.
`tsc/testdata/baselines/reference/compiler/genericRecursiveFunctionReturn.js`	Records emitted JavaScript.

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Ryan Cavanaugh (RyanCavanaugh) · 2026-08-25T17:14:01Z

TypeScript Bot (@typescript-bot) test it

typescript-automation · 2026-08-25T17:14:07Z

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

Ryan Cavanaugh (RyanCavanaugh) · 2026-08-25T17:15:15Z

when the arguments list is not assignable to the parameter list of the signature declaration

What's an intuition for why this is the correct rule?

typescript-automation · 2026-08-25T17:37:46Z

Ryan Cavanaugh (@RyanCavanaugh) Here are the results of running the user tests with tsc comparing main and refs/pull/63999/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 · 2026-08-25T17:45:17Z

Ryan Cavanaugh (@RyanCavanaugh)
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}	_{148,598k (± 0.44%)}	_{148,947k (± 0.49%)}	_~	_147,206k	_151,072k	_{p=0.478 n=12}
_{Memory Allocs}	_{2,271,289 (± 0.01%)}	_{2,271,356 (± 0.01%)}	_~	_2,271,045	_2,272,304	_{p=0.932 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.051s (± 2.68%)}	_{0.052s (± 2.53%)}	_{+0.002s (+ 3.62%)}	_0.049s	_0.056s	_{p=0.042 n=12}
_{Bind Time}	_{0.016s (±12.44%)}	_{0.015s (±10.32%)}	_~	_0.013s	_0.020s	_{p=0.381 n=12}
_{Check Time}	_{0.594s (± 0.95%)}	_{0.599s (± 0.94%)}	_~	_0.586s	_0.612s	_{p=0.164 n=12}
_{Emit Time}	_{0.330s (± 1.38%)}	_{0.333s (± 2.10%)}	_~	_0.315s	_0.352s	_{p=0.485 n=12}
_{Total Time}	_{0.997s (± 0.62%)}	_{1.006s (± 0.90%)}	_~	_0.990s	_1.033s	_{p=0.131 n=12}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_{846,939 (± 0.06%)}	_{846,888 (± 0.08%)}	_~	_845,015	_848,107	_{p=0.977 n=12}
_Types	_{249,859 (± 0.00%)}	_{249,859 (± 0.00%)}	_~	_249,856	_249,870	_{p=0.381 n=12}
_{Memory Used}	_{793,470k (± 0.06%)}	_{793,201k (± 0.04%)}	_~	_792,455k	_794,104k	_{p=0.478 n=12}
_{Memory Allocs}	_{12,959,768 (± 0.07%)}	_{12,963,741 (± 0.12%)}	_~	_12,950,176	_13,017,020	_{p=0.843 n=12}
_{Config Time}	_0.020s	_{0.020s (± 3.57%)}	_~	_0.020s	_0.024s	_{p=1.000 n=12}
_{Parse Time}	_{0.349s (± 1.98%)}	_{0.345s (± 4.09%)}	_~	_0.312s	_0.388s	_{p=0.723 n=12}
_{Bind Time}	_{0.077s (±11.09%)}	_{0.097s (±18.99%)}	_{🔻+0.020s (+25.49%)}	_0.073s	_0.152s	_{p=0.010 n=12}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=12}
_{Emit Time}	_{1.951s (± 0.68%)}	_{2.035s (± 1.46%)}	_{🔻+0.085s (+ 4.34%)}	_1.974s	_2.102s	_{p=0.000 n=12}
_{Total Time}	_{2.414s (± 0.65%)}	_{2.521s (± 1.80%)}	_{🔻+0.107s (+ 4.43%)}	_2.423s	_2.610s	_{p=0.000 n=12}
_{mui-docs - native}
_Errors	_{11,276 (± 0.04%)}	_{11,272 (± 0.05%)}	_~	_11,247	_11,282	_{p=0.256 n=12}
_Symbols	_4,456,324	_4,456,324	_~	_~	_~	_{p=1.000 n=12}
_Types	_1,669,869	_1,669,869	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{5,405,086k (± 0.03%)}	_{5,404,335k (± 0.03%)}	_~	_5,400,834k	_5,411,836k	_{p=0.478 n=12}
_{Memory Allocs}	_{50,709,606 (± 0.03%)}	_{50,690,782 (± 0.03%)}	_~	_50,651,767	_50,746,249	_{p=0.101 n=12}
_{Config Time}	_0.019s	_0.019s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.684s (± 2.06%)}	_{0.685s (± 1.56%)}	_~	_0.648s	_0.710s	_{p=0.967 n=12}
_{Bind Time}	_{0.002s (±12.64%)}	_{0.003s (±28.35%)}	_~	_0.002s	_0.006s	_{p=0.730 n=12}
_{Check Time}	_{20.053s (± 0.11%)}	_{19.942s (± 0.10%)}	_{-0.111s (- 0.55%)}	_19.902s	_20.009s	_{p=0.000 n=12}
_{Emit Time}	_{0.516s (± 0.12%)}	_{0.518s (± 0.24%)}	_{+0.002s (+ 0.39%)}	_0.515s	_0.523s	_{p=0.001 n=12}
_{Total Time}	_{22.147s (± 0.15%)}	_{22.070s (± 0.13%)}	_{-0.078s (- 0.35%)}	_21.995s	_22.143s	_{p=0.002 n=12}
_{strada-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,394,339	_1,394,339	_~	_~	_~	_{p=1.000 n=12}
_Types	_443,465	_443,465	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{1,645,209k (± 0.20%)}	_{1,648,134k (± 0.26%)}	_~	_1,638,539k	_1,658,171k	_{p=0.291 n=12}
_{Memory Allocs}	_{97,061,639 (± 0.03%)}	_{97,080,241 (± 0.04%)}	_~	_97,004,612	_97,230,246	_{p=0.713 n=12}
_{Config Time}	_{0.006s (±16.00%)}	_{0.006s (±17.24%)}	_~	_0.003s	_0.008s	_{p=0.465 n=12}
_{Parse Time}	_{0.240s (± 1.97%)}	_{0.240s (± 2.05%)}	_~	_0.230s	_0.252s	_{p=0.898 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{2.149s (± 0.40%)}	_{2.150s (± 0.44%)}	_~	_2.131s	_2.187s	_{p=0.876 n=12}
_{Emit Time}	_{0.324s (± 6.25%)}	_{0.354s (± 3.94%)}	_{🔻+0.030s (+ 9.37%)}	_0.308s	_0.378s	_{p=0.015 n=12}
_{Total Time}	_{29.666s (± 0.42%)}	_{29.893s (± 0.68%)}	_~	_29.491s	_30.589s	_{p=0.068 n=12}
_{strada-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_337,586	_337,586	_~	_~	_~	_{p=1.000 n=12}
_Types	_198,869	_198,869	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{319,504k (± 0.04%)}	_{319,494k (± 0.04%)}	_~	_319,070k	_319,857k	_{p=0.887 n=12}
_{Memory Allocs}	_{4,673,092 (± 0.01%)}	_{4,673,082 (± 0.01%)}	_~	_4,671,231	_4,674,406	_{p=1.000 n=12}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.113s (± 4.83%)}	_{0.118s (± 3.63%)}	_~	_0.107s	_0.128s	_{p=0.123 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.042s (± 0.51%)}	_{1.045s (± 0.85%)}	_~	_1.031s	_1.085s	_{p=0.876 n=12}
_{Emit Time}	_{0.140s (±11.05%)}	_{0.152s (± 8.57%)}	_~	_0.102s	_0.176s	_{p=0.235 n=12}
_{Total Time}	_{1.347s (± 1.03%)}	_{1.366s (± 1.12%)}	_~	_1.328s	_1.418s	_{p=0.088 n=12}
_{ts-pre-modules - native}
_Errors	₈₇	₈₇	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_305,317	_305,317	_~	_~	_~	_{p=1.000 n=12}
_Types	_181,984	_181,984	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{277,286k (± 0.01%)}	_{277,274k (± 0.01%)}	_~	_277,156k	_277,315k	_{p=0.478 n=12}
_{Memory Allocs}	_{1,642,277 (± 0.02%)}	_{1,642,276 (± 0.01%)}	_~	_1,641,881	_1,642,917	_{p=0.843 n=12}
_{Config Time}	_{0.000s (±146.91%)}	_{0.000s (±113.79%)}	_~	_0.000s	_0.001s	_{p=1.000 n=12}
_{Parse Time}	_{0.098s (± 3.86%)}	_{0.100s (± 3.79%)}	_~	_0.089s	_0.108s	_{p=0.468 n=12}
_{Bind Time}	_{0.042s (± 8.69%)}	_{0.041s (±14.15%)}	_~	_0.027s	_0.052s	_{p=0.989 n=12}
_{Check Time}	_{0.822s (± 0.69%)}	_{0.828s (± 0.95%)}	_~	_0.808s	_0.858s	_{p=0.311 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{0.977s (± 0.68%)}	_{0.982s (± 1.07%)}	_~	_0.960s	_1.009s	_{p=0.599 n=12}
_{vscode - native}
_Errors	₃₄₇	₃₄₇	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_7,934,940	_7,934,940	_~	_~	_~	_{p=1.000 n=12}
_Types	_2,734,507	_2,734,507	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{5,356,456k (± 0.01%)}	_{5,355,814k (± 0.01%)}	_~	_5,354,307k	_5,357,881k	_{p=0.178 n=12}
_{Memory Allocs}	_{41,369,638 (± 0.01%)}	_{41,372,148 (± 0.03%)}	_~	_41,356,822	_41,400,974	_{p=0.713 n=12}
_{Config Time}	_{0.063s (± 0.51%)}	_{0.063s (± 0.58%)}	_~	_0.062s	_0.064s	_{p=0.856 n=12}
_{Parse Time}	_{1.632s (± 2.46%)}	_{1.629s (± 2.31%)}	_~	_1.491s	_1.696s	_{p=0.898 n=12}
_{Bind Time}	_{0.362s (±13.45%)}	_{0.361s (±12.79%)}	_~	_0.336s	_0.594s	_{p=0.783 n=12}
_{Check Time}	_{9.961s (± 1.15%)}	_{10.005s (± 1.08%)}	_~	_9.462s	_10.099s	_{p=0.147 n=12}
_{Emit Time}	_{3.166s (± 7.32%)}	_{3.158s (± 7.79%)}	_~	_2.987s	_4.393s	_{p=0.486 n=12}
_{Total Time}	_{15.274s (± 1.04%)}	_{15.305s (± 1.10%)}	_~	_15.114s	_16.115s	_{p=0.744 n=12}
_{webpack - native}
_Errors	₅₃₀	₅₃₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_843,220	_843,220	_~	_~	_~	_{p=1.000 n=12}
_Types	_378,778	_378,778	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{647,461k (± 0.02%)}	_{647,234k (± 0.02%)}	_{-227k (- 0.04%)}	_646,923k	_647,697k	_{p=0.039 n=12}
_{Memory Allocs}	_{4,286,283 (± 0.01%)}	_{4,287,428 (± 0.04%)}	_~	_4,284,887	_4,295,597	_{p=0.291 n=12}
_{Config Time}	_0.008s	_{0.008s (± 5.88%)}	_~	_0.008s	_0.010s	_{p=0.478 n=12}
_{Parse Time}	_{0.209s (± 2.46%)}	_{0.211s (± 2.29%)}	_~	_0.202s	_0.228s	_{p=0.967 n=12}
_{Bind Time}	_{0.045s (±13.86%)}	_{0.048s (±19.99%)}	_~	_0.038s	_0.091s	_{p=0.986 n=12}
_{Check Time}	_{1.285s (± 0.74%)}	_{1.303s (± 2.36%)}	_~	_1.257s	_1.443s	_{p=0.370 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.572s (± 0.82%)}	_{1.596s (± 2.45%)}	_~	_1.542s	_1.775s	_{p=0.418 n=12}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,022,508	_1,022,508	_~	_~	_~	_{p=1.000 n=12}
_Types	_376,977	_376,977	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{603,512k (± 0.01%)}	_{603,499k (± 0.01%)}	_~	_603,359k	_603,602k	_{p=0.468 n=12}
_{Memory Allocs}	_{4,755,524 (± 0.05%)}	_{4,754,275 (± 0.04%)}	_~	_4,749,433	_4,760,987	_{p=0.443 n=12}
_{Config Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.113s (± 2.63%)}	_{0.115s (± 3.62%)}	_~	_0.107s	_0.128s	_{p=0.485 n=12}
_{Bind Time}	_{0.038s (±16.17%)}	_{0.033s (±14.63%)}	_~	_0.026s	_0.049s	_{p=0.143 n=12}
_{Check Time}	_{0.854s (± 0.60%)}	_{0.853s (± 1.11%)}	_~	_0.839s	_0.888s	_{p=0.339 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.017s (± 0.83%)}	_{1.012s (± 1.14%)}	_~	_0.988s	_1.050s	_{p=0.523 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 · 2026-08-25T18:19:49Z

Ryan Cavanaugh (@RyanCavanaugh) Here are the results of running the top 400 repos with tsc comparing main and refs/pull/63999/merge:

Everything looks good!

Don't ignore all generic self tail calls when collecting the return t…

4cc66cc

…ype of a function

Copilot AI balanced review requested due to automatic review settings August 25, 2026 09:53

github-project-automation Bot added this to PR Backlog Aug 25, 2026

github-project-automation Bot moved this to Not started in PR Backlog Aug 25, 2026

Copilot started reviewing on behalf of Mateusz Burzyński (Andarist) August 25, 2026 09:53 View session

Copilot AI reviewed Aug 25, 2026

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

Handle tuple spreads in recursive return inference

0a56620

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Don't ignore all generic self tail calls when collecting the return type of a function - #63999

Don't ignore all generic self tail calls when collecting the return type of a function#63999
Mateusz Burzyński (Andarist) wants to merge 2 commits into
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Andarist:fix-63990

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