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!

Mateusz Burzyński (Andarist) · 2026-08-26T09:14:01Z

Ryan Cavanaugh (@RyanCavanaugh) it seems to me the only possible instantiation of such a recursive self-tail call (when supplied arg type match the exact parameter types) is one that can't contribute anything new to the possible return type. That's because, by nature, the recursion would reach the same branches, in the same way, when analyzing (in abstract) that recursive call.

So if we think about the overall return type of this signature, it's like:

Return<Fn> = Base | Return<Fn>

This means that Return<Fn> can be safely ignored because it doesn't provide any new branching. It all boils down to just the "Base" (same reasoning here as with your original PR).

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
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