Widen types assigned to auto-typed variables by Andarist · Pull Request #3497 · microsoft/typescript-go

Andarist · 2026-04-22T12:47:15Z

ahejlsberg · 2026-04-22T15:20:54Z

typescript-bot · 2026-04-22T15:20:59Z

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-bot · 2026-04-22T15:42:33Z

@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	_{81,818 (± 0.01%)}	_{81,804 (± 0.02%)}	_~	_81,775	_81,810	_{p=0.079 n=6}
_Types	_98,821	_98,821	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{175,086k (± 0.07%)}	_{175,040k (± 0.05%)}	_~	_174,968k	_175,216k	_{p=0.471 n=6}
_{Memory Allocs}	_{1,771,550 (± 0.02%)}	_{1,771,671 (± 0.01%)}	_~	_1,771,436	_1,772,026	_{p=0.471 n=6}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=6}
_{Parse Time}	_{0.056s (± 5.57%)}	_{0.057s (± 6.83%)}	_~	_0.054s	_0.064s	_{p=0.936 n=6}
_{Bind Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_{0.856s (± 0.74%)}	_{0.871s (± 1.01%)}	_{+0.015s (+ 1.73%)}	_0.860s	_0.881s	_{p=0.013 n=6}
_{Total Time}	_{0.913s (± 0.52%)}	_{0.928s (± 1.14%)}	_{+0.015s (+ 1.70%)}	_0.917s	_0.946s	_{p=0.010 n=6}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_{884,979 (± 0.10%)}	_{884,934 (± 0.07%)}	_~	_883,759	_885,351	_{p=0.810 n=6}
_Types	_{263,959 (± 0.00%)}	_{263,958 (± 0.00%)}	_~	_263,958	_263,959	_{p=0.248 n=6}
_{Memory Used}	_{829,023k (± 0.07%)}	_{828,754k (± 0.09%)}	_~	_827,917k	_829,784k	_{p=0.689 n=6}
_{Memory Allocs}	_{6,719,629 (± 0.51%)}	_{6,716,181 (± 0.25%)}	_~	_6,696,988	_6,744,701	_{p=0.689 n=6}
_{Config Time}	_{0.029s (± 7.92%)}	_{0.028s (± 5.75%)}	_~	_0.027s	_0.031s	_{p=0.383 n=6}
_{Parse Time}	_{0.233s (± 1.90%)}	_{0.241s (± 5.54%)}	_~	_0.225s	_0.256s	_{p=0.290 n=6}
_{Bind Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_{1.911s (± 2.27%)}	_{1.925s (± 1.13%)}	_~	_1.900s	_1.954s	_{p=0.575 n=6}
_{Total Time}	_{2.174s (± 2.05%)}	_{2.196s (± 1.48%)}	_~	_2.162s	_2.230s	_{p=0.748 n=6}
_{mui-docs - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_4,308,625	_4,308,626	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_1,537,195	_1,537,198	_{+3 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory Used}	_{5,454,036k (± 0.04%)}	_{5,456,344k (± 0.03%)}	_~	_5,454,731k	_5,459,189k	_{p=0.230 n=6}
_{Memory Allocs}	_{86,504,869 (±15.27%)}	_{84,481,981 (±11.05%)}	_~	_73,316,615	_101,497,663	_{p=1.000 n=6}
_{Config Time}	_{0.022s (± 5.35%)}	_{0.023s (± 4.55%)}	_~	_0.021s	_0.024s	_{p=0.214 n=6}
_{Parse Time}	_{0.928s (±24.06%)}	_{0.875s (±19.04%)}	_~	_0.726s	_1.200s	_{p=0.936 n=6}
_{Bind Time}	_{0.002s (±22.11%)}	_0.002s	_~	_~	_~	_{p=0.174 n=6}
_{Check Time}	_{16.267s (± 1.17%)}	_{16.237s (± 0.80%)}	_~	_16.139s	_16.487s	_{p=0.810 n=6}
_{Emit Time}	_{0.435s (± 3.70%)}	_{0.444s (± 3.67%)}	_~	_0.421s	_0.459s	_{p=0.470 n=6}
_{Total Time}	_{18.133s (± 0.95%)}	_{18.002s (± 1.51%)}	_~	_17.782s	_18.442s	_{p=0.230 n=6}
_{self-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,396,661	_1,396,664	_{+3 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_442,174	_442,177	_{+3 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory Used}	_{1,647,340k (± 0.21%)}	_{1,648,737k (± 0.18%)}	_~	_1,644,871k	_1,652,978k	_{p=0.575 n=6}
_{Memory Allocs}	_{57,281,290 (± 0.06%)}	_{57,266,484 (± 0.09%)}	_~	_57,201,898	_57,332,305	_{p=0.748 n=6}
_{Config Time}	_{0.020s (±43.89%)}	_{0.013s (±44.87%)}	_~	_0.006s	_0.022s	_{p=0.173 n=6}
_{Parse Time}	_{0.283s (± 7.31%)}	_{0.288s (± 2.73%)}	_~	_0.274s	_0.298s	_{p=0.471 n=6}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{2.310s (± 0.52%)}	_{2.330s (± 1.01%)}	_~	_2.308s	_2.375s	_{p=0.078 n=6}
_{Emit Time}	_{0.219s (± 5.43%)}	_{0.220s (± 2.93%)}	_~	_0.211s	_0.230s	_{p=0.520 n=6}
_{Total Time}	_{29.723s (± 1.01%)}	_{29.834s (± 0.68%)}	_~	_29.492s	_30.099s	_{p=0.471 n=6}
_{self-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_338,176	_338,176	_~	_~	_~	_{p=1.000 n=6}
_Types	_199,525	_199,526	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory Used}	_{331,846k (± 0.05%)}	_{331,775k (± 0.04%)}	_~	_331,597k	_331,923k	_{p=0.575 n=6}
_{Memory Allocs}	_{2,551,260 (± 0.03%)}	_{2,550,834 (± 0.04%)}	_~	_2,549,594	_2,552,763	_{p=0.471 n=6}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=6}
_{Parse Time}	_{0.134s (± 3.41%)}	_{0.133s (± 3.90%)}	_~	_0.127s	_0.140s	_{p=0.517 n=6}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{1.394s (± 0.81%)}	_{1.396s (± 0.85%)}	_~	_1.379s	_1.409s	_{p=0.748 n=6}
_{Emit Time}	_{0.091s (±13.04%)}	_{0.080s (± 9.03%)}	_~	_0.074s	_0.091s	_{p=0.090 n=6}
_{Total Time}	_{1.669s (± 0.84%)}	_{1.658s (± 0.69%)}	_~	_1.647s	_1.678s	_{p=0.298 n=6}
_{ts-pre-modules - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=6}
_Symbols	₇₅₂	₇₅₂	_~	_~	_~	_{p=1.000 n=6}
_Types	₃₅₆	₃₅₆	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{104,603k (± 0.04%)}	_{104,607k (± 0.04%)}	_~	_104,550k	_104,664k	_{p=0.936 n=6}
_{Memory Allocs}	_{111,133 (± 0.31%)}	_{111,036 (± 0.25%)}	_~	_110,584	_111,378	_{p=0.810 n=6}
_{Config Time}	_{0.001s (±48.94%)}	_{0.001s (±48.94%)}	_~	_0.000s	_0.001s	_{p=1.000 n=6}
_{Parse Time}	_{0.123s (± 6.01%)}	_{0.117s (± 4.29%)}	_~	_0.113s	_0.124s	_{p=0.103 n=6}
_{Bind Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{0.124s (± 5.96%)}	_{0.118s (± 4.25%)}	_~	_0.114s	_0.125s	_{p=0.103 n=6}
_{vscode - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_6,026,217	_6,026,143	_{-74 (- 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_2,218,346	_2,218,359	_{+13 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory Used}	_{4,081,708k (± 0.02%)}	_{4,083,576k (± 0.06%)}	_~	_4,081,632k	_4,086,925k	_{p=0.471 n=6}
_{Memory Allocs}	_{29,439,929 (± 0.08%)}	_{29,436,105 (± 0.05%)}	_~	_29,420,622	_29,459,889	_{p=1.000 n=6}
_{Config Time}	_{0.123s (± 7.77%)}	_{0.127s (± 2.41%)}	_~	_0.122s	_0.130s	_{p=0.468 n=6}
_{Parse Time}	_{0.709s (± 4.57%)}	_{0.709s (± 3.25%)}	_~	_0.681s	_0.738s	_{p=0.689 n=6}
_{Bind Time}	_{0.133s (±36.78%)}	_{0.129s (±19.16%)}	_~	_0.111s	_0.177s	_{p=0.468 n=6}
_{Check Time}	_{7.883s (± 1.70%)}	_{7.882s (± 1.31%)}	_~	_7.769s	_8.062s	_{p=1.000 n=6}
_{Emit Time}	_{1.843s (±12.49%)}	_{1.783s (±11.24%)}	_~	_1.645s	_2.124s	_{p=0.936 n=6}
_{Total Time}	_{10.706s (± 1.15%)}	_{10.653s (± 1.31%)}	_~	_10.516s	_10.894s	_{p=0.689 n=6}
_{webpack - native}
_Errors	₉₄₄	₉₄₅	_{+1 (+ 0.11%)}	_~	_~	_{p=0.001 n=6}
_Symbols	_695,186	_695,185	_{-1 (- 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_332,898	_332,913	_{+15 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory Used}	_{547,335k (± 0.03%)}	_{547,275k (± 0.03%)}	_~	_546,967k	_547,488k	_{p=0.575 n=6}
_{Memory Allocs}	_{4,152,894 (± 0.12%)}	_{4,156,619 (± 0.23%)}	_~	_4,144,254	_4,172,863	_{p=0.575 n=6}
_{Config Time}	_{0.013s (±22.81%)}	_{0.015s (±21.23%)}	_~	_0.010s	_0.018s	_{p=0.625 n=6}
_{Parse Time}	_{0.140s (± 6.52%)}	_{0.145s (± 3.35%)}	_~	_0.140s	_0.152s	_{p=0.090 n=6}
_{Bind Time}	_{0.033s (±23.99%)}	_{0.031s (±28.15%)}	_~	_0.023s	_0.042s	_{p=0.629 n=6}
_{Check Time}	_{1.096s (± 0.80%)}	_{1.096s (± 1.12%)}	_~	_1.078s	_1.110s	_{p=1.000 n=6}
_{Emit Time}	_{0.001s (±38.69%)}	_{0.001s (±34.96%)}	_~	_0.001s	_0.002s	_{p=0.595 n=6}
_{Total Time}	_{1.303s (± 1.30%)}	_{1.305s (± 1.42%)}	_~	_1.291s	_1.332s	_{p=1.000 n=6}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_991,654	_991,654	_~	_~	_~	_{p=1.000 n=6}
_Types	_350,842	_350,843	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory Used}	_{603,337k (± 0.01%)}	_{603,335k (± 0.01%)}	_~	_603,252k	_603,378k	_{p=0.936 n=6}
_{Memory Allocs}	_{4,948,795 (± 0.11%)}	_{4,949,532 (± 0.24%)}	_~	_4,935,544	_4,969,490	_{p=0.936 n=6}
_{Config Time}	_{0.007s (±25.25%)}	_{0.008s (±20.43%)}	_~	_0.005s	_0.009s	_{p=0.155 n=6}
_{Parse Time}	_{0.133s (± 5.26%)}	_{0.131s (± 4.07%)}	_~	_0.124s	_0.136s	_{p=0.810 n=6}
_{Bind Time}	_{0.032s (±20.09%)}	_{0.042s (± 8.19%)}	_{🔻+0.010s (+29.38%)}	_0.037s	_0.047s	_{p=0.016 n=6}
_{Check Time}	_{1.122s (± 1.88%)}	_{1.131s (± 0.71%)}	_~	_1.118s	_1.138s	_{p=0.470 n=6}
_{Emit Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{1.300s (± 1.78%)}	_{1.317s (± 0.74%)}	_~	_1.303s	_1.327s	_{p=0.335 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-bot · 2026-04-22T15:49:52Z

@ahejlsberg Here are the results of running the user tests with tsc comparing main and refs/pull/3497/merge:

There were infrastructure failures potentially unrelated to your change:

1 instance of "Git clone failed"

Otherwise...

Something interesting changed - please have a look.

Details

webpack

tsconfig.types.test.json

[NEW] error TS7053: Element implicitly has an 'any' type because expression of type 'string' can't be used to index type '{}'.
- lib/EnvironmentPlugin.js#L52

tsconfig.types.json

[NEW] error TS7053: Element implicitly has an 'any' type because expression of type 'string' can't be used to index type '{}'.
- lib/EnvironmentPlugin.js#L52

tsconfig.json

[NEW] error TS7053: Element implicitly has an 'any' type because expression of type 'string' can't be used to index type '{}'.
- lib/EnvironmentPlugin.js#L52

typescript-bot · 2026-04-22T16:37:06Z

@ahejlsberg Here are the results of running the top 400 repos with tsc comparing main and refs/pull/3497/merge:

Something interesting changed - please have a look.

Details

RooCodeInc/Roo-Code

8 of 15 projects failed to build with the old tsc and were ignored

src/tsconfig.json

error TS2339: Property 'warnings' does not exist on type '{ error?: undefined; providerProfiles: { currentApiConfigName: string; apiConfigs: { [x: string]: { codeIndexOpenAiKey?: string | undefined; codeIndexQdrantApiKey?: string | undefined; codebaseIndexOpenAiCompatibleBaseUrl?: string | undefined; ... 104 more ...; id?: string | undefined; }; }; modeApiConfigs: { ...; }...'.

ahejlsberg · 2026-04-22T22:06:18Z

Looks like we're fine on perf, but wondering what the changes in user tests and top 400 are about?

Andarist · 2026-04-23T08:59:36Z

webpack repro

This one makes Record<string, CodeValue> | {} to be reduced to {} (where previously it would reduce to Record<string, CodeValue>). But note that {} is a special JSLiteral type that has an implicit index signature - widening loses some flags on the type (but IIRC from my debugging session it doesn't lose JSLiteral). And because the flags are different, the strict subtype relationship checks in the union subtype reduction change the result. I need to look a little bit more into this to understand the exact rules there.

But if we strip the fact this has a mix of different sources for the this.defaultValues and reduce it to one unannotated one, then we can see it does error in the very same way: TS playground

And we can further demo the difference between the subtype reduction rules here (TS playground):

/** @typedef {string | number | boolean} CodeValue */

/**
 * @param {Record<string, CodeValue>} arg
 */
function test(arg) {
	const obj = {};
	const result1 = Math.random() ? arg : obj
	//    ^? const result1: {}
	const result2 = Math.random() ? arg : {}
	//    ^? const result2: Record<string, CodeValue>

}

The original webpack break is really close to the result1, in my opinion. The final subtype reduction for a constructor-declared type happens in a "non-fresh" context. So I think this one is actually mildly surprising at first but correct.

RooCodeInc/Roo-Code repro

This one is weird. I didn't yet have the time to debug why this has changed. However, we can clearly see that result contains { success: boolean; error: string; } at all relevant control flow points. So the fact that it result.warnings access didn't error before this change feels just wrong.

So overall, I think both breaks are expected. I'll push relevant tests to this PR later.

Andarist · 2026-04-23T17:18:51Z

New tests are in now.

Andarist added 2 commits April 22, 2026 14:46

Widen types assigned to auto-typed variables

5dc3c4e

add tests

bc2264c

add tests

cc0116f

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Widen types assigned to auto-typed variables#3497

Widen types assigned to auto-typed variables#3497
Andarist wants to merge 3 commits intomicrosoft:mainfrom
Andarist:fix/auto-type-lack-of-widening

Andarist commented Apr 22, 2026

Uh oh!

ahejlsberg commented Apr 22, 2026

Uh oh!

typescript-bot commented Apr 22, 2026 •

edited

Loading

Uh oh!

typescript-bot commented Apr 22, 2026

tsc

Uh oh!

typescript-bot commented Apr 22, 2026

webpack

tsconfig.types.test.json

tsconfig.types.json

tsconfig.json

Uh oh!

typescript-bot commented Apr 22, 2026

RooCodeInc/Roo-Code

src/tsconfig.json

Uh oh!

ahejlsberg commented Apr 22, 2026

Uh oh!

Andarist commented Apr 23, 2026

Uh oh!

Andarist commented Apr 23, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

Conversation

Andarist commented Apr 22, 2026

Uh oh!

ahejlsberg commented Apr 22, 2026

Uh oh!

typescript-bot commented Apr 22, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

typescript-bot commented Apr 22, 2026

tsc

Uh oh!

typescript-bot commented Apr 22, 2026

webpack

tsconfig.types.test.json

tsconfig.types.json

tsconfig.json

Uh oh!

typescript-bot commented Apr 22, 2026

RooCodeInc/Roo-Code

src/tsconfig.json

Uh oh!

ahejlsberg commented Apr 22, 2026

Uh oh!

Andarist commented Apr 23, 2026

Uh oh!

Andarist commented Apr 23, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

typescript-bot commented Apr 22, 2026 •

edited

Loading