fix: `[Symbol.iterator]()` lost on union with `never` #62661

nrainhart · 2025-10-22T17:57:05Z

A type's [Symbol.iterator]() method is "lost" when union-ing that type with another type that's reducible to never.

The reason is that getReducedType was not getting called by getIterationTypesOfIterable, so non-iterable types that are reducible to never were not being erased out.

nrainhart · 2025-10-22T18:00:58Z

@microsoft-github-policy-service agree

jakebailey · 2025-10-22T18:01:43Z

@typescript-bot test it

typescript-bot · 2025-10-22T18:01:49Z

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-bot · 2025-10-22T18:24:49Z

Hey @jakebailey, the results of running the DT tests are ready.

Everything looks the same!

You can check the log here.

typescript-bot · 2025-10-22T18:31:42Z

@jakebailey Here are the results of running the user tests with tsc comparing main and refs/pull/62661/merge:

There were infrastructure failures potentially unrelated to your change:

1 instance of "Git clone failed"

Otherwise...

Everything looks good!

typescript-bot · 2025-10-22T18:54:13Z

@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 - node (v18.15.0, x64)}
_Errors	₃₄	₃₄	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_62,370	_62,370	_~	_~	_~	_{p=1.000 n=6}
_Types	_50,386	_50,386	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{193,939k (± 0.95%)}	_{193,275k (± 0.59%)}	_~	_192,696k	_195,584k	_{p=0.689 n=6}
_{Parse Time}	_{1.30s (± 0.49%)}	_{1.30s (± 0.93%)}	_~	_1.28s	_1.31s	_{p=0.673 n=6}
_{Bind Time}	_{0.72s (± 0.57%)}	_{0.72s (± 0.71%)}	_~	_0.72s	_0.73s	_{p=0.595 n=6}
_{Check Time}	_{9.76s (± 0.29%)}	_{9.75s (± 0.29%)}	_~	_9.70s	_9.78s	_{p=0.806 n=6}
_{Emit Time}	_{2.73s (± 0.68%)}	_{2.74s (± 1.47%)}	_~	_2.67s	_2.79s	_{p=0.466 n=6}
_{Total Time}	_{14.52s (± 0.30%)}	_{14.51s (± 0.36%)}	_~	_14.44s	_14.60s	_{p=0.809 n=6}
_{angular-1 - node (v18.15.0, x64)}
_Errors	₂	₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_955,117	_955,117	_~	_~	_~	_{p=1.000 n=6}
_Types	_415,619	_415,619	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{1,254,609k (± 0.00%)}	_{1,254,571k (± 0.00%)}	_~	_1,254,514k	_1,254,623k	_{p=0.261 n=6}
_{Parse Time}	_{6.52s (± 0.54%)}	_{6.52s (± 1.07%)}	_~	_6.45s	_6.63s	_{p=0.629 n=6}
_{Bind Time}	_{1.88s (± 0.34%)}	_{1.88s (± 0.43%)}	_~	_1.87s	_1.89s	_{p=0.432 n=6}
_{Check Time}	_{32.22s (± 0.23%)}	_{32.21s (± 0.22%)}	_~	_32.08s	_32.30s	_{p=0.809 n=6}
_{Emit Time}	_{14.90s (± 0.25%)}	_{14.88s (± 0.30%)}	_~	_14.83s	_14.94s	_{p=0.520 n=6}
_{Total Time}	_{55.52s (± 0.08%)}	_{55.48s (± 0.24%)}	_~	_55.28s	_55.63s	_{p=0.810 n=6}
_{mui-docs - node (v18.15.0, x64)}
_Errors	₁	₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_552,458	_552,458	_~	_~	_~	_{p=1.000 n=6}
_Types	₈₉	₈₉	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{827,398k (± 0.00%)}	_{827,398k (± 0.00%)}	_~	_827,386k	_827,404k	_{p=0.687 n=6}
_{Parse Time}	_{8.60s (± 0.15%)}	_{8.59s (± 0.26%)}	_~	_8.57s	_8.62s	_{p=0.684 n=6}
_{Bind Time}	_{2.19s (± 0.25%)}	_{2.19s (± 0.58%)}	_~	_2.17s	_2.20s	_{p=0.663 n=6}
_{Check Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_{0.27s (± 1.89%)}	_{0.27s (± 3.06%)}	_~	_0.26s	_0.28s	_{p=0.140 n=6}
_{Total Time}	_{11.07s (± 0.16%)}	_{11.06s (± 0.23%)}	_~	_11.02s	_11.09s	_{p=0.329 n=6}
_{self-build-src - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,237,247	_1,237,247	_~	_~	_~	_{p=1.000 n=6}
_Types	_259,866	_259,866	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{2,546,779k (±11.94%)}	_{2,487,181k (± 7.56%)}	_~	_2,364,610k	_2,732,121k	_{p=0.689 n=6}
_{Parse Time}	_{5.17s (± 1.56%)}	_{5.16s (± 0.76%)}	_~	_5.09s	_5.21s	_{p=1.000 n=6}
_{Bind Time}	_{1.77s (± 0.91%)}	_{1.76s (± 0.47%)}	_~	_1.76s	_1.78s	_{p=0.742 n=6}
_{Check Time}	_{35.09s (± 0.93%)}	_{34.92s (± 0.75%)}	_~	_34.60s	_35.28s	_{p=0.422 n=6}
_{Emit Time}	_{3.01s (± 1.78%)}	_{3.01s (± 1.41%)}	_~	_2.95s	_3.08s	_{p=0.688 n=6}
_{Total Time}	_{45.05s (± 0.74%)}	_{44.86s (± 0.50%)}	_~	_44.56s	_45.12s	_{p=0.378 n=6}
_{self-build-src-public-api - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,237,247	_1,237,247	_~	_~	_~	_{p=1.000 n=6}
_Types	_259,866	_259,866	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,159,395k (± 0.02%)}	_{3,158,535k (± 0.02%)}	_~	_3,157,769k	_3,159,362k	_{p=0.128 n=6}
_{Parse Time}	_{6.88s (± 0.55%)}	_{6.88s (± 0.76%)}	_~	_6.83s	_6.98s	_{p=0.872 n=6}
_{Bind Time}	_{2.15s (± 1.35%)}	_{2.16s (± 1.83%)}	_~	_2.13s	_2.22s	_{p=0.686 n=6}
_{Check Time}	_{42.55s (± 0.28%)}	_{42.54s (± 0.45%)}	_~	_42.29s	_42.77s	_{p=1.000 n=6}
_{Emit Time}	_{3.53s (± 3.04%)}	_{3.56s (± 2.25%)}	_~	_3.42s	_3.65s	_{p=0.470 n=6}
_{Total Time}	_{55.11s (± 0.22%)}	_{55.15s (± 0.42%)}	_~	_54.91s	_55.42s	_{p=0.810 n=6}
_{self-compiler - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_262,536	_262,536	_~	_~	_~	_{p=1.000 n=6}
_Types	_104,041	_104,041	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{440,665k (± 0.01%)}	_{440,682k (± 0.00%)}	_~	_440,656k	_440,716k	_{p=0.936 n=6}
_{Parse Time}	_{3.50s (± 0.94%)}	_{3.50s (± 1.10%)}	_~	_3.47s	_3.57s	_{p=1.000 n=6}
_{Bind Time}	_{1.33s (± 0.67%)}	_{1.32s (± 0.92%)}	_~	_1.31s	_1.34s	_{p=0.057 n=6}
_{Check Time}	_{19.04s (± 0.45%)}	_{19.01s (± 0.51%)}	_~	_18.89s	_19.12s	_{p=0.748 n=6}
_{Emit Time}	_{1.52s (± 0.68%)}	_{1.53s (± 0.83%)}	_~	_1.51s	_1.54s	_{p=0.359 n=6}
_{Total Time}	_{25.39s (± 0.38%)}	_{25.36s (± 0.42%)}	_~	_25.23s	_25.49s	_{p=0.575 n=6}
_{ts-pre-modules - node (v18.15.0, x64)}
_Errors	₇₂	₇₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_225,367	_225,367	_~	_~	_~	_{p=1.000 n=6}
_Types	_94,290	_94,290	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{370,065k (± 0.04%)}	_{369,993k (± 0.02%)}	_~	_369,869k	_370,070k	_{p=0.471 n=6}
_{Parse Time}	_{2.83s (± 0.53%)}	_{2.82s (± 1.10%)}	_~	_2.77s	_2.86s	_{p=1.000 n=6}
_{Bind Time}	_{1.60s (± 1.42%)}	_{1.59s (± 1.30%)}	_~	_1.57s	_1.63s	_{p=0.806 n=6}
_{Check Time}	_{16.45s (± 0.43%)}	_{16.45s (± 0.37%)}	_~	_16.39s	_16.55s	_{p=0.747 n=6}
_{Emit Time}	_{0.00s (±244.70%)}	_{0.00s (±244.70%)}	_~	_0.00s	_0.01s	_{p=1.000 n=6}
_{Total Time}	_{20.88s (± 0.35%)}	_{20.86s (± 0.34%)}	_~	_20.78s	_20.95s	_{p=1.000 n=6}
_{vscode - node (v18.15.0, x64)}
_Errors	₅	₅	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_3,966,343	_3,966,343	_~	_~	_~	_{p=1.000 n=6}
_Types	_1,244,764	_1,244,764	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,755,067k (± 0.01%)}	_{3,754,958k (± 0.01%)}	_~	_3,754,709k	_3,755,258k	_{p=0.810 n=6}
_{Parse Time}	_{15.38s (± 0.98%)}	_{15.25s (± 0.94%)}	_~	_15.12s	_15.53s	_{p=0.109 n=6}
_{Bind Time}	_{5.05s (± 2.27%)}	_{5.01s (± 0.63%)}	_~	_4.97s	_5.06s	_{p=0.688 n=6}
_{Check Time}	_{105.72s (± 3.61%)}	_{106.48s (± 6.32%)}	_~	_100.17s	_115.04s	_{p=0.936 n=6}
_{Emit Time}	_{35.31s (±10.37%)}	_{39.65s (±13.62%)}	_~	_31.35s	_46.98s	_{p=0.230 n=6}
_{Total Time}	_{161.47s (± 1.28%)}	_{166.39s (± 5.15%)}	_~	_159.12s	_179.59s	_{p=0.936 n=6}
_{webpack - node (v18.15.0, x64)}
_Errors	₄₀	₄₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_373,614	_373,614	_~	_~	_~	_{p=1.000 n=6}
_Types	_163,495	_163,495	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{530,025k (± 0.02%)}	_{530,034k (± 0.01%)}	_~	_529,975k	_530,124k	_{p=0.470 n=6}
_{Parse Time}	_{4.50s (± 0.98%)}	_{4.51s (± 0.79%)}	_~	_4.48s	_4.57s	_{p=0.683 n=6}
_{Bind Time}	_{1.92s (± 1.57%)}	_{1.93s (± 1.20%)}	_~	_1.89s	_1.96s	_{p=0.256 n=6}
_{Check Time}	_{22.03s (± 0.24%)}	_{22.11s (± 0.40%)}	_~	_22.02s	_22.27s	_{p=0.106 n=6}
_{Emit Time}	_{0.00s (±154.76%)}	_0.00s	_~	_~	_~	_{p=0.174 n=6}
_{Total Time}	_{28.45s (± 0.25%)}	_{28.55s (± 0.45%)}	_~	_28.45s	_28.80s	_{p=0.147 n=6}
_{xstate-main - node (v18.15.0, x64)}
_Errors	₃₀	₃₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_676,516	_676,516	_~	_~	_~	_{p=1.000 n=6}
_Types	_203,680	_203,680	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{578,416k (± 0.03%)}	_{578,440k (± 0.03%)}	_~	_578,195k	_578,730k	_{p=1.000 n=6}
_{Parse Time}	_{4.20s (± 0.39%)}	_{4.17s (± 0.56%)}	_{-0.03s (- 0.67%)}	_4.14s	_4.20s	_{p=0.046 n=6}
_{Bind Time}	_{1.36s (± 1.20%)}	_{1.35s (± 1.60%)}	_~	_1.32s	_1.38s	_{p=0.418 n=6}
_{Check Time}	_{20.54s (± 1.68%)}	_{20.58s (± 1.75%)}	_~	_20.29s	_21.06s	_{p=0.471 n=6}
_{Emit Time}	_{0.00s (±244.70%)}	_{0.00s (±244.70%)}	_~	_0.00s	_0.01s	_{p=1.000 n=6}
_{Total Time}	_{26.10s (± 1.35%)}	_{26.11s (± 1.48%)}	_~	_25.79s	_26.61s	_{p=1.000 n=6}

System info unknown

Hosts

node (v18.15.0, x64)

Scenarios

Compiler-Unions - node (v18.15.0, x64)
angular-1 - node (v18.15.0, x64)
mui-docs - node (v18.15.0, x64)
self-build-src - node (v18.15.0, x64)
self-build-src-public-api - node (v18.15.0, x64)
self-compiler - node (v18.15.0, x64)
ts-pre-modules - node (v18.15.0, x64)
vscode - node (v18.15.0, x64)
webpack - node (v18.15.0, x64)
xstate-main - node (v18.15.0, x64)

_Benchmark	_Name	_Iterations
_Current	_pr	₆
_Baseline	_baseline	₆

Developer Information:

Download Benchmarks

typescript-bot · 2025-10-22T19:19:48Z

@jakebailey Here are the results of running the top 400 repos with tsc comparing main and refs/pull/62661/merge:

Everything looks good!

RyanCavanaugh · 2025-10-22T20:31:01Z

@typescript-bot pack this

typescript-bot · 2025-10-22T20:31:06Z

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
`pack this`	✅ Started	✅ Results

typescript-bot · 2025-10-22T20:35:43Z

Hey @RyanCavanaugh, I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/166352/artifacts?artifactName=tgz&fileId=4467F597BAF6A42523EF954AA24F887067CCF4FAC4160952BFCAD0D31CEFC2A302&fileName=/typescript-6.0.0-insiders.20251022.tgz"
    }
}

and then running npm install.

There is also a playground for this build and an npm module you can use via "typescript": "npm:@typescript-deploys/pr-build@6.0.0-pr-62661-9".;

RyanCavanaugh · 2025-10-22T21:15:20Z

This seems right. I can't construct anything that didn't error before but does now due to a circularity

Andarist · 2025-10-22T21:48:16Z

src/compiler/checker.ts


        let allIterationTypes: IterationTypes[] | undefined;
        for (const constituent of (type as UnionType).types) {
+            if (!!(getReducedType(constituent).flags & TypeFlags.Never)) {


q: what if all union members reduce to never? wouldn't this break a case like this?

declare const o2: ({ a: "foo" } & { a: "bar" }) | ({ b: "qwe" } & { b: "rty" }); const [el2] = o2; // error

You're right, in that case TS allows the destructuring and types el2 as never.

I went with an alternative solution: reduce the whole iterable-like type instead of the union members: 93fb716 (#62661).

Added this case to the test as well

Andarist · 2025-10-22T21:51:15Z

tests/cases/compiler/iterableWithNeverAsUnionMember.ts

@@ -0,0 +1,30 @@
+// @strict: true
+// @noEmit: true
+// @target: es5,esnext


I think the es5 target should produce similar errors but right now it doesnt produce any

jakebailey · 2025-10-24T16:52:20Z

@typescript-bot test it
@typescript-bot pack this

typescript-bot · 2025-10-24T16:52:26Z

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
`pack this`	✅ Started	✅ Results
`test top400`	✅ Started	✅ Results
`user test this`	✅ Started	✅ Results
`run dt`	✅ Started	✅ Results
`perf test this faster`	✅ Started	👀 Results

typescript-bot · 2025-10-24T16:56:56Z

Hey @jakebailey, I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/166368/artifacts?artifactName=tgz&fileId=F90EC6886CB9060BB61CE905106796344DDAB41EF14C325F83B18EBA13BE17BE02&fileName=/typescript-6.0.0-insiders.20251024.tgz"
    }
}

and then running npm install.

There is also a playground for this build and an npm module you can use via "typescript": "npm:@typescript-deploys/pr-build@6.0.0-pr-62661-13".;

typescript-bot · 2025-10-24T17:17:49Z

Hey @jakebailey, the results of running the DT tests are ready.

Everything looks the same!

You can check the log here.

typescript-bot · 2025-10-24T17:21:45Z

@jakebailey Here are the results of running the user tests with tsc comparing main and refs/pull/62661/merge:

There were infrastructure failures potentially unrelated to your change:

1 instance of "Git clone failed"

Otherwise...

Everything looks good!

typescript-bot · 2025-10-24T17:42:44Z

@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 - node (v18.15.0, x64)}
_Errors	₃₄	₃₄	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_62,370	_62,370	_~	_~	_~	_{p=1.000 n=6}
_Types	_50,386	_50,386	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{194,592k (± 1.04%)}	_{195,258k (± 0.92%)}	_~	_192,741k	_196,547k	_{p=0.689 n=6}
_{Parse Time}	_{1.30s (± 0.40%)}	_{1.30s (± 0.69%)}	_~	_1.29s	_1.31s	_{p=0.541 n=6}
_{Bind Time}	_0.72s	_{0.72s (± 0.57%)}	_~	_0.72s	_0.73s	_{p=0.405 n=6}
_{Check Time}	_{9.74s (± 0.17%)}	_{9.76s (± 0.35%)}	_~	_9.73s	_9.81s	_{p=0.460 n=6}
_{Emit Time}	_{2.75s (± 0.44%)}	_{2.75s (± 0.74%)}	_~	_2.72s	_2.78s	_{p=1.000 n=6}
_{Total Time}	_{14.52s (± 0.17%)}	_{14.53s (± 0.22%)}	_~	_14.50s	_14.57s	_{p=0.292 n=6}
_{angular-1 - node (v18.15.0, x64)}
_Errors	₂	₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_955,117	_955,117	_~	_~	_~	_{p=1.000 n=6}
_Types	_415,619	_415,619	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{1,254,556k (± 0.00%)}	_{1,254,565k (± 0.00%)}	_~	_1,254,488k	_1,254,657k	_{p=0.936 n=6}
_{Parse Time}	_{6.53s (± 0.77%)}	_{6.55s (± 0.63%)}	_~	_6.48s	_6.58s	_{p=0.334 n=6}
_{Bind Time}	_{1.88s (± 0.40%)}	_{1.88s (± 0.34%)}	_~	_1.87s	_1.89s	_{p=0.718 n=6}
_{Check Time}	_{32.17s (± 0.44%)}	_{32.14s (± 0.35%)}	_~	_32.02s	_32.30s	_{p=0.575 n=6}
_{Emit Time}	_{14.86s (± 0.56%)}	_{14.89s (± 0.26%)}	_~	_14.87s	_14.97s	_{p=0.332 n=6}
_{Total Time}	_{55.43s (± 0.27%)}	_{55.47s (± 0.28%)}	_~	_55.31s	_55.69s	_{p=0.810 n=6}
_{mui-docs - node (v18.15.0, x64)}
_Errors	₁	₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_552,486	_552,486	_~	_~	_~	_{p=1.000 n=6}
_Types	₈₉	₈₉	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{827,427k (± 0.00%)}	_{827,433k (± 0.00%)}	_~	_827,428k	_827,442k	_{p=0.332 n=6}
_{Parse Time}	_{8.61s (± 0.14%)}	_{8.62s (± 0.15%)}	_~	_8.61s	_8.64s	_{p=0.139 n=6}
_{Bind Time}	_{2.19s (± 0.29%)}	_{2.19s (± 0.62%)}	_~	_2.17s	_2.21s	_{p=0.438 n=6}
_{Check Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_{0.27s (± 3.31%)}	_{0.27s (± 2.06%)}	_~	_0.26s	_0.27s	_{p=0.341 n=6}
_{Total Time}	_{11.07s (± 0.08%)}	_{11.08s (± 0.08%)}	_{+0.01s (+ 0.14%)}	_11.07s	_11.09s	_{p=0.021 n=6}
_{self-build-src - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,237,242	_1,237,243	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_259,863	_259,863	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{2,546,952k (±11.93%)}	_{2,425,251k (± 6.12%)}	_~	_2,364,175k	_2,728,549k	_{p=0.093 n=6}
_{Parse Time}	_{5.17s (± 1.42%)}	_{5.16s (± 1.08%)}	_~	_5.11s	_5.25s	_{p=0.936 n=6}
_{Bind Time}	_{1.77s (± 0.99%)}	_{1.78s (± 0.94%)}	_~	_1.76s	_1.80s	_{p=0.254 n=6}
_{Check Time}	_{35.18s (± 0.77%)}	_{35.09s (± 0.64%)}	_~	_34.68s	_35.32s	_{p=0.378 n=6}
_{Emit Time}	_{3.02s (± 1.16%)}	_{2.98s (± 0.55%)}	_~	_2.96s	_3.01s	_{p=0.065 n=6}
_{Total Time}	_{45.14s (± 0.61%)}	_{45.02s (± 0.39%)}	_~	_44.70s	_45.19s	_{p=0.471 n=6}
_{self-build-src-public-api - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,237,242	_1,237,243	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_259,863	_259,863	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,159,285k (± 0.01%)}	_{3,037,475k (± 9.72%)}	_{🟩-121,810k (- 3.86%)}	_2,434,235k	_3,158,528k	_{p=0.005 n=6}
_{Parse Time}	_{8.49s (± 0.97%)}	_{8.45s (± 1.22%)}	_~	_8.26s	_8.56s	_{p=0.807 n=6}
_{Bind Time}	_{2.64s (± 0.89%)}	_{2.65s (± 0.94%)}	_~	_2.62s	_2.68s	_{p=0.630 n=6}
_{Check Time}	_{52.67s (± 0.39%)}	_{52.64s (± 0.79%)}	_~	_51.81s	_52.94s	_{p=0.873 n=6}
_{Emit Time}	_{4.30s (± 1.77%)}	_{4.42s (± 1.96%)}	_~	_4.27s	_4.51s	_{p=0.066 n=6}
_{Total Time}	_{68.10s (± 0.39%)}	_{68.18s (± 0.86%)}	_~	_67.01s	_68.61s	_{p=0.230 n=6}
_{self-compiler - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_262,531	_262,532	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_104,038	_104,038	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{440,669k (± 0.02%)}	_{440,686k (± 0.02%)}	_~	_440,553k	_440,805k	_{p=0.810 n=6}
_{Parse Time}	_{3.50s (± 1.11%)}	_{3.51s (± 1.00%)}	_~	_3.47s	_3.57s	_{p=0.629 n=6}
_{Bind Time}	_{1.32s (± 1.04%)}	_{1.32s (± 1.01%)}	_~	_1.30s	_1.34s	_{p=0.801 n=6}
_{Check Time}	_{18.94s (± 0.51%)}	_{18.90s (± 0.42%)}	_~	_18.77s	_19.00s	_{p=0.575 n=6}
_{Emit Time}	_{1.52s (± 0.49%)}	_{1.52s (± 0.80%)}	_~	_1.50s	_1.53s	_{p=0.503 n=6}
_{Total Time}	_{25.28s (± 0.50%)}	_{25.24s (± 0.35%)}	_~	_25.08s	_25.34s	_{p=1.000 n=6}
_{ts-pre-modules - node (v18.15.0, x64)}
_Errors	₇₂	₇₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_225,374	_225,374	_~	_~	_~	_{p=1.000 n=6}
_Types	_94,297	_94,297	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{370,043k (± 0.02%)}	_{370,004k (± 0.02%)}	_~	_369,897k	_370,126k	_{p=0.471 n=6}
_{Parse Time}	_{2.81s (± 1.31%)}	_{2.82s (± 1.50%)}	_~	_2.77s	_2.90s	_{p=0.418 n=6}
_{Bind Time}	_{1.63s (± 1.15%)}	_{1.60s (± 0.83%)}	_{-0.03s (- 1.64%)}	_1.59s	_1.62s	_{p=0.026 n=6}
_{Check Time}	_{16.44s (± 0.34%)}	_{16.51s (± 0.41%)}	_~	_16.45s	_16.62s	_{p=0.092 n=6}
_{Emit Time}	_0.00s	_{0.00s (±244.70%)}	_~	_0.00s	_0.01s	_{p=0.405 n=6}
_{Total Time}	_{20.87s (± 0.37%)}	_{20.94s (± 0.24%)}	_~	_20.88s	_20.99s	_{p=0.127 n=6}
_{vscode - node (v18.15.0, x64)}
_Errors	₅	₅	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_3,974,366	_3,974,366	_~	_~	_~	_{p=1.000 n=6}
_Types	_1,247,316	_1,247,316	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,760,607k (± 0.01%)}	_{3,760,685k (± 0.00%)}	_~	_3,760,451k	_3,760,946k	_{p=0.471 n=6}
_{Parse Time}	_{15.22s (± 0.53%)}	_{15.25s (± 0.61%)}	_~	_15.15s	_15.41s	_{p=0.686 n=6}
_{Bind Time}	_{4.96s (± 0.42%)}	_{4.97s (± 0.97%)}	_~	_4.94s	_5.07s	_{p=0.745 n=6}
_{Check Time}	_{102.22s (± 3.35%)}	_{104.51s (± 3.61%)}	_~	_100.08s	_109.25s	_{p=0.336 n=6}
_{Emit Time}	_{37.97s (±13.14%)}	_{37.43s (±18.06%)}	_~	_30.83s	_49.31s	_{p=0.689 n=6}
_{Total Time}	_{160.38s (± 3.02%)}	_{162.16s (± 4.53%)}	_~	_157.44s	_176.86s	_{p=0.471 n=6}
_{webpack - node (v18.15.0, x64)}
_Errors	₄₀	₄₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_373,563	_373,563	_~	_~	_~	_{p=1.000 n=6}
_Types	_163,454	_163,454	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{530,008k (± 0.01%)}	_{530,034k (± 0.01%)}	_~	_529,960k	_530,133k	_{p=0.470 n=6}
_{Parse Time}	_{4.53s (± 0.94%)}	_{4.54s (± 0.79%)}	_~	_4.50s	_4.60s	_{p=0.746 n=6}
_{Bind Time}	_{1.95s (± 2.95%)}	_{1.93s (± 1.04%)}	_~	_1.91s	_1.96s	_{p=0.571 n=6}
_{Check Time}	_{22.09s (± 0.29%)}	_{22.15s (± 0.34%)}	_~	_22.05s	_22.24s	_{p=0.172 n=6}
_{Emit Time}	_{0.00s (±244.70%)}	_0.00s	_~	_~	_~	_{p=0.405 n=6}
_{Total Time}	_{28.57s (± 0.35%)}	_{28.62s (± 0.34%)}	_~	_28.48s	_28.75s	_{p=0.378 n=6}
_{xstate-main - node (v18.15.0, x64)}
_Errors	₃₀	₃₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_676,516	_676,516	_~	_~	_~	_{p=1.000 n=6}
_Types	_203,680	_203,680	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{578,405k (± 0.03%)}	_{578,440k (± 0.02%)}	_~	_578,267k	_578,535k	_{p=0.575 n=6}
_{Parse Time}	_{4.16s (± 0.42%)}	_{4.19s (± 0.47%)}	_{+0.03s (+ 0.72%)}	_4.17s	_4.22s	_{p=0.042 n=6}
_{Bind Time}	_{1.35s (± 0.90%)}	_{1.36s (± 1.02%)}	_~	_1.34s	_1.37s	_{p=0.323 n=6}
_{Check Time}	_{20.74s (± 2.04%)}	_{20.61s (± 1.99%)}	_~	_20.30s	_21.15s	_{p=0.688 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{26.25s (± 1.65%)}	_{26.16s (± 1.54%)}	_~	_25.85s	_26.69s	_{p=0.688 n=6}

System info unknown

Hosts

node (v18.15.0, x64)

Scenarios

Compiler-Unions - node (v18.15.0, x64)
angular-1 - node (v18.15.0, x64)
mui-docs - node (v18.15.0, x64)
self-build-src - node (v18.15.0, x64)
self-build-src-public-api - node (v18.15.0, x64)
self-compiler - node (v18.15.0, x64)
ts-pre-modules - node (v18.15.0, x64)
vscode - node (v18.15.0, x64)
webpack - node (v18.15.0, x64)
xstate-main - node (v18.15.0, x64)

_Benchmark	_Name	_Iterations
_Current	_pr	₆
_Baseline	_baseline	₆

Developer Information:

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typescript-bot · 2025-10-24T18:10:52Z

@jakebailey Here are the results of running the top 400 repos with tsc comparing main and refs/pull/62661/merge:

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nrainhart added 2 commits October 22, 2025 14:26

test: iterable with type reducible to never as union member

ba03419

fix: exclude union members reducible to 'never' when searching iterator

20bd02d

github-project-automation bot moved this to Not started in PR Backlog Oct 22, 2025

github-project-automation bot added this to PR Backlog Oct 22, 2025

nrainhart mentioned this pull request Oct 22, 2025

T[] | { t: 'a' } & { t: 'b' } is not iterable (TS2488) #62462

Open

RyanCavanaugh requested a review from ahejlsberg October 22, 2025 21:11

Andarist reviewed Oct 22, 2025

View reviewed changes

reduce the whole iterable-like type instead of the union members

93fb716

fix: [Symbol.iterator]() lost on union with never #62661

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fix: [Symbol.iterator]() lost on union with never #62661

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fix: `[Symbol.iterator]()` lost on union with `never` #62661

fix: `[Symbol.iterator]()` lost on union with `never` #62661

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