Disallow `[await] using` statements under case/default clauses #62709

Renegade334 · 2025-11-02T18:59:46Z

As per tc39/ecma262@8e5d501, [await] using VariableStatements cannot be immediate children of case/default clauses; they must be contained within a block.

Resolves #62708.

npm go brrrrr

jakebailey · 2025-11-17T23:43:46Z

src/compiler/checker.ts

                );
            }
+            else if (isVariableStatement(declarationList.parent) && isCaseOrDefaultClause(declarationList.parent.parent)) {
+                return grammarErrorOnNode(


Surprised that this is not a parser error, but I guess the error above it isn't either. Fun that this means that we leave the bad node in the tree, but, that's what happens already:

//// [tests/cases/conformance/statements/VariableStatements/usingDeclarations/awaitUsingDeclarationsInForIn.ts] //// //// [awaitUsingDeclarationsInForIn.ts] async function main() { for (await using x in {}) { } } //// [awaitUsingDeclarationsInForIn.js] async function main() { for (await using x in {}) { } }

Aye. Technically it is allowed in the ES grammar, it just triggers an early error.

jakebailey

I think this is correct to my eye.

jakebailey · 2025-11-17T23:48:40Z

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

typescript-bot · 2025-11-17T23:48:45Z

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 · 2025-11-17T23:52:51Z

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/166565/artifacts?artifactName=tgz&fileId=6F13AC4CEB9E5076116C52E6E6C97C98A035AFC7605E7FE3524AF84CA670936702&fileName=/typescript-6.0.0-insiders.20251117.tgz"
    }
}

and then running npm install.

typescript-bot · 2025-11-18T00:13:58Z

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

Everything looks the same!

You can check the log here.

typescript-bot · 2025-11-18T00:18:08Z

@jakebailey Here are the results of running the user tests with tsc comparing main and refs/pull/62709/merge:

There were infrastructure failures potentially unrelated to your change:

1 instance of "Git clone failed"

Otherwise...

Everything looks good!

typescript-bot · 2025-11-18T00:42:06Z

@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	₁	₁	_~	_~	_~	_{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}	_{193,922k (± 0.94%)}	_{193,862k (± 0.96%)}	_~	_192,633k	_196,304k	_{p=0.748 n=6}
_{Parse Time}	_{1.31s (± 0.39%)}	_{1.31s (± 0.64%)}	_~	_1.29s	_1.31s	_{p=0.923 n=6}
_{Bind Time}	_{0.72s (± 0.57%)}	_{0.72s (± 0.75%)}	_~	_0.72s	_0.73s	_{p=0.282 n=6}
_{Check Time}	_{9.91s (± 0.43%)}	_{9.93s (± 0.34%)}	_~	_9.89s	_9.98s	_{p=0.333 n=6}
_{Emit Time}	_{2.74s (± 0.64%)}	_{2.74s (± 0.67%)}	_~	_2.73s	_2.77s	_{p=0.869 n=6}
_{Total Time}	_{14.68s (± 0.31%)}	_{14.70s (± 0.26%)}	_~	_14.66s	_14.77s	_{p=0.165 n=6}
_{angular-1 - node (v18.15.0, x64)}
_Errors	₂	₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_956,047	_956,047	_~	_~	_~	_{p=1.000 n=6}
_Types	_415,881	_415,881	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{1,255,013k (± 0.01%)}	_{1,255,008k (± 0.00%)}	_~	_1,254,966k	_1,255,083k	_{p=0.936 n=6}
_{Parse Time}	_{6.54s (± 0.51%)}	_{6.54s (± 0.68%)}	_~	_6.48s	_6.58s	_{p=1.000 n=6}
_{Bind Time}	_{1.88s (± 0.58%)}	_{1.89s (± 0.33%)}	_~	_1.88s	_1.90s	_{p=0.081 n=6}
_{Check Time}	_{32.25s (± 0.37%)}	_{32.47s (± 0.32%)}	_{+0.22s (+ 0.68%)}	_32.33s	_32.59s	_{p=0.010 n=6}
_{Emit Time}	_{14.88s (± 0.26%)}	_{14.92s (± 0.34%)}	_~	_14.87s	_15.00s	_{p=0.334 n=6}
_{Total Time}	_{55.55s (± 0.27%)}	_{55.81s (± 0.30%)}	_{+0.26s (+ 0.47%)}	_55.61s	_56.02s	_{p=0.045 n=6}
_{mui-docs - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_2,665,841	_2,665,841	_~	_~	_~	_{p=1.000 n=6}
_Types	_922,464	_922,464	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{2,980,350k (± 0.00%)}	_{2,980,292k (± 0.00%)}	_~	_2,980,218k	_2,980,356k	_{p=0.298 n=6}
_{Parse Time}	_{8.42s (± 0.32%)}	_{8.41s (± 0.31%)}	_~	_8.38s	_8.45s	_{p=1.000 n=6}
_{Bind Time}	_{2.17s (± 0.48%)}	_{2.17s (± 0.25%)}	_~	_2.16s	_2.17s	_{p=0.663 n=6}
_{Check Time}	_{91.82s (± 0.35%)}	_{91.78s (± 0.37%)}	_~	_91.17s	_92.08s	_{p=0.688 n=6}
_{Emit Time}	_{0.30s (± 1.79%)}	_0.31s	_~	_~	_~	_{p=0.071 n=6}
_{Total Time}	_{102.71s (± 0.33%)}	_{102.67s (± 0.34%)}	_~	_102.06s	_103.00s	_{p=0.810 n=6}
_{self-build-src - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,237,141	_1,237,110	_{-31 (- 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_259,801	_259,809	_{+8 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,732,965k (±14.56%)}	_{2,853,224k (±13.15%)}	_~	_2,367,942k	_3,096,005k	_{p=0.575 n=6}
_{Parse Time}	_{6.50s (± 2.01%)}	_{6.52s (± 2.04%)}	_~	_6.31s	_6.66s	_{p=1.000 n=6}
_{Bind Time}	_{2.16s (± 2.33%)}	_{2.17s (± 1.38%)}	_~	_2.13s	_2.21s	_{p=1.000 n=6}
_{Check Time}	_{42.84s (± 0.37%)}	_{42.88s (± 0.37%)}	_~	_42.65s	_43.09s	_{p=0.936 n=6}
_{Emit Time}	_{3.46s (± 1.19%)}	_{3.51s (± 1.98%)}	_~	_3.39s	_3.56s	_{p=0.172 n=6}
_{Total Time}	_{54.95s (± 0.49%)}	_{55.08s (± 0.52%)}	_~	_54.70s	_55.37s	_{p=0.378 n=6}
_{self-build-src-public-api - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,237,141	_1,237,110	_{-31 (- 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_259,801	_259,809	_{+8 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{3,039,549k (± 9.68%)}	_{3,160,030k (± 0.03%)}	_~	_3,158,753k	_3,161,489k	_{p=0.575 n=6}
_{Parse Time}	_{6.73s (± 1.81%)}	_{6.73s (± 0.76%)}	_~	_6.65s	_6.79s	_{p=0.630 n=6}
_{Bind Time}	_{2.14s (± 2.06%)}	_{2.13s (± 0.86%)}	_~	_2.11s	_2.16s	_{p=0.745 n=6}
_{Check Time}	_{42.99s (± 0.39%)}	_{43.08s (± 0.24%)}	_~	_42.97s	_43.26s	_{p=0.810 n=6}
_{Emit Time}	_{3.48s (± 2.68%)}	_{3.48s (± 1.32%)}	_~	_3.42s	_3.53s	_{p=0.471 n=6}
_{Total Time}	_{55.35s (± 0.49%)}	_{55.45s (± 0.12%)}	_~	_55.36s	_55.55s	_{p=0.688 n=6}
_{self-compiler - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_262,364	_262,311	_{-53 (- 0.02%)}	_~	_~	_{p=0.001 n=6}
_Types	_103,960	_103,966	_{+6 (+ 0.01%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{440,551k (± 0.01%)}	_{440,535k (± 0.01%)}	_~	_440,488k	_440,581k	_{p=0.575 n=6}
_{Parse Time}	_{2.82s (± 0.62%)}	_{2.82s (± 0.49%)}	_~	_2.80s	_2.84s	_{p=0.803 n=6}
_{Bind Time}	_{1.13s (± 0.74%)}	_{1.13s (± 0.56%)}	_~	_1.12s	_1.14s	_{p=0.340 n=6}
_{Check Time}	_{16.06s (± 0.29%)}	_{16.05s (± 0.25%)}	_~	_15.99s	_16.10s	_{p=0.872 n=6}
_{Emit Time}	_{1.32s (± 0.75%)}	_{1.30s (± 0.93%)}	_{-0.02s (- 1.64%)}	_1.28s	_1.31s	_{p=0.016 n=6}
_{Total Time}	_{21.32s (± 0.22%)}	_{21.30s (± 0.23%)}	_~	_21.21s	_21.34s	_{p=0.291 n=6}
_{ts-pre-modules - node (v18.15.0, x64)}
_Errors	₇₂	₇₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_225,386	_225,386	_~	_~	_~	_{p=1.000 n=6}
_Types	_94,304	_94,304	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{370,198k (± 0.07%)}	_{370,042k (± 0.02%)}	_~	_369,958k	_370,134k	_{p=0.471 n=6}
_{Parse Time}	_{3.51s (± 0.96%)}	_{3.53s (± 1.34%)}	_~	_3.45s	_3.59s	_{p=0.377 n=6}
_{Bind Time}	_{1.97s (± 1.52%)}	_{1.97s (± 1.31%)}	_~	_1.93s	_2.00s	_{p=0.517 n=6}
_{Check Time}	_{20.64s (± 0.14%)}	_{20.58s (± 0.28%)}	_{-0.07s (- 0.33%)}	_20.51s	_20.66s	_{p=0.035 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{26.13s (± 0.12%)}	_{26.08s (± 0.26%)}	_~	_26.02s	_26.19s	_{p=0.199 n=6}
_{vscode - node (v18.15.0, x64)}
_Errors	₁₁	₁₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_4,037,266	_4,037,266	_~	_~	_~	_{p=1.000 n=6}
_Types	_1,269,667	_1,269,667	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,819,442k (± 0.00%)}	_{3,819,554k (± 0.01%)}	_~	_3,819,339k	_3,819,904k	_{p=0.471 n=6}
_{Parse Time}	_{15.52s (± 0.59%)}	_{15.48s (± 0.30%)}	_~	_15.40s	_15.53s	_{p=0.336 n=6}
_{Bind Time}	_{5.07s (± 0.34%)}	_{5.07s (± 0.52%)}	_~	_5.03s	_5.10s	_{p=1.000 n=6}
_{Check Time}	_{109.74s (± 4.57%)}	_{108.13s (± 3.85%)}	_~	_103.26s	_112.83s	_{p=0.936 n=6}
_{Emit Time}	_{43.50s (±22.60%)}	_{43.12s (±24.30%)}	_~	_34.72s	_61.69s	_{p=1.000 n=6}
_{Total Time}	_{173.83s (± 4.98%)}	_{171.80s (± 4.96%)}	_~	_162.94s	_185.49s	_{p=0.936 n=6}
_{webpack - node (v18.15.0, x64)}
_Errors	₄₀	₄₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_379,737	_379,737	_~	_~	_~	_{p=1.000 n=6}
_Types	_166,566	_166,566	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{539,372k (± 0.03%)}	_{539,397k (± 0.01%)}	_~	_539,308k	_539,503k	_{p=0.936 n=6}
_{Parse Time}	_{4.62s (± 1.09%)}	_{4.63s (± 0.52%)}	_~	_4.59s	_4.65s	_{p=0.748 n=6}
_{Bind Time}	_{1.96s (± 1.09%)}	_{1.97s (± 1.46%)}	_~	_1.95s	_2.03s	_{p=0.139 n=6}
_{Check Time}	_{23.04s (± 1.12%)}	_{22.85s (± 0.19%)}	_{-0.18s (- 0.80%)}	_22.79s	_22.90s	_{p=0.030 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{29.62s (± 0.78%)}	_{29.45s (± 0.19%)}	_~	_29.38s	_29.52s	_{p=0.109 n=6}
_{xstate-main - node (v18.15.0, x64)}
_Errors	₃₀	₃₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_690,485	_690,485	_~	_~	_~	_{p=1.000 n=6}
_Types	_208,757	_208,757	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{586,475k (± 0.02%)}	_{586,606k (± 0.02%)}	_{+132k (+ 0.02%)}	_586,457k	_586,763k	_{p=0.045 n=6}
_{Parse Time}	_{4.20s (± 0.52%)}	_{4.19s (± 0.78%)}	_~	_4.15s	_4.24s	_{p=0.746 n=6}
_{Bind Time}	_{1.36s (± 1.42%)}	_{1.35s (± 0.66%)}	_~	_1.34s	_1.36s	_{p=0.222 n=6}
_{Check Time}	_{20.94s (± 1.43%)}	_{20.56s (± 1.81%)}	_~	_20.27s	_21.11s	_{p=0.064 n=6}
_{Emit Time}	_{0.00s (±154.76%)}	_{0.01s (±109.43%)}	_~	_0.00s	_0.01s	_{p=0.640 n=6}
_{Total Time}	_{26.51s (± 1.10%)}	_{26.11s (± 1.47%)}	_~	_25.76s	_26.64s	_{p=0.092 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 · 2025-11-18T01:16:09Z

@jakebailey Here are the results of running the top 400 repos with tsc comparing main and refs/pull/62709/merge:

Everything looks good!

jakebailey · 2025-11-19T22:08:42Z

Babel and acorn already did this, so I'll merge this. Thanks for the PR!

disallow [await] using declarations under case/default clauses

dec3a5b

github-project-automation bot moved this to Not started in PR Backlog Nov 2, 2025

github-project-automation bot added this to PR Backlog Nov 2, 2025

typescript-bot added the For Uncommitted Bug PR for untriaged, rejected, closed or missing bug label Nov 2, 2025

Renegade334 changed the title ~~Disallow [await] using declarations under case/default clauses~~ Disallow [await] using statements under case/default clauses Nov 2, 2025

force CI rerun

5533269

npm go brrrrr

typescript-bot added For Backlog Bug PRs that fix a backlog bug and removed For Uncommitted Bug PR for untriaged, rejected, closed or missing bug labels Nov 5, 2025

jakebailey mentioned this pull request Nov 7, 2025

fix(parser): reject using/await using in switch case/default clauses #62732

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jakebailey reviewed Nov 17, 2025

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jakebailey approved these changes Nov 17, 2025

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github-project-automation bot moved this from Not started to Needs merge in PR Backlog Nov 17, 2025

jakebailey merged commit 60e99ec into microsoft:main Nov 19, 2025
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github-project-automation bot moved this from Needs merge to Done in PR Backlog Nov 19, 2025

Renegade334 deleted the erm-switch-statements branch November 19, 2025 22:13

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Disallow `[await] using` statements under case/default clauses #62709

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Disallow [await] using statements under case/default clauses #62709

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Disallow `[await] using` statements under case/default clauses #62709

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