Mark class property initializers as outside of CFA containers by Copilot · Pull Request #63310 · microsoft/TypeScript

Copilot · 2026-03-27T23:30:27Z

Investigate root cause: PR Improve inference by not considering thisless functions to be context-sensitive #62243 removed PropertyDeclaration from ContainerFlags.IsControlFlowContainer in binder.ts
Root cause confirmed: property initializers no longer have isolated flow scopes, causing module-level narrowing (const DEFAULT: AB = 'A' → "A") to leak into class property type inference
Remove debug code from checker.ts
Fix: restore PropertyDeclaration as IsControlFlowContainer in binder.ts
Add regression test classPropertyInferenceFromBroaderTypeConst.ts
Verify test passes with correct types (D: AB, this.D: AB)
Run full test suite

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Agent-Logs-Url: https://github.com/microsoft/TypeScript/sessions/c865d3f5-aaf0-4dfc-8051-47e4fe1b3e86 Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>

Mateusz Burzyński (Andarist) · 2026-03-28T07:24:29Z

 return ContainerFlags.IsControlFlowContainer;

+ case SyntaxKind.PropertyDeclaration:
+ return (node as PropertyDeclaration).initializer ? ContainerFlags.IsControlFlowContainer : ContainerFlags.None;


It would be worth rechecking if this now shouldn't include PropagatesThisKeyword too

Mateusz Burzyński (@Andarist) how does this manifest? I tried to cook up a repro but failed

At the time of writing the comment, I didn't think of any specific case. I just knew this change might change the presence of this flag on some property declarations so it would be worth double-checking if that can lead to some adverse effects.
But like you and your agents, my agents and I can't find any either. 🤪
So my vote goes for "internal change without public-facing downsides"

Ryan Cavanaugh (RyanCavanaugh) · 2026-03-28T16:01:28Z

TypeScript Bot (@typescript-bot) test it

TypeScript Bot (typescript-bot) · 2026-03-28T16:01:34Z

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 (typescript-bot) · 2026-03-28T16:25:20Z

Hey Ryan Cavanaugh (@RyanCavanaugh), the results of running the DT tests are ready.

Everything looks the same!

You can check the log here.

TypeScript Bot (typescript-bot) · 2026-03-28T16:31:12Z

Ryan Cavanaugh (@RyanCavanaugh) Here are the results of running the user tests with tsc comparing main and refs/pull/63310/merge:

There were infrastructure failures potentially unrelated to your change:

1 instance of "Git clone failed"

Otherwise...

Everything looks good!

Copilot

Pull request overview

Fixes a TypeScript 6.0 regression where module-level narrowing from a broader-typed const could leak into class property initializer inference, by restoring an isolated control-flow scope for property initializers in the binder.

Changes:

Restore SyntaxKind.PropertyDeclaration as a control-flow container (when it has an initializer) in getContainerFlags.
Add a compiler regression test ensuring class/static properties infer the declared wider type (AB) rather than a narrowed literal.
Add new baselines (.types, .symbols, .js) for the regression test.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/compiler/binder.ts	Restores property declarations with initializers as isolated control-flow containers to prevent narrowing leakage into inference.
tests/cases/compiler/classPropertyInferenceFromBroaderTypeConst.ts	New regression test covering instance and static class property inference from a broader-typed `const`.
tests/baselines/reference/classPropertyInferenceFromBroaderTypeConst.types	Baseline verifying inferred types (e.g., `D: AB`, `SD: AB`) and switch narrowing behavior.
tests/baselines/reference/classPropertyInferenceFromBroaderTypeConst.symbols	Baseline verifying symbol binding for the new test.
tests/baselines/reference/classPropertyInferenceFromBroaderTypeConst.js	Baseline verifying emit output for the new test.

TypeScript Bot (typescript-bot) · 2026-03-28T17:15:18Z

Ryan Cavanaugh (@RyanCavanaugh) Here are the results of running the top 400 repos with tsc comparing main and refs/pull/63310/merge:

Everything looks good!

TypeScript Bot (typescript-bot) · 2026-03-28T21:31:32Z

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 - node (v18.15.0, x64)}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_82,521	_82,521	_~	_~	_~	_{p=1.000 n=6}
_Types	_125,280	_125,280	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{275,005k (± 0.50%)}	_{276,127k (± 0.68%)}	_~	_274,392k	_277,890k	_{p=0.936 n=6}
_{Parse Time}	_{1.31s (± 0.79%)}	_{1.31s (± 0.80%)}	_~	_1.29s	_1.32s	_{p=0.801 n=6}
_{Bind Time}	_0.76s	_{0.76s (± 0.54%)}	_~	_0.75s	_0.76s	_{p=0.405 n=6}
_{Check Time}	_{14.28s (± 1.15%)}	_{14.19s (± 0.49%)}	_~	_14.10s	_14.26s	_{p=0.336 n=6}
_{Emit Time}	_{2.69s (± 0.61%)}	_{2.69s (± 0.61%)}	_~	_2.67s	_2.72s	_{p=0.370 n=6}
_{Total Time}	_{19.04s (± 0.91%)}	_{18.94s (± 0.43%)}	_~	_18.83s	_19.02s	_{p=0.296 n=6}
_{angular-1 - node (v18.15.0, x64)}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_959,140	_959,140	_~	_~	_~	_{p=1.000 n=6}
_Types	_416,033	_416,033	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{1,261,053k (± 0.01%)}	_{1,261,321k (± 0.00%)}	_{+268k (+ 0.02%)}	_1,261,239k	_1,261,396k	_{p=0.005 n=6}
_{Parse Time}	_{8.01s (± 1.05%)}	_{8.05s (± 0.85%)}	_~	_7.98s	_8.16s	_{p=0.520 n=6}
_{Bind Time}	_{2.40s (± 0.82%)}	_{2.39s (± 1.10%)}	_~	_2.35s	_2.42s	_{p=0.359 n=6}
_{Check Time}	_{38.81s (± 0.42%)}	_{38.72s (± 0.53%)}	_~	_38.49s	_38.96s	_{p=0.470 n=6}
_{Emit Time}	_{18.11s (± 0.48%)}	_{18.08s (± 0.65%)}	_~	_17.85s	_18.18s	_{p=0.689 n=6}
_{Total Time}	_{67.33s (± 0.29%)}	_{67.23s (± 0.40%)}	_~	_67.02s	_67.63s	_{p=0.689 n=6}
_{mui-docs - node (v18.15.0, x64)}
_Errors	_11,136	_11,136	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_2,202,148	_2,202,148	_~	_~	_~	_{p=1.000 n=6}
_Types	_764,674	_764,674	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{2,642,677k (± 0.00%)}	_{2,642,601k (± 0.00%)}	_~	_2,642,416k	_2,642,791k	_{p=0.298 n=6}
_{Parse Time}	_{9.52s (± 0.25%)}	_{9.52s (± 0.18%)}	_~	_9.49s	_9.54s	_{p=1.000 n=6}
_{Bind Time}	_{2.50s (± 1.40%)}	_{2.53s (± 0.63%)}	_~	_2.51s	_2.55s	_{p=0.256 n=6}
_{Check Time}	_{86.29s (± 1.63%)}	_{87.59s (± 2.03%)}	_~	_85.49s	_90.33s	_{p=0.298 n=6}
_{Emit Time}	_{0.36s (± 2.48%)}	_{0.37s (± 3.17%)}	_~	_0.35s	_0.38s	_{p=0.214 n=6}
_{Total Time}	_{98.68s (± 1.41%)}	_{100.01s (± 1.78%)}	_~	_97.92s	_102.78s	_{p=0.173 n=6}
_{self-build-src - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,227,363	_1,227,363	_~	_~	_~	_{p=1.000 n=6}
_Types	_267,197	_267,198	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,779,598k (± 9.86%)}	_{2,779,994k (± 5.35%)}	_~	_2,718,790k	_3,083,850k	_{p=0.378 n=6}
_{Parse Time}	_{6.58s (± 1.21%)}	_{6.55s (± 0.81%)}	_~	_6.51s	_6.65s	_{p=0.689 n=6}
_{Bind Time}	_{2.24s (± 1.18%)}	_{2.24s (± 0.68%)}	_~	_2.21s	_2.25s	_{p=0.872 n=6}
_{Check Time}	_{43.09s (± 0.51%)}	_{42.85s (± 0.83%)}	_~	_42.41s	_43.40s	_{p=0.173 n=6}
_{Emit Time}	_{3.56s (± 2.90%)}	_{3.59s (± 3.14%)}	_~	_3.48s	_3.76s	_{p=0.873 n=6}
_{Total Time}	_{55.48s (± 0.55%)}	_{55.22s (± 0.75%)}	_~	_54.69s	_55.73s	_{p=0.575 n=6}
_{self-build-src-public-api - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,227,363	_1,227,363	_~	_~	_~	_{p=1.000 n=6}
_Types	_267,197	_267,198	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,785,493k (± 8.21%)}	_{2,785,042k (± 0.02%)}	_~	_2,784,403k	_2,785,587k	_{p=1.000 n=6}
_{Parse Time}	_{6.73s (± 1.23%)}	_{6.69s (± 0.73%)}	_~	_6.63s	_6.75s	_{p=0.689 n=6}
_{Bind Time}	_{2.23s (± 0.52%)}	_{2.26s (± 1.74%)}	_~	_2.22s	_2.32s	_{p=0.127 n=6}
_{Check Time}	_{43.00s (± 0.85%)}	_{42.82s (± 0.55%)}	_~	_42.52s	_43.22s	_{p=0.298 n=6}
_{Emit Time}	_{3.52s (± 4.16%)}	_{3.58s (± 5.07%)}	_~	_3.41s	_3.88s	_{p=0.521 n=6}
_{Total Time}	_{55.52s (± 0.69%)}	_{55.35s (± 0.75%)}	_~	_54.85s	_55.91s	_{p=0.575 n=6}
_{self-compiler - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_261,660	_261,660	_~	_~	_~	_{p=1.000 n=6}
_Types	_104,322	_104,323	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{439,336k (± 0.01%)}	_{439,347k (± 0.01%)}	_~	_439,248k	_439,402k	_{p=0.630 n=6}
_{Parse Time}	_{3.52s (± 0.72%)}	_{3.52s (± 0.54%)}	_~	_3.49s	_3.54s	_{p=0.933 n=6}
_{Bind Time}	_{1.34s (± 1.05%)}	_{1.38s (± 1.33%)}	_{+0.04s (+ 2.86%)}	_1.36s	_1.40s	_{p=0.008 n=6}
_{Check Time}	_{19.32s (± 0.48%)}	_{19.30s (± 0.39%)}	_~	_19.21s	_19.39s	_{p=0.689 n=6}
_{Emit Time}	_{1.55s (± 1.49%)}	_{1.56s (± 0.94%)}	_~	_1.54s	_1.58s	_{p=0.869 n=6}
_{Total Time}	_{25.74s (± 0.28%)}	_{25.76s (± 0.35%)}	_~	_25.63s	_25.88s	_{p=0.688 n=6}
_{ts-pre-modules - node (v18.15.0, x64)}
_Errors	₂₇₁	₂₇₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_225,897	_225,897	_~	_~	_~	_{p=1.000 n=6}
_Types	_93,564	_93,564	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{371,594k (± 0.01%)}	_{371,632k (± 0.02%)}	_~	_371,573k	_371,807k	_{p=0.471 n=6}
_{Parse Time}	_{2.85s (± 0.79%)}	_{2.85s (± 0.60%)}	_~	_2.83s	_2.87s	_{p=0.807 n=6}
_{Bind Time}	_{1.64s (± 0.99%)}	_{1.64s (± 0.71%)}	_~	_1.62s	_1.65s	_{p=0.568 n=6}
_{Check Time}	_{17.46s (± 0.22%)}	_{17.44s (± 0.50%)}	_~	_17.29s	_17.53s	_{p=0.685 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{21.95s (± 0.26%)}	_{21.94s (± 0.43%)}	_~	_21.77s	_22.01s	_{p=0.686 n=6}
_{vscode - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=NaN n=0}
_Symbols	₀	₀	_~	_~	_~	_{p=NaN n=0}
_Types	₀	₀	_~	_~	_~	_{p=NaN n=0}
_{Memory used}	_0k	_0k	_~	_~	_~	_{p=NaN n=0}
_{Parse Time}	_0s	_0s	_~	_~	_~	_{p=NaN n=0}
_{Bind Time}	_0s	_0s	_~	_~	_~	_{p=NaN n=0}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=NaN n=0}
_{Emit Time}	_0s	_0s	_~	_~	_~	_{p=NaN n=0}
_{Total Time}	_0s	_0s	_~	_~	_~	_{p=NaN n=0}
_{webpack - node (v18.15.0, x64)}
_Errors	₈	₈	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_408,835	_408,835	_~	_~	_~	_{p=1.000 n=6}
_Types	_186,908	_186,908	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{570,889k (± 0.02%)}	_{570,961k (± 0.02%)}	_~	_570,797k	_571,065k	_{p=0.298 n=6}
_{Parse Time}	_{4.75s (± 0.64%)}	_{4.73s (± 0.84%)}	_~	_4.66s	_4.78s	_{p=0.517 n=6}
_{Bind Time}	_{2.02s (± 0.88%)}	_{2.05s (± 1.62%)}	_~	_2.00s	_2.09s	_{p=0.063 n=6}
_{Check Time}	_{24.48s (± 0.42%)}	_{24.47s (± 0.54%)}	_~	_24.29s	_24.62s	_{p=0.873 n=6}
_{Emit Time}	_{0.00s (±154.76%)}	_{0.01s (±77.38%)}	_~	_0.00s	_0.01s	_{p=0.311 n=6}
_{Total Time}	_{31.25s (± 0.41%)}	_{31.25s (± 0.58%)}	_~	_30.99s	_31.49s	_{p=1.000 n=6}
_{xstate-main - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_781,066	_781,066	_~	_~	_~	_{p=1.000 n=6}
_Types	_227,795	_227,795	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{697,180k (± 0.01%)}	_{697,166k (± 0.01%)}	_~	_697,097k	_697,250k	_{p=0.575 n=6}
_{Parse Time}	_{3.98s (± 0.44%)}	_{3.98s (± 0.63%)}	_~	_3.95s	_4.01s	_{p=1.000 n=6}
_{Bind Time}	_{1.34s (± 0.47%)}	_{1.35s (± 0.56%)}	_~	_1.34s	_1.36s	_{p=0.081 n=6}
_{Check Time}	_{19.37s (± 0.45%)}	_{19.43s (± 0.46%)}	_~	_19.33s	_19.53s	_{p=0.261 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{24.68s (± 0.41%)}	_{24.75s (± 0.34%)}	_~	_24.64s	_24.85s	_{p=0.228 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

Jake Bailey (jakebailey) · 2026-03-31T16:21:29Z

TypeScript Bot (@typescript-bot) cherry-pick this to release-6.0

TypeScript Bot (typescript-bot) · 2026-03-31T16:21:34Z

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

Command	Status	Results
`cherry-pick this to release-6.0`	✅ Started	✅ Results

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com> Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>

TypeScript Bot (typescript-bot) · 2026-03-31T16:23:25Z

Hey, Jake Bailey (@jakebailey)! I've created #63327 for you.

…e-6.0 (#63327) Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com> Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>

Initial plan
f8dd2db

CopilotAI assigned Copilot and Ryan Cavanaugh (RyanCavanaugh)Mar 27, 2026

CopilotAI linked an issue Mar 27, 2026 that may be closed by this pull request
Regression in 6.0: type inferred from const with broader type is too narrow #63309
Closed

Copilotstarted work on behalf of Ryan Cavanaugh (RyanCavanaugh)March 27, 2026 23:30 View session

CopilotAIand others added 2 commits March 28, 2026 00:13

Initial plan and debug investigation
e491322
Agent-Logs-Url: https://github.com/microsoft/TypeScript/sessions/c865d3f5-aaf0-4dfc-8051-47e4fe1b3e86 Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>

Fix regression: restore PropertyDeclaration as flow container, add test
f7b579a
Agent-Logs-Url: https://github.com/microsoft/TypeScript/sessions/c865d3f5-aaf0-4dfc-8051-47e4fe1b3e86 Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>

CopilotAI requested a review from Ryan Cavanaugh (RyanCavanaugh)March 28, 2026 00:31

Copilot stopped work on behalf of Ryan Cavanaugh (RyanCavanaugh) due to an error March 28, 2026 00:31
Copilot had to stop work due to a timeout.

Mateusz Burzyński (Andarist) reviewed Mar 28, 2026
View reviewed changes

Ryan Cavanaugh (RyanCavanaugh) marked this pull request as ready for review March 28, 2026 16:50

CopilotAI review requested due to automatic review settings March 28, 2026 16:50

Copilotstarted reviewing on behalf of Ryan Cavanaugh (RyanCavanaugh)March 28, 2026 16:54 View session

CopilotAI reviewed Mar 28, 2026
View reviewed changes

Comment threadsrc/compiler/binder.ts

Ryan Cavanaugh (RyanCavanaugh) changed the title ~~[WIP] Fix regression in 6.0: Broader type inference issue~~Mark class property initializers as outside of CFA containersMar 30, 2026

Ryan Cavanaugh (RyanCavanaugh) added this to the TypeScript 6.0.2 milestone Mar 30, 2026

Jake Bailey (jakebailey) approved these changes Mar 30, 2026
View reviewed changes

Ryan Cavanaugh (RyanCavanaugh) added this pull request to the merge queue Mar 31, 2026

Merged via the queue into main with commit 0844c43Mar 31, 2026
27 checks passed

Ryan Cavanaugh (RyanCavanaugh) deleted the copilot/fix-narrow-inference-regression branch March 31, 2026 15:54

TypeScript Bot (typescript-bot) mentioned this pull request Mar 31, 2026
🤖 Pick PR #63310 (Mark class property initializers as...) into release-6.0 #63327
Merged

coderabbitaiBot mentioned this pull request Apr 10, 2026
chore(deps): update dependency typescript to v6 hiroppy/mf-dashboard#75
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Daniel Rosenwasser (DanielRosenwasser) modified the milestones: TypeScript 6.0.2, TypeScript 6.0.3 Apr 17, 2026

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