[WIP] Fix regression in 6.0: Broader type inference issue 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>

Andarist · 2026-03-28T07:24:29Z

src/compiler/binder.ts

            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

RyanCavanaugh · 2026-03-28T16:01:28Z

@typescript-bot test it

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 · 2026-03-28T16:25:20Z

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

Everything looks the same!

You can check the log here.

typescript-bot · 2026-03-28T16:31:12Z

@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.

Copilot · 2026-03-28T16:59:53Z

src/compiler/binder.ts

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


PR description indicates the full test suite hasn’t been run yet. Please run the repo’s required verification (at least npx hereby runtests-parallel, plus npx hereby lint and npx hereby format if applicable) and update the PR before this is merged, since binder control-flow container changes can have broad impact.

typescript-bot · 2026-03-28T17:15:18Z

@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 · 2026-03-28T21:31:32Z

@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:

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Initial plan

f8dd2db

Copilot AI assigned Copilot and RyanCavanaugh Mar 27, 2026

Copilot AI 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

Open

Copilot started work on behalf of RyanCavanaugh March 27, 2026 23:30 View session

Copilot AI and 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>

Copilot AI requested a review from RyanCavanaugh March 28, 2026 00:31

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

Andarist reviewed Mar 28, 2026

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

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[WIP] Fix regression in 6.0: Broader type inference issue#63310

[WIP] Fix regression in 6.0: Broader type inference issue#63310
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		case SyntaxKind.PropertyDeclaration:
		return (node as PropertyDeclaration).initializer ? ContainerFlags.IsControlFlowContainer : ContainerFlags.None;

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