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fix: tree-shake unused default export in dynamic-reexport mode#20557

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shoryax wants to merge 1 commit intowebpack:mainfrom
shoryax:fix/tree-shaking-unused-default-export
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fix: tree-shake unused default export in dynamic-reexport mode#20557
shoryax wants to merge 1 commit intowebpack:mainfrom
shoryax:fix/tree-shaking-unused-default-export

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@shoryax
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@shoryax shoryax commented Feb 28, 2026 •
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this fixes the issue #20537, i:e,

when vue-loader generates an intermediary file that does export * from "./module", webpack falls into dynamic-reexport mode because it can't statically determine all exports. In this mode, it was returning EXPORTS_OBJECT_REFERENCED — marking ALL exports as used, including default.
But export * never re-exports default per the ES spec. The code already had mode.ignored which contains "default", but getReferencedExports wasn't checking it in the dynamic-reexport case.

What kind of change does this PR introduce?
Bug fix

Did you add tests for your changes?
Yes, added test/cases/optimize/tree-shaking-unused-default-export/

Does this PR introduce a breaking change?
Shouldn't be — this makes webpack correctly exclude default from export * re-exports, which is what the ES spec requires.

@shoryax
fix: tree-shake unused default export in dynamic-reexport mode
73aa64d 
 Fix: in dynamic-reexport mode, iterate over known exports and skip any that are in mode.ignored, so 'default' is correctly excluded and can be tree-shaken.
Fixes: webpack#20537
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⚠️ No Changeset found

Latest commit: 73aa64d

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linux-foundation-easycla bot commented Feb 28, 2026

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@alexander-akait
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alexander-akait commented Feb 28, 2026

@shoryax What you fixed? Your code look like AI generated, describe changes in logic

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codspeed-hq bot commented Feb 28, 2026 •
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Merging this PR will degrade performance by 89.31%

⚡ 6 improved benchmarks
❌ 4 regressed benchmarks
✅ 134 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

Mode Benchmark BASE HEAD Efficiency
❌ Memory benchmark "asset-modules-inline", scenario '{"name":"mode-development-rebuild","mode":"development","watch":true}' 731.9 KB 6,844.1 KB -89.31%
⚡ Memory benchmark "json-modules", scenario '{"name":"mode-development","mode":"development"}' 668.4 KB 541.7 KB +23.4%
⚡ Memory benchmark "asset-modules-resource", scenario '{"name":"mode-development","mode":"development"}' 3.2 MB 2 MB +56.79%
❌ Memory benchmark "many-modules-esm", scenario '{"name":"mode-development-rebuild","mode":"development","watch":true}' 298.9 KB 800.9 KB -62.68%
⚡ Memory benchmark "many-modules-commonjs", scenario '{"name":"mode-production","mode":"production"}' 9.2 MB 7.4 MB +23.91%
⚡ Memory benchmark "many-chunks-esm", scenario '{"name":"mode-production","mode":"production"}' 9.2 MB 7.2 MB +27.73%
❌ Memory benchmark "asset-modules-inline", scenario '{"name":"mode-development","mode":"development"}' 2.3 MB 3.6 MB -35.86%
⚡ Memory benchmark "many-modules-commonjs", scenario '{"name":"mode-development-rebuild","mode":"development","watch":true}' 798 KB 393 KB ×2
❌ Memory benchmark "many-modules-commonjs", scenario '{"name":"mode-development","mode":"development"}' 980.1 KB 1,293.4 KB -24.23%
⚡ Memory benchmark "asset-modules-resource", scenario '{"name":"mode-development-rebuild","mode":"development","watch":true}' 6,845.3 KB 733.3 KB ×9.3

Comparing shoryax:fix/tree-shaking-unused-default-export (73aa64d) with main (a3d7839)

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@hai-x
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hai-x commented Mar 3, 2026

I can’t see the issue based on the test case, and I don’t think this is a valid change.

dynamic-reexport dependency, means that provided info of the imported module exports is null, so we can only determine those exports at runtime. So for getReferencedExports, it will always return Dependency.EXPORTS_OBJECT_REFERENCED;

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@shoryax @alexander-akait @hai-x