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Implement LWG-3891 LWG-3870 breaks std::expected<cv T, E> - #6233

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Stephan T. Lavavej (StephanTLavavej) merged 3 commits into
microsoft:mainfrom
frederick-vs-ja:lwg-3891
Apr 17, 2026
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Implement LWG-3891 LWG-3870 breaks std::expected<cv T, E>#6233
Stephan T. Lavavej (StephanTLavavej) merged 3 commits into
microsoft:mainfrom
frederick-vs-ja:lwg-3891

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@frederick-vs-ja A. Jiang (frederick-vs-ja) commented Apr 7, 2026

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Fixes #6220.

Remarks:

  • The final resolution of LWG-3891 introduced some potential inconsistency - some member functions are detected but not actually called when value_type is cv-qualified. The assert(false); occurrences correspond to such inconsistency.
  • MSVC may fail to activate union member of an empty class type during constant evaluation, and DevCom-11070772 is reported for this. For MSVC, FakeCvAssignable is made non-empty for now.

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A. Jiang (frederick-vs-ja) requested a review from a team as a code owner April 7, 2026 08:02
@github-project-automation github-project-automation Bot moved this to Initial Review in STL Code Reviews Apr 7, 2026
@StephanTLavavej Stephan T. Lavavej (StephanTLavavej) added the LWG Library Working Group issue label Apr 7, 2026
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Stephan T. Lavavej (StephanTLavavej) reviewed Apr 14, 2026
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Comment thread tests/std/tests/P0323R12_expected/test.cpp Outdated
Comment thread tests/std/tests/P0323R12_expected/test.cpp
@StephanTLavavej Stephan T. Lavavej (StephanTLavavej) moved this from Initial Review to Ready To Merge in STL Code Reviews Apr 14, 2026
@StephanTLavavej Stephan T. Lavavej (StephanTLavavej) moved this from Ready To Merge to Merging in STL Code Reviews Apr 15, 2026
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Stephan T. Lavavej (StephanTLavavej) commented Apr 15, 2026

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I'm mirroring this to the MSVC-internal repo. Please notify me if any further changes are pushed, otherwise no action is required.

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Stephan T. Lavavej (StephanTLavavej) merged commit a519509 into microsoft:main Apr 17, 2026
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@github-project-automation github-project-automation Bot moved this from Merging to Done in STL Code Reviews Apr 17, 2026
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Stephan T. Lavavej (StephanTLavavej) commented Apr 17, 2026

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🏚️ 🛠️ 🏡

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A. Jiang (frederick-vs-ja) deleted the lwg-3891 branch April 17, 2026 10:23
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LWG-3891 LWG-3870 breaks std::expected<cv T, E>

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