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Add Opt-in Clang Explicitly Built Modules Support for C++/ObjC++ Sources - #1599

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Add Opt-in Clang Explicitly Built Modules Support for C++/ObjC++ Sources#1599
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@qiongsiwu qiongsiwu commented Aug 5, 2026 •
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This PR enables explicitly built modules (EBM) for C++/ObjC++ files when modules are enabled, and the clang flag -fcxx-modules is in effect. It additionally adds a check against a block list, so that for projects that do not work with EBM well at the moment to opt out.

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qiongsiwu force-pushed the eng_172304064_block_list branch from 9463859 to 84b213c Compare August 5, 2026 23:39
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qiongsiwu commented Aug 5, 2026

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@swift-ci please test

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qiongsiwu requested a review from cyndyishida August 6, 2026 15:45
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cyndyishida commented Aug 6, 2026 •
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Why do we need separate settings? My feeling is that it adds more staging steps than we likely need. I think we should have a blocklist just for CLANG_ENABLE_EXPLICIT_MODULES and opt objective-c++/c++ sources in the same setting.

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qiongsiwu commented Aug 6, 2026

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Why do we need separate settings? My feeling is that it adds more staging steps than we likely need.

My understanding of the staging process is that we want to turn on CLANG_ENABLE_EXPLICIT_MODULES for C++/ObjC++ sources, and run world builds to figure out the content of the block list. I checked with @owenv and it seems to be rather cumbersome to run world builds with a custom swift-build. The suggestion is to land the enablement with the gating option default to off, and then run world build using a submitted swift-build with the gating option turned on.

I think we should have a blocklist just for CLANG_ENABLE_EXPLICIT_MODULES and opt objective-c++/c++ sources in the same setting.

What do we mean exactly by opt objective-c++/c++ sources in the same setting? Does this mean we want to flip CLANG_ENABLE_EXPLICIT_MODULES on only conditioning on EBM on and the block list? I don't quite see how that can work with an empty block list.

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qiongsiwu marked this pull request as draft August 6, 2026 18:20
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[Modules] Enable Explicitly Built Modules for C++/ObjC++ Projects By
124ca36 
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Enables EBM for C++/ObjC++ source files and adding a block list to
disable explicit modules for C++/ObjC++ source files.
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qiongsiwu force-pushed the eng_172304064_block_list branch from 84b213c to 124ca36 Compare September 24, 2026 23:13
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qiongsiwu commented Sep 24, 2026

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@swift-ci please test.

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@owenv owenv owenv approved these changes
@cmcgee1024 cmcgee1024 Awaiting requested review from cmcgee1024 cmcgee1024 is a code owner
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