On Tue, Oct 21, 2025 at 3:40 PM ClearScript Developers <
[email protected]> wrote:

> An update: We were able to get around these issues by migrating our Linux
> builds to the bundled C++ library.
>
> Support for system C++ libraries seems to be broken in 14.1, at least for
> Linux x64 and arm64. MacOS isn't affected but was easy to migrate, but we
> haven't been able to migrate our Windows build.
>
> Question for the V8 team: Is the removal of use_custom_libcxx imminent?
>

We inherit that flag from Chromium's build configuration, so this would be
a question for [email protected].
I only just found out that it is indeed marked deprecated since
https://chromium-review.googlesource.com/c/chromium/src/+/5963336. M138 has
passed, I don't know what the current plan is. The tracking bug
<https://issues.chromium.org/issues/372852985> doesn't look like lots of
activity recently.

Personally I'd be fine with accepting (reasonably non-intrusive)
community-supported patches that keep the system-libc++ build alive for the
time being.


> On Tuesday, October 14, 2025 at 1:42:29 PM UTC-4 ClearScript Developers
> wrote:
>
>> Greetings!
>>
>> While building the 14.1 branch tip on Linux, we're seeing compilation
>> errors. Our configuration:
>>
>> fatal_linker_warnings = false
>> is_cfi = false
>> is_component_build = false
>> is_debug = false
>> target_cpu = "x64"
>> use_custom_libcxx = false
>> use_thin_lto = false
>> v8_enable_fuzztest = false
>> v8_enable_pointer_compression = false
>> v8_enable_31bit_smis_on_64bit_arch = false
>> v8_enable_temporal_support = false
>> v8_monolithic = true
>> v8_use_external_startup_data = false
>> v8_target_cpu = "x64"
>>
>> The errors:
>>
>> FAILED: obj/build/modules/linux-x64/std_core/module.pcm
>> ../../../third_party/llvm-build/Release+Asserts/bin/clang++ -MMD -MF
>> obj/build/modules/linux-x64/std_core/module.pcm.d -D_FILE_OFFSET_BITS=64
>> -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_GNU_SOURCE
>> -DCR_CLANG_REVISION=\"llvmorg-21-init-16348-gbd809ffb-17\"
>> -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_NONE
>> -D_GLIBCXX_ASSERTIONS=1 -DCR_SYSROOT_KEY=20250129T203412Z-1 -DUSE_UDEV
>> -DUSE_AURA=1 -DUSE_GLIB=1 -DUSE_OZONE=1 -DNDEBUG -DNVALGRIND
>> -DDYNAMIC_ANNOTATIONS_ENABLED=0 -D_LIBCPP_BUILDING_LIBRARY -I../../.. -Igen
>> -fno-implicit-module-maps -fno-delete-null-pointer-checks
>> -fno-strict-overflow -fno-ident -fno-math-errno -fno-strict-aliasing
>> -fstack-protector -funwind-tables -fPIC -pthread -fcolor-diagnostics
>> -fmerge-all-constants -fno-sized-deallocation
>> -fcrash-diagnostics-dir=../../../tools/clang/crashreports -mllvm
>> -instcombine-lower-dbg-declare=0 -mllvm
>> -split-threshold-for-reg-with-hint=0 -ffp-contract=off
>> -Wa,--crel,--allow-experimental-crel --target=x86_64-unknown-linux-gnu
>> -msse3 -Wno-builtin-macro-redefined -D__DATE__= -D__TIME__=
>> -D__TIMESTAMP__= -ffile-compilation-dir=. -no-canonical-prefixes -Xclang
>> -fmodule-file-home-is-cwd -ftrivial-auto-var-init=pattern -O2
>> -fdata-sections -ffunction-sections -fno-unique-section-names
>> -fno-omit-frame-pointer -g0 -fvisibility=hidden -Wheader-hygiene
>> -Wstring-conversion -Wtautological-overlap-compare -Xclang -add-plugin
>> -Xclang find-bad-constructs -Xclang -plugin-arg-find-bad-constructs -Xclang
>> check-stack-allocated -Xclang -plugin-arg-find-bad-constructs -Xclang
>> check-ipc -Xclang -add-plugin -Xclang raw-ptr-plugin -Xclang
>> -plugin-arg-raw-ptr-plugin -Xclang check-raw-ptr-to-stack-allocated -Xclang
>> -plugin-arg-raw-ptr-plugin -Xclang
>> disable-check-raw-ptr-to-stack-allocated-error -Xclang
>> -plugin-arg-raw-ptr-plugin -Xclang
>> raw-ptr-exclude-path=../../../third_party/dawn/ -Xclang
>> -plugin-arg-raw-ptr-plugin -Xclang
>> raw-ptr-exclude-path=../../../build/config/clang/chrome/test/fuzzing/webidl_fuzzing/
>> -fstrict-aliasing -Wundef -fPIC -Wall -Wno-unused-variable
>> -Wno-c++11-narrowing -Wno-unused-but-set-variable
>> -Wno-missing-field-initializers -Wno-unused-parameter -Wno-psabi
>> -Wloop-analysis -Wno-unneeded-internal-declaration -Wno-cast-function-type
>> -Wno-thread-safety-reference-return -Wno-nontrivial-memcall -Werror
>> -Wno-exit-time-destructors -std=c++20 -Wno-trigraphs
>> -gsimple-template-names -fno-exceptions -fno-rtti
>> --sysroot=../../../build/linux/debian_bullseye_amd64-sysroot
>> -fvisibility-inlines-hidden -fbuiltin-module-map
>> -fmodule-map-file=gen/third_party/libc++/src/include/module.modulemap
>> -fmodule-map-file=../../../build/modules/linux-x64/module.modulemap
>> -std=c++20 -Wno-invalid-offsetof -Wenum-compare-conditional
>> -Wno-nullability-completeness
>> -fmodule-map-file=gen/third_party/libc++/src/include/module.modulemap
>> -fbuiltin-module-map
>> -fmodule-map-file=../../../build/modules/linux-x64/module.modulemap
>> -fmodule-file=obj/build/modules/linux-x64/_Builtin_stdarg/module.pcm
>> -fmodule-file=obj/build/modules/linux-x64/_Builtin_stddef/module.pcm
>> -fmodule-file=obj/build/modules/linux-x64/_Builtin_stdint/module.pcm
>> -fmodule-file=obj/build/modules/linux-x64/sys_stage1/module.pcm
>> -fmodule-name=std_core -c -x c++ -Xclang -emit-module
>> gen/third_party/libc++/src/include/module.modulemap -o
>> obj/build/modules/linux-x64/std_core/module.pcm
>> While building module 'std_core':
>> In file included from <module-includes>:1:
>> gen/third_party/libc++/src/include/__cstddef/byte.h:12:10: fatal error:
>> '__config' file not found
>>    12 | #include <__config>
>>       |          ^~~~~~~~~~
>> 1 error generated.
>> [1116/3269] CXX_MODULE obj/build/modules/linux-x64/std_ctype_h/module.pcm
>> FAILED: obj/build/modules/linux-x64/std_ctype_h/module.pcm
>> ../../../third_party/llvm-build/Release+Asserts/bin/clang++ -MMD -MF
>> obj/build/modules/linux-x64/std_ctype_h/module.pcm.d -D_FILE_OFFSET_BITS=64
>> -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_GNU_SOURCE
>> -DCR_CLANG_REVISION=\"llvmorg-21-init-16348-gbd809ffb-17\"
>> -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_NONE
>> -D_GLIBCXX_ASSERTIONS=1 -DCR_SYSROOT_KEY=20250129T203412Z-1 -DUSE_UDEV
>> -DUSE_AURA=1 -DUSE_GLIB=1 -DUSE_OZONE=1 -DNDEBUG -DNVALGRIND
>> -DDYNAMIC_ANNOTATIONS_ENABLED=0 -D_LIBCPP_BUILDING_LIBRARY -I../../.. -Igen
>> -fno-implicit-module-maps -fno-delete-null-pointer-checks
>> -fno-strict-overflow -fno-ident -fno-math-errno -fno-strict-aliasing
>> -fstack-protector -funwind-tables -fPIC -pthread -fcolor-diagnostics
>> -fmerge-all-constants -fno-sized-deallocation
>> -fcrash-diagnostics-dir=../../../tools/clang/crashreports -mllvm
>> -instcombine-lower-dbg-declare=0 -mllvm
>> -split-threshold-for-reg-with-hint=0 -ffp-contract=off
>> -Wa,--crel,--allow-experimental-crel --target=x86_64-unknown-linux-gnu
>> -msse3 -Wno-builtin-macro-redefined -D__DATE__= -D__TIME__=
>> -D__TIMESTAMP__= -ffile-compilation-dir=. -no-canonical-prefixes -Xclang
>> -fmodule-file-home-is-cwd -ftrivial-auto-var-init=pattern -O2
>> -fdata-sections -ffunction-sections -fno-unique-section-names
>> -fno-omit-frame-pointer -g0 -fvisibility=hidden -Wheader-hygiene
>> -Wstring-conversion -Wtautological-overlap-compare -Xclang -add-plugin
>> -Xclang find-bad-constructs -Xclang -plugin-arg-find-bad-constructs -Xclang
>> check-stack-allocated -Xclang -plugin-arg-find-bad-constructs -Xclang
>> check-ipc -Xclang -add-plugin -Xclang raw-ptr-plugin -Xclang
>> -plugin-arg-raw-ptr-plugin -Xclang check-raw-ptr-to-stack-allocated -Xclang
>> -plugin-arg-raw-ptr-plugin -Xclang
>> disable-check-raw-ptr-to-stack-allocated-error -Xclang
>> -plugin-arg-raw-ptr-plugin -Xclang
>> raw-ptr-exclude-path=../../../third_party/dawn/ -Xclang
>> -plugin-arg-raw-ptr-plugin -Xclang
>> raw-ptr-exclude-path=../../../build/config/clang/chrome/test/fuzzing/webidl_fuzzing/
>> -fstrict-aliasing -Wundef -fPIC -Wall -Wno-unused-variable
>> -Wno-c++11-narrowing -Wno-unused-but-set-variable
>> -Wno-missing-field-initializers -Wno-unused-parameter -Wno-psabi
>> -Wloop-analysis -Wno-unneeded-internal-declaration -Wno-cast-function-type
>> -Wno-thread-safety-reference-return -Wno-nontrivial-memcall -Werror
>> -Wno-exit-time-destructors -std=c++20 -Wno-trigraphs
>> -gsimple-template-names -fno-exceptions -fno-rtti
>> --sysroot=../../../build/linux/debian_bullseye_amd64-sysroot
>> -fvisibility-inlines-hidden -fbuiltin-module-map
>> -fmodule-map-file=gen/third_party/libc++/src/include/module.modulemap
>> -fmodule-map-file=../../../build/modules/linux-x64/module.modulemap
>> -std=c++20 -Wno-invalid-offsetof -Wenum-compare-conditional
>> -Wno-nullability-completeness
>> -fmodule-map-file=gen/third_party/libc++/src/include/module.modulemap
>> -fmodule-map-file=../../../build/modules/linux-x64/module.modulemap
>> -fbuiltin-module-map
>> -fmodule-file=obj/build/modules/linux-x64/_Builtin_stdarg/module.pcm
>> -fmodule-file=obj/build/modules/linux-x64/_Builtin_stddef/module.pcm
>> -fmodule-file=obj/build/modules/linux-x64/sys_stage1/module.pcm
>> -fmodule-name=std_ctype_h -c -x c++ -Xclang -emit-module
>> gen/third_party/libc++/src/include/module.modulemap -o
>> obj/build/modules/linux-x64/std_ctype_h/module.pcm
>> While building module 'std_ctype_h':
>> In file included from <module-includes>:1:
>> gen/third_party/libc++/src/include/ctype.h:35:12: error: '__config' file
>> not found with <angled> include; use "quotes" instead
>>    35 | #  include <__config>
>>       |            ^
>> While building module 'std_ctype_h':
>> In file included from <module-includes>:1:
>> In file included from gen/third_party/libc++/src/include/ctype.h:35:
>> gen/third_party/libc++/src/include/__config:13:10: error: '__config_site'
>> file not found with <angled> include; use "quotes" instead
>>    13 | #include <__config_site>
>>       |          ^
>> gen/third_party/libc++/src/include/__config:14:10: error:
>> '__configuration/abi.h' file not found with <angled> include; use "quotes"
>> instead
>>    14 | #include <__configuration/abi.h>
>>       |          ^
>> While building module 'std_ctype_h':
>> In file included from <module-includes>:1:
>> In file included from gen/third_party/libc++/src/include/ctype.h:35:
>> In file included from gen/third_party/libc++/src/include/__config:14:
>> gen/third_party/libc++/src/include/__configuration/abi.h:13:10: fatal
>> error: '__config_site' file not found
>>    13 | #include <__config_site>
>>       |          ^~~~~~~~~~~~~~~
>> 4 errors generated.
>>
>> Full log attached. Can someone suggest a solution?
>>
> --
>

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