V8Update bundles Clang 18, but Visual Studio requires Clang 19, what do?

[AnsisMalins]

Building V8 fails with errors like this. What do I do?

(Why do build instructions ask to install Clang for Windows when V8Update downloads and uses its own copy?)

[1/2292] CXX obj/cppgc_base/compaction-worklists.obj
FAILED: obj/cppgc_base/compaction-worklists.obj 
..\..\..\third_party\llvm-build\Release+Asserts\bin\clang-cl.exe /c ../../../src/heap/cppgc/compaction-worklists.cc /Foobj/cppgc_base/compaction-worklists.obj /nologo /showIncludes:user "-imsvc../../../../../../../../../../Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/MSVC/14.44.35207/include" "-imsvc../../../../../../../../../../Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/MSVC/14.44.35207/ATLMFC/include" "-imsvc../../../../../../../../../../Program Files/Microsoft Visual Studio/2022/Community/VC/Auxiliary/VS/include" "-imsvc../../../../../../../../../../Program Files (x86)/Windows Kits/10/include/10.0.22621.0/ucrt" "-imsvc../../../../../../../../../../Program Files (x86)/Windows Kits/10/include/10.0.22621.0/um" "-imsvc../../../../../../../../../../Program Files (x86)/Windows Kits/10/include/10.0.22621.0/shared" "-imsvc../../../../../../../../../../Program Files (x86)/Windows Kits/10/include/10.0.22621.0/winrt" "-imsvc../../../../../../../../../../Program Files (x86)/Windows Kits/10/include/10.0.22621.0/cppwinrt" "-imsvc../../../../../../../../../../Program Files (x86)/Windows Kits/NETFXSDK/4.8/include/um" -DUSE_AURA=1 -D_HAS_NODISCARD -D_CRT_NONSTDC_NO_WARNINGS -D_WINSOCK_DEPRECATED_NO_WARNINGS -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_NONE "-DCR_CLANG_REVISION=\"llvmorg-18-init-17730-gf670112a-5\"" -D_HAS_EXCEPTIONS=0 -D__STD_C -D_CRT_RAND_S -D_CRT_SECURE_NO_DEPRECATE -D_SCL_SECURE_NO_DEPRECATE -D_ATL_NO_OPENGL -D_WINDOWS -DCERT_CHAIN_PARA_HAS_EXTRA_FIELDS -DPSAPI_VERSION=2 -DWIN32 -D_SECURE_ATL -DWINAPI_FAMILY=WINAPI_FAMILY_DESKTOP_APP -DWIN32_LEAN_AND_MEAN -DNOMINMAX -D_UNICODE -DUNICODE -DNTDDI_VERSION=NTDDI_WIN10_NI -D_WIN32_WINNT=0x0A00 -DWINVER=0x0A00 -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 "-DV8_EMBEDDER_STRING=\"-ClearScript\"" -DV8_TYPED_ARRAY_MAX_SIZE_IN_HEAP=64 -DV8_INTL_SUPPORT -DV8_ATOMIC_OBJECT_FIELD_WRITES -DV8_ENABLE_LAZY_SOURCE_POSITIONS -DV8_SHARED_RO_HEAP -DV8_WIN64_UNWINDING_INFO -DV8_ENABLE_REGEXP_INTERPRETER_THREADED_DISPATCH -DV8_ENABLE_FUZZTEST -DV8_ENABLE_SPARKPLUG -DV8_ENABLE_TURBOFAN -DV8_ENABLE_SYSTEM_INSTRUMENTATION -DV8_ENABLE_ETW_STACK_WALKING -DV8_ENABLE_WEBASSEMBLY -DV8_ENABLE_CONTINUATION_PRESERVED_EMBEDDER_DATA -DV8_ALLOCATION_FOLDING -DV8_ALLOCATION_SITE_TRACKING -DV8_ADVANCED_BIGINT_ALGORITHMS -DV8_USE_ZLIB -DV8_USE_LIBM_TRIG_FUNCTIONS -DV8_ENABLE_MAGLEV_GRAPH_PRINTER -DV8_ENABLE_EXTENSIBLE_RO_SNAPSHOT -DV8_DEPRECATION_WARNINGS -DV8_IMMINENT_DEPRECATION_WARNINGS -DCPPGC_SLIM_WRITE_BARRIER -DV8_TARGET_ARCH_IA32 -DV8_HAVE_TARGET_OS -DV8_TARGET_OS_WIN -I../../.. -Igen -I../../../include /W4 -Wimplicit-fallthrough -Wextra-semi -Wunreachable-code-aggressive -Wthread-safety -Wno-missing-field-initializers -Wno-unused-parameter -Wno-psabi -Wloop-analysis -Wno-unneeded-internal-declaration -Wno-nonportable-include-path -Wenum-compare-conditional -Wno-ignored-pragma-optimize -Wno-deprecated-builtins -Wno-bitfield-constant-conversion -Wno-deprecated-this-capture -Wno-invalid-offsetof -Wno-vla-extension -Wno-thread-safety-reference-return -Wshadow /WX -fno-delete-null-pointer-checks -fno-ident -fcolor-diagnostics -fmerge-all-constants -fcrash-diagnostics-dir=../../../tools/clang/crashreports -mllvm -instcombine-lower-dbg-declare=0 -mllvm -split-threshold-for-reg-with-hint=0 /clang:-ffp-contract=off -fcomplete-member-pointers /Gy /FS /bigobj /utf-8 /Zc:twoPhase -ffile-reproducible /Zc:sizedDealloc- /D__WRL_ENABLE_FUNCTION_STATICS__ -fmsc-version=1934 -m32 -msse3 /Brepro -Wno-builtin-macro-redefined -D__DATE__= -D__TIME__= -D__TIMESTAMP__= -ffile-compilation-dir=. -no-canonical-prefixes /Z7 -gno-codeview-command-line -gcodeview-ghash -Xclang -fuse-ctor-homing /guard:cf /MT -Wheader-hygiene -Wstring-conversion -Wtautological-overlap-compare -D_SILENCE_CXX20_OLD_SHARED_PTR_ATOMIC_SUPPORT_DEPRECATION_WARNING /arch:SSE2 -Wunreachable-code -Wno-shadow -Wctad-maybe-unsupported -Wno-invalid-offsetof /wd4245 /wd4267 /wd4324 /wd4701 /wd4702 /wd4703 /wd4709 /wd4714 /wd4715 /wd4718 /wd4723 /wd4724 /wd4800 -Wmissing-field-initializers /O2 /Ob2 /Oy- /Zc:inline /Gw /clang:-fno-math-errno -Wexit-time-destructors -Wno-c++11-narrowing-const-reference /std:c++20 -Wno-trigraphs /TP /GR- /Fd"obj/cppgc_base_cc.pdb"
In file included from ../../../src/heap/cppgc/compaction-worklists.cc:6:
In file included from ../../..\src/heap/cppgc/compaction-worklists.h:8:
In file included from ../../../../../../../../../../Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/MSVC/14.44.35207/include\unordered_set:8:
../../../../../../../../../../Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/MSVC/14.44.35207/include\yvals_core.h(908,1): error: static assertion failed: error STL1000: Unexpected compiler version, expected Clang 19.0.0 or newer.
  908 | _EMIT_STL_ERROR(STL1000, "Unexpected compiler version, expected Clang 19.0.0 or newer.");
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../../../../../../../../Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/MSVC/14.44.35207/include\yvals_core.h(534,58): note: expanded from macro '_EMIT_STL_ERROR'
  534 | #define _EMIT_STL_ERROR(NUMBER, MESSAGE)   static_assert(false, "error " #NUMBER ": " MESSAGE)
      |                                                          ^~~~~
1 error generated.

[ClearScriptLib]

Hi @AnsisMalins,

Hmm, we haven't encountered that specific issue, but something nearly always breaks when we upgrade V8 or Visual Studio.

In this case, the only solution might be to build V8 on a machine or VM with an older Visual Studio installed. On the other hand, that assertion is in an #ifndef _ALLOW_COMPILER_AND_STL_VERSION_MISMATCH block, so defining that macro somewhere might work too.

Why do build instructions ask to install Clang for Windows when V8Update downloads and uses its own copy?

ClearScript uses Visual Studio's Clang to build the native code that's linked with the static V8 library to yield the native V8 assembly.

Cheers!

[AnsisMalins]

What is the bundled Clang used for?

[ClearScriptLib]

What is the bundled Clang used for?

V8's build environment uses a private copy of Clang to compile V8. ClearScript configures it to generate a static library (v8_monolith.lib). Visual Studio is then used to build the ClearScriptV8 code using its own Clang. That code is linked with the V8 static library to create ClearScriptV8.win-[arch].dll.

BTW, we've upgraded to V8 14.1 internally, and it uses Clang 21, which seems to work fine with the current Visual Studio.

[AnsisMalins]

The solution is that I was a dumbass. git pull had not gone through, and I did not notice, so I was trying to build ClearScript 7.4 instead of 7.5. Sorry for wasting your time.