clangd crashes when including some standard library headers in the global module fragment

[marwing]

The following code currently reliably crashes clangd.
I am using a nightly llvm build (d9850fe) with current neovim HEAD and the built-in lsp client.

I bisected this crash to a0b6747.

Considering this is only using libc++ headers from the same commit, I don't include preprocessed sources. If you need them anyway, I can provide them.

Reproducer:

// test.cpp
module;

// any one off these is enough to crash clangd
// #include <iostream>
#include <string>
// #include <string_view>
// #include <cmath>
// #include <system_error>
// #include <new>
// #include <bit>
// probably many more

// only ok with libc++, not the system provided libstdc++ 13.2.1
#include <iosfwd>

// these are ok
#include <cstddef>
#include <cerrno>

export module test;

// compile_flags.txt
-std=c++23
-stdlib=libc++

clangd log

I[18:32:44.313] clangd version 19.0.0git (https://github.com/llvm/llvm-project.git d9850fe23cc69b784cf655405cdfae7defa44573)
I[18:32:44.313] Features: linux+debug
I[18:32:44.313] PID: 526801
I[18:32:44.313] Working directory: /home/user/workspace
I[18:32:44.313] argv[0]: /home/user/workspace/install/llvm/bin/clangd
I[18:32:44.313] argv[1]: -query-driver=/usr/**/arm-none-eabi*
I[18:32:44.313] argv[2]: --completion-style=detailed
I[18:32:44.313] argv[3]: --malloc-trim
I[18:32:44.313] argv[4]: --enable-config
I[18:32:44.313] argv[5]: --use-dirty-headers
I[18:32:44.313] Starting LSP over stdin/stdout
I[18:32:44.314] <-- initialize(1)
I[18:32:44.315] --> reply:initialize(1) 0 ms
I[18:32:44.523] <-- initialized
I[18:32:44.523] <-- textDocument/didOpen
I[18:32:44.526] --> textDocument/publishDiagnostics
I[18:32:44.526] Loaded compilation database from /home/user/workspace/compile_flags.txt
I[18:32:44.526] ASTWorker building file /home/user/workspace/test.cpp version 0 with command [/home/user/workspace] /home/user/workspace/install/llvm/bin/clang-tool -std=c++23 -stdlib=libc++ -resource-dir=/home/user/workspace/install/llvm/lib/clang/19 -- /home/user/workspace/test.cpp
I[18:32:44.528] <-- textDocument/documentSymbol(2)
I[18:32:44.535] <-- textDocument/semanticTokens/full(3)
I[18:32:44.855] <-- textDocument/documentSymbol(4)
I[18:32:44.855] <-- textDocument/documentHighlight(5)
I[18:32:52.632] Built preamble of size 7092296 for file /home/user/workspace/test.cpp version 0 in 8.10 seconds
I[18:32:52.634] Indexing c++23 standard library in the context of /home/user/workspace/test.cpp
I[18:32:52.634] --> workspace/semanticTokens/refresh(0)
I[18:32:52.635] <-- $/cancelRequest
I[18:32:52.635] <-- textDocument/semanticTokens/full(6)
I[18:32:52.635] <-- reply(0)
clangd: /home/user/workspace/llvm-project/llvm/include/llvm/ADT/SmallVector.h:304: reference llvm::SmallVectorTemplateCommon<unsigned long>::operator[](size_type) [T = unsigned long]: Assertion `idx < size()' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
 #0 0x000064685fd8b346 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /home/user/workspace/llvm-project/llvm/lib/Support/Unix/Signals.inc:723:11
 #1 0x000064685fd8b8eb PrintStackTraceSignalHandler(void*) /home/user/workspace/llvm-project/llvm/lib/Support/Unix/Signals.inc:798:1
 #2 0x000064685fd89803 llvm::sys::RunSignalHandlers() /home/user/workspace/llvm-project/llvm/lib/Support/Signals.cpp:105:5
 #3 0x000064685fd8bfd1 SignalHandler(int) /home/user/workspace/llvm-project/llvm/lib/Support/Unix/Signals.inc:413:1
 #4 0x000075ec9af2b710 (/usr/lib/libc.so.6+0x3e710)
 #5 0x000075ec9af7b83c (/usr/lib/libc.so.6+0x8e83c)
 #6 0x000075ec9af2b668 gsignal (/usr/lib/libc.so.6+0x3e668)
 #7 0x000075ec9af134b8 abort (/usr/lib/libc.so.6+0x264b8)
 #8 0x000075ec9af133dc (/usr/lib/libc.so.6+0x263dc)
 #9 0x000075ec9af23d26 (/usr/lib/libc.so.6+0x36d26)
#10 0x000064685ff15af9 llvm::SmallVectorTemplateCommon<unsigned long, void>::operator[](unsigned long) /home/user/workspace/llvm-project/llvm/include/llvm/ADT/SmallVector.h:0:5
#11 0x0000646865f14dcb clang::ASTRecordReader::readInt() /home/user/workspace/llvm-project/clang/include/clang/Serialization/ASTRecordReader.h:74:31
#12 0x0000646865fe535d clang::ASTDeclReader::ReadLocalOffset() /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:107:37
#13 0x0000646865fad1c7 clang::ASTDeclReader::VisitDeclContext(clang::DeclContext*) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:2771:12
#14 0x0000646865fda0f2 clang::ASTReader::ReadDeclRecord(unsigned int) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:4129:52
#15 0x0000646865eec3b0 clang::ASTReader::GetDecl(unsigned int) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:7783:5
#16 0x0000646865f19d42 clang::ASTReader::GetLocalDecl(clang::serialization::ModuleFile&, unsigned int) /home/user/workspace/llvm-project/clang/include/clang/Serialization/ASTReader.h:1920:5
#17 0x0000646865fdcd7a clang::ASTReader::loadPendingDeclChain(clang::Decl*, unsigned long) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:4317:15
#18 0x0000646865efc15c clang::ASTReader::finishPendingActions() /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:9585:57
#19 0x0000646865eff156 clang::ASTReader::FinishedDeserializing() /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:0:5
#20 0x000064686018993c clang::ExternalASTSource::Deserializing::~Deserializing() /home/user/workspace/llvm-project/clang/include/clang/AST/ExternalASTSource.h:87:5
#21 0x0000646865ee2050 clang::ASTReader::ReadAST(llvm::StringRef, clang::serialization::ModuleKind, clang::SourceLocation, unsigned int, clang::serialization::ModuleFile**) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:4663:1
#22 0x0000646865a18b1a clang::CompilerInstance::createPCHExternalASTSource(llvm::StringRef, llvm::StringRef, clang::DisableValidationForModuleKind, bool, clang::Preprocessor&, clang::InMemoryModuleCache&, clang::ASTContext&, clang::PCHContainerReader const&, llvm::ArrayRef<std::__1::shared_ptr<clang::ModuleFileExtension>>, llvm::ArrayRef<std::__1::shared_ptr<clang::DependencyCollector>>, void*, bool, bool, bool) /home/user/workspace/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:660:3
#23 0x0000646865a1862b clang::CompilerInstance::createPCHExternalASTSource(llvm::StringRef, clang::DisableValidationForModuleKind, bool, void*, bool) /home/user/workspace/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:616:18
#24 0x0000646865b0d6c4 clang::FrontendAction::BeginSourceFile(clang::CompilerInstance&, clang::FrontendInputFile const&) /home/user/workspace/llvm-project/clang/lib/Frontend/FrontendAction.cpp:990:14
#25 0x0000646862c20f46 clang::clangd::ParsedAST::build(llvm::StringRef, clang::clangd::ParseInputs const&, std::__1::unique_ptr<clang::CompilerInvocation, std::__1::default_delete<clang::CompilerInvocation>>, llvm::ArrayRef<clang::clangd::Diag>, std::__1::shared_ptr<clang::clangd::PreambleData const>) /home/user/workspace/llvm-project/clang-tools-extra/clangd/ParsedAST.cpp:505:7
#26 0x0000646862eb7daa clang::clangd::(anonymous namespace)::ASTWorker::generateDiagnostics(std::__1::unique_ptr<clang::CompilerInvocation, std::__1::default_delete<clang::CompilerInvocation>>, clang::clangd::ParseInputs, std::__1::vector<clang::clangd::Diag, std::__1::allocator<clang::clangd::Diag>>) /home/user/workspace/llvm-project/clang-tools-extra/clangd/TUScheduler.cpp:1210:39
#27 0x0000646862eb7a10 clang::clangd::(anonymous namespace)::ASTWorker::updatePreamble(std::__1::unique_ptr<clang::CompilerInvocation, std::__1::default_delete<clang::CompilerInvocation>>, clang::clangd::ParseInputs, std::__1::shared_ptr<clang::clangd::PreambleData const>, std::__1::vector<clang::clangd::Diag, std::__1::allocator<clang::clangd::Diag>>, clang::clangd::WantDiagnostics)::$_0::operator()() /home/user/workspace/llvm-project/clang-tools-extra/clangd/TUScheduler.cpp:1144:5
#28 0x0000646862ebaded void llvm::detail::UniqueFunctionBase<void>::CallImpl<clang::clangd::(anonymous namespace)::ASTWorker::updatePreamble(std::__1::unique_ptr<clang::CompilerInvocation, std::__1::default_delete<clang::CompilerInvocation>>, clang::clangd::ParseInputs, std::__1::shared_ptr<clang::clangd::PreambleData const>, std::__1::vector<clang::clangd::Diag, std::__1::allocator<clang::clangd::Diag>>, clang::clangd::WantDiagnostics)::$_0>(void*) /home/user/workspace/llvm-project/llvm/include/llvm/ADT/FunctionExtras.h:221:5
#29 0x000064686279a5af llvm::unique_function<void ()>::operator()() /home/user/workspace/llvm-project/llvm/include/llvm/ADT/FunctionExtras.h:385:5
#30 0x0000646862ec3225 void llvm::function_ref<void ()>::callback_fn<llvm::unique_function<void ()>>(long) /home/user/workspace/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:45:5
#31 0x000064685fc8da39 llvm::function_ref<void ()>::operator()() const /home/user/workspace/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:68:5
#32 0x0000646862eadd39 clang::clangd::(anonymous namespace)::ASTWorker::runTask(llvm::StringRef, llvm::function_ref<void ()>) /home/user/workspace/llvm-project/clang-tools-extra/clangd/TUScheduler.cpp:1324:3
#33 0x0000646862ead2ed clang::clangd::(anonymous namespace)::ASTWorker::run() /home/user/workspace/llvm-project/clang-tools-extra/clangd/TUScheduler.cpp:1459:5
#34 0x0000646862eacb4d clang::clangd::(anonymous namespace)::ASTWorker::create(llvm::StringRef, clang::clangd::GlobalCompilationDatabase const&, clang::clangd::TUScheduler::ASTCache&, clang::clangd::TUScheduler::HeaderIncluderCache&, clang::clangd::AsyncTaskRunner*, clang::clangd::Semaphore&, clang::clangd::TUScheduler::Options const&, clang::clangd::ParsingCallbacks&)::$_0::operator()() const /home/user/workspace/llvm-project/clang-tools-extra/clangd/TUScheduler.cpp:822:49
#35 0x0000646862eacacd void llvm::detail::UniqueFunctionBase<void>::CallImpl<clang::clangd::(anonymous namespace)::ASTWorker::create(llvm::StringRef, clang::clangd::GlobalCompilationDatabase const&, clang::clangd::TUScheduler::ASTCache&, clang::clangd::TUScheduler::HeaderIncluderCache&, clang::clangd::AsyncTaskRunner*, clang::clangd::Semaphore&, clang::clangd::TUScheduler::Options const&, clang::clangd::ParsingCallbacks&)::$_0>(void*) /home/user/workspace/llvm-project/llvm/include/llvm/ADT/FunctionExtras.h:221:5
#36 0x000064686279a5af llvm::unique_function<void ()>::operator()() /home/user/workspace/llvm-project/llvm/include/llvm/ADT/FunctionExtras.h:385:5
#37 0x000064686334ad
8b clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::$_1::operator()() /home/user/workspace/llvm-project/clang-tools-extra/clangd/support/Threading.cpp:101:5
#38 0x000064686334ad39 auto void llvm::thread::GenericThreadProxy<std::__1::tuple<clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::$_1>>(void*)::'lambda'(auto&&, auto&&...)::operator()<clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::$_1&>(auto&&, auto&&...) const /home/user/workspace/llvm-project/llvm/include/llvm/Support/thread.h:44:9
#39 0x000064686334aced decltype(std::declval<auto>()(std::declval<auto>()...)) std::__1::__invoke[abi:v160006]<void llvm::thread::GenericThreadProxy<std::__1::tuple<clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::$_1>>(void*)::'lambda'(auto&&, auto&&...), clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::$_1&>(auto&&, auto&&...) /usr/bin/../include/c++/v1/__functional/invoke.h:394:7
#40 0x000064686334acbd decltype(auto) std::__1::__apply_tuple_impl[abi:v160006]<void llvm::thread::GenericThreadProxy<std::__1::tuple<clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::$_1>>(void*)::'lambda'(auto&&, auto&&...), std::__1::tuple<clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::$_1>&, 0ul>(auto&&, std::__1::tuple<clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::$_1>&, std::__1::__tuple_indices<0ul>) /usr/bin/../include/c++/v1/tuple:1816:1
#41 0x000064686334ac5d decltype(auto) std::__1::apply[abi:v160006]<void llvm::thread::GenericThreadProxy<std::__1::tuple<clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::$_1>>(void*)::'lambda'(auto&&, auto&&...), std::__1::tuple<clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::$_1>&>(auto&&, std::__1::tuple<clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::$_1>&) /usr/bin/../include/c++/v1/tuple:1825:1
#42 0x000064686334ac0f void llvm::thread::GenericThreadProxy<std::__1::tuple<clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::$_1>>(void*) /home/user/workspace/llvm-project/llvm/include/llvm/Support/thread.h:41:5
#43 0x000064686334aa15 void* llvm::thread::ThreadProxy<std::__1::tuple<clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::$_1>>(void*) /home/user/workspace/llvm-project/llvm/include/llvm/Support/thread.h:56:5
#44 0x000075ec9af799eb (/usr/lib/libc.so.6+0x8c9eb)
#45 0x000075ec9affd7cc (/usr/lib/libc.so.6+0x1107cc)
Signalled during AST worker action: Build AST
  Filename: /home/user/workspace/test.cpp
  Directory: /home/user/workspace
  Command Line: /home/user/workspace/install/llvm/bin/clang-tool -std=c++23 -stdlib=libc++ -resource-dir=/home/user/workspace/install/llvm/lib/clang/19 -- /home/user/workspace/test.cpp
  Version: 0

Just in case another log is helpful, clangd --check fails with the same assertion.

I[19:03:47.944] clangd version 19.0.0git (https://github.com/llvm/llvm-project.git d9850fe23cc69b784cf655405cdfae7defa44573)
I[19:03:47.944] Features: linux+debug
I[19:03:47.944] PID: 535714
I[19:03:47.945] Working directory: /home/user/workspace
I[19:03:47.945] argv[0]: clangd
I[19:03:47.945] argv[1]: --check=test.cpp
I[19:03:47.945] Entering check mode (no LSP server)
I[19:03:47.945] Testing on source file /home/user/workspace/test.cpp
I[19:03:47.945] Loading compilation database...
I[19:03:47.946] Loaded compilation database from /home/user/workspace/compile_flags.txt
I[19:03:47.947] Compile command from CDB is: [/home/user/workspace] /home/user/workspace/llvm-project/builds/Debug/bin/clang-tool -std=c++23 -stdlib=libc++ -resource-dir=/home/user/workspace/llvm-project/builds/Debug/lib/clang/19 -- /home/user/workspace/test.cpp
I[19:03:47.947] Parsing command...
I[19:03:47.951] internal (cc1) args are: -cc1 -triple x86_64-unknown-linux-gnu -fsyntax-only -disable-free -clear-ast-before-backend -main-file-name test.cpp -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=all -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -tune-cpu generic -debugger-tuning=gdb -fdebug-compilation-dir=/home/user/workspace -fcoverage-compilation-dir=/home/user/workspace -resource-dir /home/user/workspace/llvm-project/builds/Debug/lib/clang/19 -internal-isystem /home/user/workspace/llvm-project/builds/Debug/bin/../include/x86_64-unknown-linux-gnu/c++/v1 -internal-isystem /home/user/workspace/llvm-project/builds/Debug/bin/../include/c++/v1 -internal-isystem /home/user/workspace/llvm-project/builds/Debug/lib/clang/19/include -internal-isystem /usr/local/include -internal-isystem /usr/lib64/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../x86_64-pc-linux-gnu/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -std=c++23 -fdeprecated-macro -ferror-limit 19 -fgnuc-version=4.2.1 -fno-implicit-modules -fskip-odr-check-in-gmf -fcxx-exceptions -fexceptions -no-round-trip-args -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -x c++ /home/user/workspace/test.cpp
I[19:03:47.951] Building preamble...
I[19:03:55.501] Built preamble of size 7128100 for file /home/user/workspace/test.cpp version null in 7.55 seconds
I[19:03:55.501] Indexing headers...
I[19:03:56.622] Building AST...
clangd: /home/user/workspace/llvm-project/llvm/include/llvm/ADT/SmallVector.h:304: reference llvm::SmallVectorTemplateCommon<unsigned long>::operator[](size_type) [T = unsigned long]: Assertion `idx < size()' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.	Program arguments: clangd --check=test.cpp
 #0 0x0000623f105b4346 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /home/user/workspace/llvm-project/llvm/lib/Support/Unix/Signals.inc:723:11
 #1 0x0000623f105b48eb PrintStackTraceSignalHandler(void*) /home/user/workspace/llvm-project/llvm/lib/Support/Unix/Signals.inc:798:1
 #2 0x0000623f105b2803 llvm::sys::RunSignalHandlers() /home/user/workspace/llvm-project/llvm/lib/Support/Signals.cpp:105:5
 #3 0x0000623f105b4fd1 SignalHandler(int) /home/user/workspace/llvm-project/llvm/lib/Support/Unix/Signals.inc:413:1
 #4 0x0000732eb8dfd710 (/usr/lib/libc.so.6+0x3e710)
 #5 0x0000732eb8e4d83c (/usr/lib/libc.so.6+0x8e83c)
 #6 0x0000732eb8dfd668 gsignal (/usr/lib/libc.so.6+0x3e668)
 #7 0x0000732eb8de54b8 abort (/usr/lib/libc.so.6+0x264b8)
 #8 0x0000732eb8de53dc (/usr/lib/libc.so.6+0x263dc)
 #9 0x0000732eb8df5d26 (/usr/lib/libc.so.6+0x36d26)
#10 0x0000623f1073eaf9 llvm::SmallVectorTemplateCommon<unsigned long, void>::operator[](unsigned long) /home/user/workspace/llvm-project/llvm/include/llvm/ADT/SmallVector.h:0:5
#11 0x0000623f1673ddcb clang::ASTRecordReader::readInt() /home/user/workspace/llvm-project/clang/include/clang/Serialization/ASTRecordReader.h:74:31
#12 0x0000623f1680e35d clang::ASTDeclReader::ReadLocalOffset() /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:107:37
#13 0x0000623f167d61c7 clang::ASTDeclReader::VisitDeclContext(clang::DeclContext*) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:2771:12
#14 0x0000623f168030f2 clang::ASTReader::ReadDeclRecord(unsigned int) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:4129:52
#15 0x0000623f167153b0 clang::ASTReader::GetDecl(unsigned int) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:7783:5
#16 0x0000623f16742d42 clang::ASTReader::GetLocalDecl(clang::serialization::ModuleFile&, unsigned int) /home/user/workspace/llvm-project/clang/include/clang/Serialization/ASTReader.h:1920:5
#17 0x0000623f16805d7a clang::ASTReader::loadPendingDeclChain(clang::Decl*, unsigned long) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:4317:15
#18 0x0000623f1672515c clang::ASTReader::finishPendingActions() /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:9585:57
#19 0x0000623f16728156 clang::ASTReader::FinishedDeserializing() /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:0:5
#20 0x0000623f109b293c clang::ExternalASTSource::Deserializing::~Deserializing() /home/user/workspace/llvm-project/clang/include/clang/AST/ExternalASTSource.h:87:5
#21 0x0000623f1670b050 clang::ASTReader::ReadAST(llvm::StringRef, clang::serialization::ModuleKind, clang::SourceLocation, unsigned int, clang::serialization::ModuleFile**) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:4663:1
#22 0x0000623f16241b1a clang::CompilerInstance::createPCHExternalASTSource(llvm::StringRef, llvm::StringRef, clang::DisableValidationForModuleKind, bool, clang::Preprocessor&, clang::InMemoryModuleCache&, clang::ASTContext&, clang::PCHContainerReader const&, llvm::ArrayRef<std::__1::shared_ptr<clang::ModuleFileExtension>>, llvm::ArrayRef<std::__1::shared_ptr<clang::DependencyCollector>>, void*, bool, bool, bool) /home/user/workspace/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:660:3
#23 0x0000623f1624162b clang::CompilerInstance::createPCHExternalASTSource(llvm::StringRef, clang::DisableValidationForModuleKind, bool, void*, bool) /home/user/workspace/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:616:18
#24 0x0000623f163366c4 clang::FrontendAction::BeginSourceFile(clang::CompilerInstance&, clang::FrontendInputFile const&) /home/user/workspace/llvm-project/clang/lib/Frontend/FrontendAction.cpp:990:14
#25 0x0000623f13449f46 clang::clangd::ParsedAST::build(llvm::StringRef, clang::clangd::ParseInputs const&, std::__1::unique_ptr<clang::CompilerInvocation, std::__1::default_delete<clang::CompilerInvocation>>, llvm::ArrayRef<clang::clangd::Diag>, std::__1::shared_ptr<clang::clangd::PreambleData const>) /home/user/workspace/llvm-project/clang-tools-extra/clangd/ParsedAST.cpp:505:7
#26 0x0000623f12d22ae6 clang::clangd::(anonymous namespace)::Checker::buildAST() /home/user/workspace/llvm-project/clang-tools-extra/clangd/tool/Check.cpp:254:11
#27 0x0000623f12d21bf6 clang::clangd::check(llvm::StringRef, clang::clangd::ThreadsafeFS const&, clang::clangd::ClangdLSPServer::Options const&) /home/user/workspace/llvm-project/clang-tools-extra/clangd/tool/Check.cpp:508:7
#28 0x0000623f12cd336d clang::clangd::clangdMain(int, char**) /home/user/workspace/llvm-project/clang-tools-extra/clangd/tool/ClangdMain.cpp:976:12
#29 0x0000623f10479ee2 main /home/user/workspace/llvm-project/clang-tools-extra/clangd/tool/ClangdToolMain.cpp:12:3
#30 0x0000732eb8de6cd0 (/usr/lib/libc.so.6+0x27cd0)
#31 0x0000732eb8de6d8a __libc_start_main (/usr/lib/libc.so.6+0x27d8a)
#32 0x0000623f1009ef35 _start (/home/user/workspace/llvm-project/builds/Debug/bin/clangd+0x36a8f35)

[llvmbot]

@llvm/issue-subscribers-clangd

Author: None (marwing)

The following code currently reliably crashes clangd. I am using a nightly llvm build (d9850fe) with current neovim HEAD and the built-in lsp client.

I bisected this crash to a0b6747.

Considering this is only using libc++ headers from the same commit, I don't include preprocessed sources. If you need them anyway, I can provide them.

Reproducer:

// test.cpp
module;

// any one off these is enough to crash clangd
// #include <iostream>
#include <string>
// #include <string_view>
// #include <cmath>
// #include <system_error>
// #include <new>
// #include <bit>
// probably many more

// only ok with libc++, not the system provided libstdc++ 13.2.1
#include <iosfwd>

// these are ok
#include <cstddef>
#include <cerrno>

export module test;

// compile_flags.txt
-std=c++23
-stdlib=libc++

<details>
<summary>clangd log</summary>

I[18:32:44.313] clangd version 19.0.0git (https://github.com/llvm/llvm-project.git d9850fe23cc69b784cf655405cdfae7defa44573)
I[18:32:44.313] Features: linux+debug
I[18:32:44.313] PID: 526801
I[18:32:44.313] Working directory: /home/user/workspace
I[18:32:44.313] argv[0]: /home/user/workspace/install/llvm/bin/clangd
I[18:32:44.313] argv[1]: -query-driver=/usr/**/arm-none-eabi*
I[18:32:44.313] argv[2]: --completion-style=detailed
I[18:32:44.313] argv[3]: --malloc-trim
I[18:32:44.313] argv[4]: --enable-config
I[18:32:44.313] argv[5]: --use-dirty-headers
I[18:32:44.313] Starting LSP over stdin/stdout
I[18:32:44.314] <-- initialize(1)
I[18:32:44.315] --> reply:initialize(1) 0 ms
I[18:32:44.523] <-- initialized
I[18:32:44.523] <-- textDocument/didOpen
I[18:32:44.526] --> textDocument/publishDiagnostics
I[18:32:44.526] Loaded compilation database from /home/user/workspace/compile_flags.txt
I[18:32:44.526] ASTWorker building file /home/user/workspace/test.cpp version 0 with command [/home/user/workspace] /home/user/workspace/install/llvm/bin/clang-tool -std=c++23 -stdlib=libc++ -resource-dir=/home/user/workspace/install/llvm/lib/clang/19 -- /home/user/workspace/test.cpp
I[18:32:44.528] <-- textDocument/documentSymbol(2)
I[18:32:44.535] <-- textDocument/semanticTokens/full(3)
I[18:32:44.855] <-- textDocument/documentSymbol(4)
I[18:32:44.855] <-- textDocument/documentHighlight(5)
I[18:32:52.632] Built preamble of size 7092296 for file /home/user/workspace/test.cpp version 0 in 8.10 seconds
I[18:32:52.634] Indexing c++23 standard library in the context of /home/user/workspace/test.cpp
I[18:32:52.634] --> workspace/semanticTokens/refresh(0)
I[18:32:52.635] <-- $/cancelRequest
I[18:32:52.635] <-- textDocument/semanticTokens/full(6)
I[18:32:52.635] <-- reply(0)
clangd: /home/user/workspace/llvm-project/llvm/include/llvm/ADT/SmallVector.h:304: reference llvm::SmallVectorTemplateCommon<unsigned long>::operator[](size_type) [T = unsigned long]: Assertion `idx < size()' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
 #<!-- -->0 0x000064685fd8b346 llvm::sys::PrintStackTrace(llvm::raw_ostream&amp;, int) /home/user/workspace/llvm-project/llvm/lib/Support/Unix/Signals.inc:723:11
 #<!-- -->1 0x000064685fd8b8eb PrintStackTraceSignalHandler(void*) /home/user/workspace/llvm-project/llvm/lib/Support/Unix/Signals.inc:798:1
 #<!-- -->2 0x000064685fd89803 llvm::sys::RunSignalHandlers() /home/user/workspace/llvm-project/llvm/lib/Support/Signals.cpp:105:5
 #<!-- -->3 0x000064685fd8bfd1 SignalHandler(int) /home/user/workspace/llvm-project/llvm/lib/Support/Unix/Signals.inc:413:1
 #<!-- -->4 0x000075ec9af2b710 (/usr/lib/libc.so.6+0x3e710)
 #<!-- -->5 0x000075ec9af7b83c (/usr/lib/libc.so.6+0x8e83c)
 #<!-- -->6 0x000075ec9af2b668 gsignal (/usr/lib/libc.so.6+0x3e668)
 #<!-- -->7 0x000075ec9af134b8 abort (/usr/lib/libc.so.6+0x264b8)
 #<!-- -->8 0x000075ec9af133dc (/usr/lib/libc.so.6+0x263dc)
 #<!-- -->9 0x000075ec9af23d26 (/usr/lib/libc.so.6+0x36d26)
#<!-- -->10 0x000064685ff15af9 llvm::SmallVectorTemplateCommon&lt;unsigned long, void&gt;::operator[](unsigned long) /home/user/workspace/llvm-project/llvm/include/llvm/ADT/SmallVector.h:0:5
#<!-- -->11 0x0000646865f14dcb clang::ASTRecordReader::readInt() /home/user/workspace/llvm-project/clang/include/clang/Serialization/ASTRecordReader.h:74:31
#<!-- -->12 0x0000646865fe535d clang::ASTDeclReader::ReadLocalOffset() /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:107:37
#<!-- -->13 0x0000646865fad1c7 clang::ASTDeclReader::VisitDeclContext(clang::DeclContext*) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:2771:12
#<!-- -->14 0x0000646865fda0f2 clang::ASTReader::ReadDeclRecord(unsigned int) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:4129:52
#<!-- -->15 0x0000646865eec3b0 clang::ASTReader::GetDecl(unsigned int) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:7783:5
#<!-- -->16 0x0000646865f19d42 clang::ASTReader::GetLocalDecl(clang::serialization::ModuleFile&amp;, unsigned int) /home/user/workspace/llvm-project/clang/include/clang/Serialization/ASTReader.h:1920:5
#<!-- -->17 0x0000646865fdcd7a clang::ASTReader::loadPendingDeclChain(clang::Decl*, unsigned long) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:4317:15
#<!-- -->18 0x0000646865efc15c clang::ASTReader::finishPendingActions() /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:9585:57
#<!-- -->19 0x0000646865eff156 clang::ASTReader::FinishedDeserializing() /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:0:5
#<!-- -->20 0x000064686018993c clang::ExternalASTSource::Deserializing::~Deserializing() /home/user/workspace/llvm-project/clang/include/clang/AST/ExternalASTSource.h:87:5
#<!-- -->21 0x0000646865ee2050 clang::ASTReader::ReadAST(llvm::StringRef, clang::serialization::ModuleKind, clang::SourceLocation, unsigned int, clang::serialization::ModuleFile**) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:4663:1
#<!-- -->22 0x0000646865a18b1a clang::CompilerInstance::createPCHExternalASTSource(llvm::StringRef, llvm::StringRef, clang::DisableValidationForModuleKind, bool, clang::Preprocessor&amp;, clang::InMemoryModuleCache&amp;, clang::ASTContext&amp;, clang::PCHContainerReader const&amp;, llvm::ArrayRef&lt;std::__1::shared_ptr&lt;clang::ModuleFileExtension&gt;&gt;, llvm::ArrayRef&lt;std::__1::shared_ptr&lt;clang::DependencyCollector&gt;&gt;, void*, bool, bool, bool) /home/user/workspace/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:660:3
#<!-- -->23 0x0000646865a1862b clang::CompilerInstance::createPCHExternalASTSource(llvm::StringRef, clang::DisableValidationForModuleKind, bool, void*, bool) /home/user/workspace/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:616:18
#<!-- -->24 0x0000646865b0d6c4 clang::FrontendAction::BeginSourceFile(clang::CompilerInstance&amp;, clang::FrontendInputFile const&amp;) /home/user/workspace/llvm-project/clang/lib/Frontend/FrontendAction.cpp:990:14
#<!-- -->25 0x0000646862c20f46 clang::clangd::ParsedAST::build(llvm::StringRef, clang::clangd::ParseInputs const&amp;, std::__1::unique_ptr&lt;clang::CompilerInvocation, std::__1::default_delete&lt;clang::CompilerInvocation&gt;&gt;, llvm::ArrayRef&lt;clang::clangd::Diag&gt;, std::__1::shared_ptr&lt;clang::clangd::PreambleData const&gt;) /home/user/workspace/llvm-project/clang-tools-extra/clangd/ParsedAST.cpp:505:7
#<!-- -->26 0x0000646862eb7daa clang::clangd::(anonymous namespace)::ASTWorker::generateDiagnostics(std::__1::unique_ptr&lt;clang::CompilerInvocation, std::__1::default_delete&lt;clang::CompilerInvocation&gt;&gt;, clang::clangd::ParseInputs, std::__1::vector&lt;clang::clangd::Diag, std::__1::allocator&lt;clang::clangd::Diag&gt;&gt;) /home/user/workspace/llvm-project/clang-tools-extra/clangd/TUScheduler.cpp:1210:39
#<!-- -->27 0x0000646862eb7a10 clang::clangd::(anonymous namespace)::ASTWorker::updatePreamble(std::__1::unique_ptr&lt;clang::CompilerInvocation, std::__1::default_delete&lt;clang::CompilerInvocation&gt;&gt;, clang::clangd::ParseInputs, std::__1::shared_ptr&lt;clang::clangd::PreambleData const&gt;, std::__1::vector&lt;clang::clangd::Diag, std::__1::allocator&lt;clang::clangd::Diag&gt;&gt;, clang::clangd::WantDiagnostics)::$_0::operator()() /home/user/workspace/llvm-project/clang-tools-extra/clangd/TUScheduler.cpp:1144:5
#<!-- -->28 0x0000646862ebaded void llvm::detail::UniqueFunctionBase&lt;void&gt;::CallImpl&lt;clang::clangd::(anonymous namespace)::ASTWorker::updatePreamble(std::__1::unique_ptr&lt;clang::CompilerInvocation, std::__1::default_delete&lt;clang::CompilerInvocation&gt;&gt;, clang::clangd::ParseInputs, std::__1::shared_ptr&lt;clang::clangd::PreambleData const&gt;, std::__1::vector&lt;clang::clangd::Diag, std::__1::allocator&lt;clang::clangd::Diag&gt;&gt;, clang::clangd::WantDiagnostics)::$_0&gt;(void*) /home/user/workspace/llvm-project/llvm/include/llvm/ADT/FunctionExtras.h:221:5
#<!-- -->29 0x000064686279a5af llvm::unique_function&lt;void ()&gt;::operator()() /home/user/workspace/llvm-project/llvm/include/llvm/ADT/FunctionExtras.h:385:5
#<!-- -->30 0x0000646862ec3225 void llvm::function_ref&lt;void ()&gt;::callback_fn&lt;llvm::unique_function&lt;void ()&gt;&gt;(long) /home/user/workspace/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:45:5
#<!-- -->31 0x000064685fc8da39 llvm::function_ref&lt;void ()&gt;::operator()() const /home/user/workspace/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:68:5
#<!-- -->32 0x0000646862eadd39 clang::clangd::(anonymous namespace)::ASTWorker::runTask(llvm::StringRef, llvm::function_ref&lt;void ()&gt;) /home/user/workspace/llvm-project/clang-tools-extra/clangd/TUScheduler.cpp:1324:3
#<!-- -->33 0x0000646862ead2ed clang::clangd::(anonymous namespace)::ASTWorker::run() /home/user/workspace/llvm-project/clang-tools-extra/clangd/TUScheduler.cpp:1459:5
#<!-- -->34 0x0000646862eacb4d clang::clangd::(anonymous namespace)::ASTWorker::create(llvm::StringRef, clang::clangd::GlobalCompilationDatabase const&amp;, clang::clangd::TUScheduler::ASTCache&amp;, clang::clangd::TUScheduler::HeaderIncluderCache&amp;, clang::clangd::AsyncTaskRunner*, clang::clangd::Semaphore&amp;, clang::clangd::TUScheduler::Options const&amp;, clang::clangd::ParsingCallbacks&amp;)::$_0::operator()() const /home/user/workspace/llvm-project/clang-tools-extra/clangd/TUScheduler.cpp:822:49
#<!-- -->35 0x0000646862eacacd void llvm::detail::UniqueFunctionBase&lt;void&gt;::CallImpl&lt;clang::clangd::(anonymous namespace)::ASTWorker::create(llvm::StringRef, clang::clangd::GlobalCompilationDatabase const&amp;, clang::clangd::TUScheduler::ASTCache&amp;, clang::clangd::TUScheduler::HeaderIncluderCache&amp;, clang::clangd::AsyncTaskRunner*, clang::clangd::Semaphore&amp;, clang::clangd::TUScheduler::Options const&amp;, clang::clangd::ParsingCallbacks&amp;)::$_0&gt;(void*) /home/user/workspace/llvm-project/llvm/include/llvm/ADT/FunctionExtras.h:221:5
#<!-- -->36 0x000064686279a5af llvm::unique_function&lt;void ()&gt;::operator()() /home/user/workspace/llvm-project/llvm/include/llvm/ADT/FunctionExtras.h:385:5
#<!-- -->37 0x000064686334ad
8b clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1::operator()() /home/user/workspace/llvm-project/clang-tools-extra/clangd/support/Threading.cpp:101:5
#<!-- -->38 0x000064686334ad39 auto void llvm::thread::GenericThreadProxy&lt;std::__1::tuple&lt;clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1&gt;&gt;(void*)::'lambda'(auto&amp;&amp;, auto&amp;&amp;...)::operator()&lt;clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1&amp;&gt;(auto&amp;&amp;, auto&amp;&amp;...) const /home/user/workspace/llvm-project/llvm/include/llvm/Support/thread.h:44:9
#<!-- -->39 0x000064686334aced decltype(std::declval&lt;auto&gt;()(std::declval&lt;auto&gt;()...)) std::__1::__invoke[abi:v160006]&lt;void llvm::thread::GenericThreadProxy&lt;std::__1::tuple&lt;clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1&gt;&gt;(void*)::'lambda'(auto&amp;&amp;, auto&amp;&amp;...), clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1&amp;&gt;(auto&amp;&amp;, auto&amp;&amp;...) /usr/bin/../include/c++/v1/__functional/invoke.h:394:7
#<!-- -->40 0x000064686334acbd decltype(auto) std::__1::__apply_tuple_impl[abi:v160006]&lt;void llvm::thread::GenericThreadProxy&lt;std::__1::tuple&lt;clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1&gt;&gt;(void*)::'lambda'(auto&amp;&amp;, auto&amp;&amp;...), std::__1::tuple&lt;clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1&gt;&amp;, 0ul&gt;(auto&amp;&amp;, std::__1::tuple&lt;clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1&gt;&amp;, std::__1::__tuple_indices&lt;0ul&gt;) /usr/bin/../include/c++/v1/tuple:1816:1
#<!-- -->41 0x000064686334ac5d decltype(auto) std::__1::apply[abi:v160006]&lt;void llvm::thread::GenericThreadProxy&lt;std::__1::tuple&lt;clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1&gt;&gt;(void*)::'lambda'(auto&amp;&amp;, auto&amp;&amp;...), std::__1::tuple&lt;clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1&gt;&amp;&gt;(auto&amp;&amp;, std::__1::tuple&lt;clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1&gt;&amp;) /usr/bin/../include/c++/v1/tuple:1825:1
#<!-- -->42 0x000064686334ac0f void llvm::thread::GenericThreadProxy&lt;std::__1::tuple&lt;clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1&gt;&gt;(void*) /home/user/workspace/llvm-project/llvm/include/llvm/Support/thread.h:41:5
#<!-- -->43 0x000064686334aa15 void* llvm::thread::ThreadProxy&lt;std::__1::tuple&lt;clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1&gt;&gt;(void*) /home/user/workspace/llvm-project/llvm/include/llvm/Support/thread.h:56:5
#<!-- -->44 0x000075ec9af799eb (/usr/lib/libc.so.6+0x8c9eb)
#<!-- -->45 0x000075ec9affd7cc (/usr/lib/libc.so.6+0x1107cc)
Signalled during AST worker action: Build AST
  Filename: /home/user/workspace/test.cpp
  Directory: /home/user/workspace
  Command Line: /home/user/workspace/install/llvm/bin/clang-tool -std=c++23 -stdlib=libc++ -resource-dir=/home/user/workspace/install/llvm/lib/clang/19 -- /home/user/workspace/test.cpp
  Version: 0

</details>

<details>
<summary>Just in case another log is helpful, <code>clangd --check</code> fails with the same assertion.</summary>

I[19:03:47.944] clangd version 19.0.0git (https://github.com/llvm/llvm-project.git d9850fe23cc69b784cf655405cdfae7defa44573)
I[19:03:47.944] Features: linux+debug
I[19:03:47.944] PID: 535714
I[19:03:47.945] Working directory: /home/user/workspace
I[19:03:47.945] argv[0]: clangd
I[19:03:47.945] argv[1]: --check=test.cpp
I[19:03:47.945] Entering check mode (no LSP server)
I[19:03:47.945] Testing on source file /home/user/workspace/test.cpp
I[19:03:47.945] Loading compilation database...
I[19:03:47.946] Loaded compilation database from /home/user/workspace/compile_flags.txt
I[19:03:47.947] Compile command from CDB is: [/home/user/workspace] /home/user/workspace/llvm-project/builds/Debug/bin/clang-tool -std=c++23 -stdlib=libc++ -resource-dir=/home/user/workspace/llvm-project/builds/Debug/lib/clang/19 -- /home/user/workspace/test.cpp
I[19:03:47.947] Parsing command...
I[19:03:47.951] internal (cc1) args are: -cc1 -triple x86_64-unknown-linux-gnu -fsyntax-only -disable-free -clear-ast-before-backend -main-file-name test.cpp -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=all -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -tune-cpu generic -debugger-tuning=gdb -fdebug-compilation-dir=/home/user/workspace -fcoverage-compilation-dir=/home/user/workspace -resource-dir /home/user/workspace/llvm-project/builds/Debug/lib/clang/19 -internal-isystem /home/user/workspace/llvm-project/builds/Debug/bin/../include/x86_64-unknown-linux-gnu/c++/v1 -internal-isystem /home/user/workspace/llvm-project/builds/Debug/bin/../include/c++/v1 -internal-isystem /home/user/workspace/llvm-project/builds/Debug/lib/clang/19/include -internal-isystem /usr/local/include -internal-isystem /usr/lib64/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../x86_64-pc-linux-gnu/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -std=c++23 -fdeprecated-macro -ferror-limit 19 -fgnuc-version=4.2.1 -fno-implicit-modules -fskip-odr-check-in-gmf -fcxx-exceptions -fexceptions -no-round-trip-args -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -x c++ /home/user/workspace/test.cpp
I[19:03:47.951] Building preamble...
I[19:03:55.501] Built preamble of size 7128100 for file /home/user/workspace/test.cpp version null in 7.55 seconds
I[19:03:55.501] Indexing headers...
I[19:03:56.622] Building AST...
clangd: /home/user/workspace/llvm-project/llvm/include/llvm/ADT/SmallVector.h:304: reference llvm::SmallVectorTemplateCommon<unsigned long>::operator[](size_type) [T = unsigned long]: Assertion `idx < size()' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.	Program arguments: clangd --check=test.cpp
 #<!-- -->0 0x0000623f105b4346 llvm::sys::PrintStackTrace(llvm::raw_ostream&amp;, int) /home/user/workspace/llvm-project/llvm/lib/Support/Unix/Signals.inc:723:11
 #<!-- -->1 0x0000623f105b48eb PrintStackTraceSignalHandler(void*) /home/user/workspace/llvm-project/llvm/lib/Support/Unix/Signals.inc:798:1
 #<!-- -->2 0x0000623f105b2803 llvm::sys::RunSignalHandlers() /home/user/workspace/llvm-project/llvm/lib/Support/Signals.cpp:105:5
 #<!-- -->3 0x0000623f105b4fd1 SignalHandler(int) /home/user/workspace/llvm-project/llvm/lib/Support/Unix/Signals.inc:413:1
 #<!-- -->4 0x0000732eb8dfd710 (/usr/lib/libc.so.6+0x3e710)
 #<!-- -->5 0x0000732eb8e4d83c (/usr/lib/libc.so.6+0x8e83c)
 #<!-- -->6 0x0000732eb8dfd668 gsignal (/usr/lib/libc.so.6+0x3e668)
 #<!-- -->7 0x0000732eb8de54b8 abort (/usr/lib/libc.so.6+0x264b8)
 #<!-- -->8 0x0000732eb8de53dc (/usr/lib/libc.so.6+0x263dc)
 #<!-- -->9 0x0000732eb8df5d26 (/usr/lib/libc.so.6+0x36d26)
#<!-- -->10 0x0000623f1073eaf9 llvm::SmallVectorTemplateCommon&lt;unsigned long, void&gt;::operator[](unsigned long) /home/user/workspace/llvm-project/llvm/include/llvm/ADT/SmallVector.h:0:5
#<!-- -->11 0x0000623f1673ddcb clang::ASTRecordReader::readInt() /home/user/workspace/llvm-project/clang/include/clang/Serialization/ASTRecordReader.h:74:31
#<!-- -->12 0x0000623f1680e35d clang::ASTDeclReader::ReadLocalOffset() /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:107:37
#<!-- -->13 0x0000623f167d61c7 clang::ASTDeclReader::VisitDeclContext(clang::DeclContext*) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:2771:12
#<!-- -->14 0x0000623f168030f2 clang::ASTReader::ReadDeclRecord(unsigned int) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:4129:52
#<!-- -->15 0x0000623f167153b0 clang::ASTReader::GetDecl(unsigned int) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:7783:5
#<!-- -->16 0x0000623f16742d42 clang::ASTReader::GetLocalDecl(clang::serialization::ModuleFile&amp;, unsigned int) /home/user/workspace/llvm-project/clang/include/clang/Serialization/ASTReader.h:1920:5
#<!-- -->17 0x0000623f16805d7a clang::ASTReader::loadPendingDeclChain(clang::Decl*, unsigned long) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:4317:15
#<!-- -->18 0x0000623f1672515c clang::ASTReader::finishPendingActions() /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:9585:57
#<!-- -->19 0x0000623f16728156 clang::ASTReader::FinishedDeserializing() /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:0:5
#<!-- -->20 0x0000623f109b293c clang::ExternalASTSource::Deserializing::~Deserializing() /home/user/workspace/llvm-project/clang/include/clang/AST/ExternalASTSource.h:87:5
#<!-- -->21 0x0000623f1670b050 clang::ASTReader::ReadAST(llvm::StringRef, clang::serialization::ModuleKind, clang::SourceLocation, unsigned int, clang::serialization::ModuleFile**) /home/user/workspace/llvm-project/clang/lib/Serialization/ASTReader.cpp:4663:1
#<!-- -->22 0x0000623f16241b1a clang::CompilerInstance::createPCHExternalASTSource(llvm::StringRef, llvm::StringRef, clang::DisableValidationForModuleKind, bool, clang::Preprocessor&amp;, clang::InMemoryModuleCache&amp;, clang::ASTContext&amp;, clang::PCHContainerReader const&amp;, llvm::ArrayRef&lt;std::__1::shared_ptr&lt;clang::ModuleFileExtension&gt;&gt;, llvm::ArrayRef&lt;std::__1::shared_ptr&lt;clang::DependencyCollector&gt;&gt;, void*, bool, bool, bool) /home/user/workspace/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:660:3
#<!-- -->23 0x0000623f1624162b clang::CompilerInstance::createPCHExternalASTSource(llvm::StringRef, clang::DisableValidationForModuleKind, bool, void*, bool) /home/user/workspace/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:616:18
#<!-- -->24 0x0000623f163366c4 clang::FrontendAction::BeginSourceFile(clang::CompilerInstance&amp;, clang::FrontendInputFile const&amp;) /home/user/workspace/llvm-project/clang/lib/Frontend/FrontendAction.cpp:990:14
#<!-- -->25 0x0000623f13449f46 clang::clangd::ParsedAST::build(llvm::StringRef, clang::clangd::ParseInputs const&amp;, std::__1::unique_ptr&lt;clang::CompilerInvocation, std::__1::default_delete&lt;clang::CompilerInvocation&gt;&gt;, llvm::ArrayRef&lt;clang::clangd::Diag&gt;, std::__1::shared_ptr&lt;clang::clangd::PreambleData const&gt;) /home/user/workspace/llvm-project/clang-tools-extra/clangd/ParsedAST.cpp:505:7
#<!-- -->26 0x0000623f12d22ae6 clang::clangd::(anonymous namespace)::Checker::buildAST() /home/user/workspace/llvm-project/clang-tools-extra/clangd/tool/Check.cpp:254:11
#<!-- -->27 0x0000623f12d21bf6 clang::clangd::check(llvm::StringRef, clang::clangd::ThreadsafeFS const&amp;, clang::clangd::ClangdLSPServer::Options const&amp;) /home/user/workspace/llvm-project/clang-tools-extra/clangd/tool/Check.cpp:508:7
#<!-- -->28 0x0000623f12cd336d clang::clangd::clangdMain(int, char**) /home/user/workspace/llvm-project/clang-tools-extra/clangd/tool/ClangdMain.cpp:976:12
#<!-- -->29 0x0000623f10479ee2 main /home/user/workspace/llvm-project/clang-tools-extra/clangd/tool/ClangdToolMain.cpp:12:3
#<!-- -->30 0x0000732eb8de6cd0 (/usr/lib/libc.so.6+0x27cd0)
#<!-- -->31 0x0000732eb8de6d8a __libc_start_main (/usr/lib/libc.so.6+0x27d8a)
#<!-- -->32 0x0000623f1009ef35 _start (/home/user/workspace/llvm-project/builds/Debug/bin/clangd+0x36a8f35)

</details>

[zyn0217]

Please note that module support of clangd is incomplete and tracked at clangd/clangd#1293.

[marwing]

Please note that module support of clangd is incomplete and tracked at clangd/clangd#1293.

I know, thanks. I'm just toying around with modules to improve my understanding of them and a crash is bad regardless.

[ChuanqiXu9]

The reason may be that the clangd serialize the headers into preamble files with ODRHash and when we read them we skipped the ODRHash.