[clang] crash on invalid code involving constrained variable template

[XDeme1]

clangd 18 keeps crashing while typing template<typename T> above a constrained variable template when there is a explicit variable template instanciation below.

template <typename T>
concept Test = true;

template<typename T>
//! e constant.
template <typename T>
  requires(Test<T>)
constexpr T e_v = 2.718281828'4590452353'6028747135L;

constexpr double e = e_v<double>;

or

template <typename T>
concept Test = true;

template<typename T>
//! e constant.
template <Test T> constexpr T e_v = 2.718281828'4590452353'6028747135L;

constexpr double e = e_v<double>;

Crash report:

[ERROR][2023-11-29 22:28:38] .../vim/lsp/rpc.lua:675	"rpc"	"/usr/local/bin/clangd"	"stderr"	"PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.\t/home/fernando/Codes/cpp/libs/test/main.cpp:8:33: current parser token ';'
"

[ERROR][2023-11-29 22:28:38] .../vim/lsp/rpc.lua:675	"rpc"	"/usr/local/bin/clangd"	"stderr"	" #0 0x000056129f5aca88 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/usr/local/bin/clangd+0x3e95a88)
 #1 0x000056129f5aa98c llvm::sys::RunSignalHandlers() (/usr/local/bin/clangd+0x3e9398c)
 #2 0x000056129f5ad3d8 SignalHandler(int) Signals.cpp:0:0
 #3 0x00007f727985c710 (/usr/lib/libc.so.6+0x3e710)
 #4 0x000056129f7a7757 clang::StmtVisitorBase<llvm::make_const_ptr, (anonymous namespace)::IntExprEvaluator, bool>::Visit(clang::Stmt const*) ExprConstant.cpp:0:0
 #5 0x000056129f73915b Evaluate(clang::APValue&, (anonymous namespace)::EvalInfo&, clang::Expr const*) ExprConstant.cpp:0:0
 #6 0x000056129f735606 EvaluateInPlace(clang::APValue&, (anonymous namespace)::EvalInfo&, (anonymous namespace)::LValue const&, clang::Expr const*, bool) ExprConstant.cpp:0:0
 #7 0x000056129f735083 clang::Expr::EvaluateAsConstantExpr(clang::Expr::EvalResult&, clang::ASTContext const&, clang::Expr::ConstantExprKind) const (/usr/local/bin/clangd+0x401e083)
 #8 0x000056129fd89cd8 clang::ActionResult<clang::Expr*, true> calculateConstraintSatisfaction<calculateConstraintSatisfaction(clang::Sema&, clang::NamedDecl const*, clang::SourceLocation, clang::MultiLevelTemplateArgumentList const&, clang::Expr const*, clang::ConstraintSatisfaction&)::$_0>(clang::Sema&, clang::Expr const*, clang::ConstraintSatisfaction&, calculateConstraintSatisfaction(clang::Sema&, clang::NamedDecl const*, clang::SourceLocation, clang::MultiLevelTemplateArgumentList const&, clang::Expr const*, clang::ConstraintSatisfaction&)::$_0&&) SemaConcept.cpp:0:0
 #9 0x000056129fd8439f CheckConstraintSatisfaction(clang::Sema&, clang::NamedDecl const*, llvm::ArrayRef<clang::Expr const*>, llvm::SmallVectorImpl<clang::Expr*>&, clang::MultiLevelTemplateArgumentList const&, clang::SourceRange, clang::ConstraintSatisfaction&) SemaConcept.cpp:0:0
#10 0x000056129fd84130 clang::Sema::CheckConstraintSatisfaction(clang::NamedDecl const*, llvm::ArrayRef<clang::Expr const*>, llvm::SmallVectorImpl<clang::Expr*>&, clang::MultiLevelTemplateArgumentList const&, clang::SourceRange, clang::ConstraintSatisfaction&) (/usr/local/bin/clangd+0x466d130)
#11 0x000056129fd86691 clang::Sema::EnsureTemplateArgumentListConstraints(clang::TemplateDecl*, clang::MultiLevelTemplateArgumentList const&, clang::SourceRange) (/usr/local/bin/clangd+0x466f691)
#12 0x00005612a017d90c clang::Sema::CheckTemplateArgumentList(clang::TemplateDecl*, clang::SourceLocation, clang::TemplateArgumentListInfo&, bool, llvm::SmallVectorImpl<clang::TemplateArgument>&, llvm::SmallVectorImpl<clang::TemplateArgument>&, bool, bool*) (/usr/local/bin/clangd+0x4a6690c)
#13 0x00005612a018333d clang::Sema::CheckVarTemplateId(clang::VarTemplateDecl*, clang::SourceLocation, clang::SourceLocation, clang::TemplateArgumentListInfo const&) (/usr/local/bin/clangd+0x4a6c33d)
#14 0x00005612a0184863 clang::Sema::BuildTemplateIdExpr(clang::CXXScopeSpec const&, clang::SourceLocation, clang::LookupResult&, bool, clang::TemplateArgumentListInfo const*) (/usr/local/bin/clangd+0x4a6d863)
#15 0x000056129fc67358 clang::Sema::ActOnIdExpression(clang::Scope*, clang::CXXScopeSpec&, clang::SourceLocation, clang::UnqualifiedId&, bool, bool, clang::CorrectionCandidateCallback*, bool, clang::Token*) (/usr/local/bin/clangd+0x4550358)
#16 0x00005612a2e86f6a clang::Parser::tryParseCXXIdExpression(clang::CXXScopeSpec&, bool, clang::Token&) (/usr/local/bin/clangd+0x776ff6a)
#17 0x00005612a2e88041 clang::Parser::ParseCXXIdExpression(bool) (/usr/local/bin/clangd+0x7771041)
#18 0x00005612a2e7b3c7 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&, clang::Parser::TypeCastState, bool, bool*) (/usr/local/bin/clangd+0x77643c7)
#19 0x00005612a2e7bc25 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&, clang::Parser::TypeCastState, bool, bool*) (/usr/local/bin/clangd+0x7764c25)
#20 0x00005612a2e7565f clang::Parser::ParseAssignmentExpression(clang::Parser::TypeCastState) (/usr/local/bin/clangd+0x775e65f)
#21 0x00005612a2eb9a6e clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/usr/local/bin/clangd+0x77a2a6e)
#22 0x00005612a2eb71e4 clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/usr/local/bin/clangd+0x77a01e4)
#23 0x00005612a2e5b6cf clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/usr/local/bin/clangd+0x77446cf)
#24 0x00005612a2e5b0d1 clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/usr/local/bin/clangd+0x77440d1)
#25 0x00005612a2e5a3b6 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/usr/local/bin/clangd+0x77433b6)
#26 0x00005612a2e588c6 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/usr/local/bin/clangd+0x77418c6)
#27 0x00005612a2e5491e clang::ParseAST(clang::Sema&, bool, bool) (/usr/local/bin/clangd+0x773d91e)
#28 0x00005612a2b02cb7 clang::FrontendAction::Execute() (/usr/local/bin/clangd+0x73ebcb7)
#29 0x00005612a05ac887 clang::clangd::ParsedAST::build(llvm::StringRef, clang::clangd::ParseInputs const&, std::unique_ptr<clang::CompilerInvocation, std::default_delete<clang::CompilerInvocation>>, llvm::ArrayRef<clang::clangd::Diag>, std::shared_ptr<clang::clangd::PreambleData const>) (/usr/local/bin/clangd+0x4e95887)
#30 0x00005612a0642c86 clang::clangd::(anonymous namespace)::ASTWorker::generateDiagnostics(std::unique_ptr<clang::CompilerInvocation, std::default_delete<clang::CompilerInvocation>>, clang::clangd::ParseInputs, std::vector<clang::clangd::Diag, std::allocator<clang::clangd::Diag>>) TUScheduler.cpp:0:0
#31 0x00005612a06465da void llvm::detail::UniqueFunctionBase<void>::CallImpl<clang::clangd::(anonymous namespace)::ASTWorker::update(clang::clangd::ParseInputs, clang::clangd::WantDiagnostics, bool)::$_0>(void*) TUScheduler.cpp:0:0
#32 0x00005612a063f150 clang::clangd::(anonymous namespace)::ASTWorker::runTask(llvm::StringRef, llvm::function_ref<void ()>) TUScheduler.cpp:0:0
#33 0x00005612a063ebb8 void llvm::detail::UniqueFunctionBase<void>::CallImpl<clang::clangd::(anonymous namespace)::ASTWorker::create(llvm::StringRef, clang::clangd::GlobalCompilationDatabase const&, clang::clangd::TUScheduler::ASTCache&, clang::clangd::TUScheduler::HeaderIncluderCache&, clang::clangd::AsyncTaskRunner*, clang::clangd::Semaphore&, clang::clangd::TUScheduler::Options const&, clang::clangd::ParsingCallbacks&)::$_0>(void*) TUScheduler.cpp:0:0
#34 0x00005612a07a3c72 void* llvm::thread::ThreadProxy<std::tuple<clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&, llvm::unique_function<void ()>)::$_1>>(void*) Threading.cpp:0:0
#35 0x00007f72798aa9eb (/usr/lib/libc.so.6+0x8c9eb)
#36 0x00007f727992e7cc (/usr/lib/libc.so.6+0x1107cc)
Signalled during AST worker action: Update
  Filename: /home/fernando/Codes/cpp/libs/test/main.cpp
  Directory: /home/fernando/Codes/cpp/libs/test/build/Debug
  Command Line: /usr/bin/g++ --driver-mode=g++ -g -std=gnu++20 -o CMakeFiles/test_clangd.dir/main.cpp.o -c -resource-dir=/usr/local/lib/clang/18 -- /home/fernando/Codes/cpp/libs/test/main.cpp
  Version: 33
"

[llvmbot]

@llvm/issue-subscribers-clangd

Author: None (XDeme)

clangd 18 keeps crashing while typing `template<typename T>` above a constrained variable template when there is a explicit variable template instanciation below.

template <typename T>
concept Test = true;

//! e constant
template <typename T>
  requires(Test<T>)
constexpr T e_v = 2.718281828'4590452353'6028747135L;

constexpr double e = e_v<double>;

or

template <typename T>
concept Test = true;

//! e constant
template <Test T> constexpr T e_v = 2.718281828'4590452353'6028747135L;

constexpr double e = e_v<double>;

Crash report:

[ERROR][2023-11-29 22:28:38] .../vim/lsp/rpc.lua:675	"rpc"	"/usr/local/bin/clangd"	"stderr"	" #<!-- -->0 0x000056129f5aca88 llvm::sys::PrintStackTrace(llvm::raw_ostream&amp;, int) (/usr/local/bin/clangd+0x3e95a88)
 #<!-- -->1 0x000056129f5aa98c llvm::sys::RunSignalHandlers() (/usr/local/bin/clangd+0x3e9398c)
 #<!-- -->2 0x000056129f5ad3d8 SignalHandler(int) Signals.cpp:0:0
 #<!-- -->3 0x00007f727985c710 (/usr/lib/libc.so.6+0x3e710)
 #<!-- -->4 0x000056129f7a7757 clang::StmtVisitorBase&lt;llvm::make_const_ptr, (anonymous namespace)::IntExprEvaluator, bool&gt;::Visit(clang::Stmt const*) ExprConstant.cpp:0:0
 #<!-- -->5 0x000056129f73915b Evaluate(clang::APValue&amp;, (anonymous namespace)::EvalInfo&amp;, clang::Expr const*) ExprConstant.cpp:0:0
 #<!-- -->6 0x000056129f735606 EvaluateInPlace(clang::APValue&amp;, (anonymous namespace)::EvalInfo&amp;, (anonymous namespace)::LValue const&amp;, clang::Expr const*, bool) ExprConstant.cpp:0:0
 #<!-- -->7 0x000056129f735083 clang::Expr::EvaluateAsConstantExpr(clang::Expr::EvalResult&amp;, clang::ASTContext const&amp;, clang::Expr::ConstantExprKind) const (/usr/local/bin/clangd+0x401e083)
 #<!-- -->8 0x000056129fd89cd8 clang::ActionResult&lt;clang::Expr*, true&gt; calculateConstraintSatisfaction&lt;calculateConstraintSatisfaction(clang::Sema&amp;, clang::NamedDecl const*, clang::SourceLocation, clang::MultiLevelTemplateArgumentList const&amp;, clang::Expr const*, clang::ConstraintSatisfaction&amp;)::$_0&gt;(clang::Sema&amp;, clang::Expr const*, clang::ConstraintSatisfaction&amp;, calculateConstraintSatisfaction(clang::Sema&amp;, clang::NamedDecl const*, clang::SourceLocation, clang::MultiLevelTemplateArgumentList const&amp;, clang::Expr const*, clang::ConstraintSatisfaction&amp;)::$_0&amp;&amp;) SemaConcept.cpp:0:0
 #<!-- -->9 0x000056129fd8439f CheckConstraintSatisfaction(clang::Sema&amp;, clang::NamedDecl const*, llvm::ArrayRef&lt;clang::Expr const*&gt;, llvm::SmallVectorImpl&lt;clang::Expr*&gt;&amp;, clang::MultiLevelTemplateArgumentList const&amp;, clang::SourceRange, clang::ConstraintSatisfaction&amp;) SemaConcept.cpp:0:0
#<!-- -->10 0x000056129fd84130 clang::Sema::CheckConstraintSatisfaction(clang::NamedDecl const*, llvm::ArrayRef&lt;clang::Expr const*&gt;, llvm::SmallVectorImpl&lt;clang::Expr*&gt;&amp;, clang::MultiLevelTemplateArgumentList const&amp;, clang::SourceRange, clang::ConstraintSatisfaction&amp;) (/usr/local/bin/clangd+0x466d130)
#<!-- -->11 0x000056129fd86691 clang::Sema::EnsureTemplateArgumentListConstraints(clang::TemplateDecl*, clang::MultiLevelTemplateArgumentList const&amp;, clang::SourceRange) (/usr/local/bin/clangd+0x466f691)
#<!-- -->12 0x00005612a017d90c clang::Sema::CheckTemplateArgumentList(clang::TemplateDecl*, clang::SourceLocation, clang::TemplateArgumentListInfo&amp;, bool, llvm::SmallVectorImpl&lt;clang::TemplateArgument&gt;&amp;, llvm::SmallVectorImpl&lt;clang::TemplateArgument&gt;&amp;, bool, bool*) (/usr/local/bin/clangd+0x4a6690c)
#<!-- -->13 0x00005612a018333d clang::Sema::CheckVarTemplateId(clang::VarTemplateDecl*, clang::SourceLocation, clang::SourceLocation, clang::TemplateArgumentListInfo const&amp;) (/usr/local/bin/clangd+0x4a6c33d)
#<!-- -->14 0x00005612a0184863 clang::Sema::BuildTemplateIdExpr(clang::CXXScopeSpec const&amp;, clang::SourceLocation, clang::LookupResult&amp;, bool, clang::TemplateArgumentListInfo const*) (/usr/local/bin/clangd+0x4a6d863)
#<!-- -->15 0x000056129fc67358 clang::Sema::ActOnIdExpression(clang::Scope*, clang::CXXScopeSpec&amp;, clang::SourceLocation, clang::UnqualifiedId&amp;, bool, bool, clang::CorrectionCandidateCallback*, bool, clang::Token*) (/usr/local/bin/clangd+0x4550358)
#<!-- -->16 0x00005612a2e86f6a clang::Parser::tryParseCXXIdExpression(clang::CXXScopeSpec&amp;, bool, clang::Token&amp;) (/usr/local/bin/clangd+0x776ff6a)
#<!-- -->17 0x00005612a2e88041 clang::Parser::ParseCXXIdExpression(bool) (/usr/local/bin/clangd+0x7771041)
#<!-- -->18 0x00005612a2e7b3c7 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&amp;, clang::Parser::TypeCastState, bool, bool*) (/usr/local/bin/clangd+0x77643c7)
#<!-- -->19 0x00005612a2e7bc25 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&amp;, clang::Parser::TypeCastState, bool, bool*) (/usr/local/bin/clangd+0x7764c25)
#<!-- -->20 0x00005612a2e7565f clang::Parser::ParseAssignmentExpression(clang::Parser::TypeCastState) (/usr/local/bin/clangd+0x775e65f)
#<!-- -->21 0x00005612a2eb9a6e clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&amp;, clang::Parser::ParsedTemplateInfo const&amp;, clang::Parser::ForRangeInit*) (/usr/local/bin/clangd+0x77a2a6e)
#<!-- -->22 0x00005612a2eb71e4 clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&amp;, clang::DeclaratorContext, clang::ParsedAttributes&amp;, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/usr/local/bin/clangd+0x77a01e4)
#<!-- -->23 0x00005612a2e5b6cf clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;, clang::ParsingDeclSpec&amp;, clang::AccessSpecifier) (/usr/local/bin/clangd+0x77446cf)
#<!-- -->24 0x00005612a2e5b0d1 clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/usr/local/bin/clangd+0x77440d1)
#<!-- -->25 0x00005612a2e5a3b6 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;, clang::ParsingDeclSpec*) (/usr/local/bin/clangd+0x77433b6)
#<!-- -->26 0x00005612a2e588c6 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&lt;clang::DeclGroupRef&gt;&amp;, clang::Sema::ModuleImportState&amp;) (/usr/local/bin/clangd+0x77418c6)
#<!-- -->27 0x00005612a2e5491e clang::ParseAST(clang::Sema&amp;, bool, bool) (/usr/local/bin/clangd+0x773d91e)
#<!-- -->28 0x00005612a2b02cb7 clang::FrontendAction::Execute() (/usr/local/bin/clangd+0x73ebcb7)
#<!-- -->29 0x00005612a05ac887 clang::clangd::ParsedAST::build(llvm::StringRef, clang::clangd::ParseInputs const&amp;, std::unique_ptr&lt;clang::CompilerInvocation, std::default_delete&lt;clang::CompilerInvocation&gt;&gt;, llvm::ArrayRef&lt;clang::clangd::Diag&gt;, std::shared_ptr&lt;clang::clangd::PreambleData const&gt;) (/usr/local/bin/clangd+0x4e95887)
#<!-- -->30 0x00005612a0642c86 clang::clangd::(anonymous namespace)::ASTWorker::generateDiagnostics(std::unique_ptr&lt;clang::CompilerInvocation, std::default_delete&lt;clang::CompilerInvocation&gt;&gt;, clang::clangd::ParseInputs, std::vector&lt;clang::clangd::Diag, std::allocator&lt;clang::clangd::Diag&gt;&gt;) TUScheduler.cpp:0:0
#<!-- -->31 0x00005612a06465da void llvm::detail::UniqueFunctionBase&lt;void&gt;::CallImpl&lt;clang::clangd::(anonymous namespace)::ASTWorker::update(clang::clangd::ParseInputs, clang::clangd::WantDiagnostics, bool)::$_0&gt;(void*) TUScheduler.cpp:0:0
#<!-- -->32 0x00005612a063f150 clang::clangd::(anonymous namespace)::ASTWorker::runTask(llvm::StringRef, llvm::function_ref&lt;void ()&gt;) TUScheduler.cpp:0:0
#<!-- -->33 0x00005612a063ebb8 void llvm::detail::UniqueFunctionBase&lt;void&gt;::CallImpl&lt;clang::clangd::(anonymous namespace)::ASTWorker::create(llvm::StringRef, clang::clangd::GlobalCompilationDatabase const&amp;, clang::clangd::TUScheduler::ASTCache&amp;, clang::clangd::TUScheduler::HeaderIncluderCache&amp;, clang::clangd::AsyncTaskRunner*, clang::clangd::Semaphore&amp;, clang::clangd::TUScheduler::Options const&amp;, clang::clangd::ParsingCallbacks&amp;)::$_0&gt;(void*) TUScheduler.cpp:0:0
#<!-- -->34 0x00005612a07a3c72 void* llvm::thread::ThreadProxy&lt;std::tuple&lt;clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1&gt;&gt;(void*) Threading.cpp:0:0
#<!-- -->35 0x00007f72798aa9eb (/usr/lib/libc.so.6+0x8c9eb)
#<!-- -->36 0x00007f727992e7cc (/usr/lib/libc.so.6+0x1107cc)
Signalled during AST worker action: Update
  Filename: /home/fernando/Codes/cpp/libs/test/main.cpp
  Directory: /home/fernando/Codes/cpp/libs/test/build/Debug
  Command Line: /usr/bin/g++ --driver-mode=g++ -g -std=gnu++20 -o CMakeFiles/test_clangd.dir/main.cpp.o -c -resource-dir=/usr/local/lib/clang/18 -- /home/fernando/Codes/cpp/libs/test/main.cpp
  Version: 33
"

[zyn0217]

I'm not sure if this is a regression introduced recently. I couldn't reproduce it from 89ecb80, which is a three-week-ago commit.

Could you please provide more details about the crash? e.g. what commit is your clangd built from?

[XDeme1]

running clangd --version gives me:

clangd version 18.0.0 (https://github.com/llvm/llvm-project.git a6c02edd6eac476523b5c73f29619a7a9e054872)
Features: linux
Platform: x86_64-unknown-linux-gnu

About the crash report from clang 18, I missed some lines in the top of the crash report:

[ERROR][2023-11-29 22:28:38] .../vim/lsp/rpc.lua:675	"rpc"	"/usr/local/bin/clangd"	"stderr"	"PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.\t/home/fernando/Codes/cpp/libs/test/main.cpp:8:33: current parser token ';'

I also updated the original crash report to include it, hope it helps.

but i don't think it is introduced recently, i could also reproduce it with clangd 16.0.6.

Crash report from clangd 16.0.6

[ERROR][2023-12-01 12:22:11] .../vim/lsp/rpc.lua:675	"rpc"	"/usr/bin/clangd"	"stderr"	" #0 0x00007f48af21f503 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/usr/lib/libLLVM-16.so+0xe1f503)
 #1 0x00007f48af21c7bf llvm::sys::RunSignalHandlers() (/usr/lib/libLLVM-16.so+0xe1c7bf)
 #2 0x00007f48af21c90d (/usr/lib/libLLVM-16.so+0xe1c90d)
 #3 0x00007f48ade5c710 (/usr/lib/libc.so.6+0x3e710)
 #4 0x00007f48b743461e (/usr/lib/libclang-cpp.so.16+0xc3461e)
 #5 0x00007f48b742b6a0 (/usr/lib/libclang-cpp.so.16+0xc2b6a0)
 #6 0x00007f48b7453a28 clang::Expr::EvaluateAsConstantExpr(clang::Expr::EvalResult&, clang::ASTContext const&, clang::Expr::ConstantExprKind) const (/usr/lib/libclang-cpp.so.16+0xc53a28)
 #7 0x00007f48b7969050 (/usr/lib/libclang-cpp.so.16+0x1169050)
 #8 0x00007f48b796977d (/usr/lib/libclang-cpp.so.16+0x116977d)
 #9 0x00007f48b7969beb clang::Sema::CheckConstraintSatisfaction(clang::NamedDecl const*, llvm::ArrayRef<clang::Expr const*>, llvm::SmallVectorImpl<clang::Expr*>&, clang::MultiLevelTemplateArgumentList const&, clang::SourceRange, clang::ConstraintSatisfaction&) (/usr/lib/libclang-cpp.so.16+0x1169beb)
#10 0x00007f48b796a493 clang::Sema::EnsureTemplateArgumentListConstraints(clang::TemplateDecl*, clang::MultiLevelTemplateArgumentList const&, clang::SourceRange) (/usr/lib/libclang-cpp.so.16+0x116a493)
#11 0x00007f48b7d944da clang::Sema::CheckTemplateArgumentList(clang::TemplateDecl*, clang::SourceLocation, clang::TemplateArgumentListInfo&, bool, llvm::SmallVectorImpl<clang::TemplateArgument>&, llvm::SmallVectorImpl<clang::TemplateArgument>&, bool, bool*) (/usr/lib/libclang-cpp.so.16+0x15944da)
#12 0x00007f48b7da02e8 clang::Sema::CheckVarTemplateId(clang::VarTemplateDecl*, clang::SourceLocation, clang::SourceLocation, clang::TemplateArgumentListInfo const&) (/usr/lib/libclang-cpp.so.16+0x15a02e8)
#13 0x00007f48b7da0cd8 clang::Sema::BuildTemplateIdExpr(clang::CXXScopeSpec const&, clang::SourceLocation, clang::LookupResult&, bool, clang::TemplateArgumentListInfo const*) (/usr/lib/libclang-cpp.so.16+0x15a0cd8)
#14 0x00007f48b7ac13f8 clang::Sema::ActOnIdExpression(clang::Scope*, clang::CXXScopeSpec&, clang::SourceLocation, clang::UnqualifiedId&, bool, bool, clang::CorrectionCandidateCallback*, bool, clang::Token*) (/usr/lib/libclang-cpp.so.16+0x12c13f8)
#15 0x00007f48b7129670 clang::Parser::tryParseCXXIdExpression(clang::CXXScopeSpec&, bool, clang::Token&) (/usr/lib/libclang-cpp.so.16+0x929670)
#16 0x00007f48b712972a clang::Parser::ParseCXXIdExpression(bool) (/usr/lib/libclang-cpp.so.16+0x92972a)
#17 0x00007f48b7134bc4 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&, clang::Parser::TypeCastState, bool, bool*) (/usr/lib/libclang-cpp.so.16+0x934bc4)
#18 0x00007f48b7135c7d clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&, clang::Parser::TypeCastState, bool, bool*) (/usr/lib/libclang-cpp.so.16+0x935c7d)
#19 0x00007f48b900b8bd (/usr/lib/libclang-cpp.so.16+0x280b8bd)
#20 0x00007f48b900b966 (/usr/lib/libclang-cpp.so.16+0x280b966)
#21 0x00007f48b70fa4d1 clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/usr/lib/libclang-cpp.so.16+0x8fa4d1)
#22 0x00007f48b71048d5 clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/usr/lib/libclang-cpp.so.16+0x9048d5)
#23 0x00007f48b71adfc9 (/usr/lib/libclang-cpp.so.16+0x9adfc9)
#24 0x00007f48b71b8cd6 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/usr/lib/libclang-cpp.so.16+0x9b8cd6)
#25 0x00007f48b71b9073 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/usr/lib/libclang-cpp.so.16+0x9b9073)
#26 0x00007f48b70ecf49 clang::ParseAST(clang::Sema&, bool, bool) (/usr/lib/libclang-cpp.so.16+0x8ecf49)
#27 0x00007f48b891e7c9 clang::FrontendAction::Execute() (/usr/lib/libclang-cpp.so.16+0x211e7c9)
#28 0x000055bcd376ec3d (/usr/bin/clangd+0x744c3d)
#29 0x000055bcd37d1ae2 (/usr/bin/clangd+0x7a7ae2)
#30 0x000055bcd37b7657 (/usr/bin/clangd+0x78d657)
#31 0x000055bcd37b80aa (/usr/bin/clangd+0x78e0aa)
#32 0x000055bcd392f6c9 (/usr/bin/clangd+0x9056c9)
#33 0x00007f48adeaa9eb (/usr/lib/libc.so.6+0x8c9eb)
#34 0x00007f48adf2e7cc (/usr/lib/libc.so.6+0x1107cc)
Signalled during AST worker action: Build AST
  Filename: /home/fernando/Codes/cpp/libs/test/main.cpp
  Directory: /home/fernando/Codes/cpp/libs/test/build/Debug
  Command Line: /usr/local/bin/clang++ --driver-mode=g++ -g -c -std=gnu++20 -resource-dir=/usr/lib/clang/16 -- /home/fernando/Codes/cpp/libs/test/main.cpp
  Version: 24
"

Correct reproducer with template<typename T>:

template <typename T>
concept Test = true;

template<typename T>
//! e constant.
template <Test T> constexpr T e_v = 2.718281828'4590452353'6028747135L;

constexpr double e = e_v<double>;

I have also updated both original reproducers to include the template<typename T>.
Sorry about the wrong reproducer, i tought the original one was ok.

[HighCommander4]

Not clangd-specific, trying to compile the reproducer with clang -std=c++20 -fsyntax-only also crashes.

[llvmbot]

@llvm/issue-subscribers-c-20

Author: None (XDeme)

clangd 18 keeps crashing while typing `template<typename T>` above a constrained variable template when there is a explicit variable template instanciation below.

template <typename T>
concept Test = true;

template<typename T>
//! e constant.
template <typename T>
  requires(Test<T>)
constexpr T e_v = 2.718281828'4590452353'6028747135L;

constexpr double e = e_v<double>;

or

template <typename T>
concept Test = true;

template<typename T>
//! e constant.
template <Test T> constexpr T e_v = 2.718281828'4590452353'6028747135L;

constexpr double e = e_v<double>;

Crash report:

[ERROR][2023-11-29 22:28:38] .../vim/lsp/rpc.lua:675	"rpc"	"/usr/local/bin/clangd"	"stderr"	"PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.\t/home/fernando/Codes/cpp/libs/test/main.cpp:8:33: current parser token ';'
"

[ERROR][2023-11-29 22:28:38] .../vim/lsp/rpc.lua:675	"rpc"	"/usr/local/bin/clangd"	"stderr"	" #<!-- -->0 0x000056129f5aca88 llvm::sys::PrintStackTrace(llvm::raw_ostream&amp;, int) (/usr/local/bin/clangd+0x3e95a88)
 #<!-- -->1 0x000056129f5aa98c llvm::sys::RunSignalHandlers() (/usr/local/bin/clangd+0x3e9398c)
 #<!-- -->2 0x000056129f5ad3d8 SignalHandler(int) Signals.cpp:0:0
 #<!-- -->3 0x00007f727985c710 (/usr/lib/libc.so.6+0x3e710)
 #<!-- -->4 0x000056129f7a7757 clang::StmtVisitorBase&lt;llvm::make_const_ptr, (anonymous namespace)::IntExprEvaluator, bool&gt;::Visit(clang::Stmt const*) ExprConstant.cpp:0:0
 #<!-- -->5 0x000056129f73915b Evaluate(clang::APValue&amp;, (anonymous namespace)::EvalInfo&amp;, clang::Expr const*) ExprConstant.cpp:0:0
 #<!-- -->6 0x000056129f735606 EvaluateInPlace(clang::APValue&amp;, (anonymous namespace)::EvalInfo&amp;, (anonymous namespace)::LValue const&amp;, clang::Expr const*, bool) ExprConstant.cpp:0:0
 #<!-- -->7 0x000056129f735083 clang::Expr::EvaluateAsConstantExpr(clang::Expr::EvalResult&amp;, clang::ASTContext const&amp;, clang::Expr::ConstantExprKind) const (/usr/local/bin/clangd+0x401e083)
 #<!-- -->8 0x000056129fd89cd8 clang::ActionResult&lt;clang::Expr*, true&gt; calculateConstraintSatisfaction&lt;calculateConstraintSatisfaction(clang::Sema&amp;, clang::NamedDecl const*, clang::SourceLocation, clang::MultiLevelTemplateArgumentList const&amp;, clang::Expr const*, clang::ConstraintSatisfaction&amp;)::$_0&gt;(clang::Sema&amp;, clang::Expr const*, clang::ConstraintSatisfaction&amp;, calculateConstraintSatisfaction(clang::Sema&amp;, clang::NamedDecl const*, clang::SourceLocation, clang::MultiLevelTemplateArgumentList const&amp;, clang::Expr const*, clang::ConstraintSatisfaction&amp;)::$_0&amp;&amp;) SemaConcept.cpp:0:0
 #<!-- -->9 0x000056129fd8439f CheckConstraintSatisfaction(clang::Sema&amp;, clang::NamedDecl const*, llvm::ArrayRef&lt;clang::Expr const*&gt;, llvm::SmallVectorImpl&lt;clang::Expr*&gt;&amp;, clang::MultiLevelTemplateArgumentList const&amp;, clang::SourceRange, clang::ConstraintSatisfaction&amp;) SemaConcept.cpp:0:0
#<!-- -->10 0x000056129fd84130 clang::Sema::CheckConstraintSatisfaction(clang::NamedDecl const*, llvm::ArrayRef&lt;clang::Expr const*&gt;, llvm::SmallVectorImpl&lt;clang::Expr*&gt;&amp;, clang::MultiLevelTemplateArgumentList const&amp;, clang::SourceRange, clang::ConstraintSatisfaction&amp;) (/usr/local/bin/clangd+0x466d130)
#<!-- -->11 0x000056129fd86691 clang::Sema::EnsureTemplateArgumentListConstraints(clang::TemplateDecl*, clang::MultiLevelTemplateArgumentList const&amp;, clang::SourceRange) (/usr/local/bin/clangd+0x466f691)
#<!-- -->12 0x00005612a017d90c clang::Sema::CheckTemplateArgumentList(clang::TemplateDecl*, clang::SourceLocation, clang::TemplateArgumentListInfo&amp;, bool, llvm::SmallVectorImpl&lt;clang::TemplateArgument&gt;&amp;, llvm::SmallVectorImpl&lt;clang::TemplateArgument&gt;&amp;, bool, bool*) (/usr/local/bin/clangd+0x4a6690c)
#<!-- -->13 0x00005612a018333d clang::Sema::CheckVarTemplateId(clang::VarTemplateDecl*, clang::SourceLocation, clang::SourceLocation, clang::TemplateArgumentListInfo const&amp;) (/usr/local/bin/clangd+0x4a6c33d)
#<!-- -->14 0x00005612a0184863 clang::Sema::BuildTemplateIdExpr(clang::CXXScopeSpec const&amp;, clang::SourceLocation, clang::LookupResult&amp;, bool, clang::TemplateArgumentListInfo const*) (/usr/local/bin/clangd+0x4a6d863)
#<!-- -->15 0x000056129fc67358 clang::Sema::ActOnIdExpression(clang::Scope*, clang::CXXScopeSpec&amp;, clang::SourceLocation, clang::UnqualifiedId&amp;, bool, bool, clang::CorrectionCandidateCallback*, bool, clang::Token*) (/usr/local/bin/clangd+0x4550358)
#<!-- -->16 0x00005612a2e86f6a clang::Parser::tryParseCXXIdExpression(clang::CXXScopeSpec&amp;, bool, clang::Token&amp;) (/usr/local/bin/clangd+0x776ff6a)
#<!-- -->17 0x00005612a2e88041 clang::Parser::ParseCXXIdExpression(bool) (/usr/local/bin/clangd+0x7771041)
#<!-- -->18 0x00005612a2e7b3c7 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&amp;, clang::Parser::TypeCastState, bool, bool*) (/usr/local/bin/clangd+0x77643c7)
#<!-- -->19 0x00005612a2e7bc25 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&amp;, clang::Parser::TypeCastState, bool, bool*) (/usr/local/bin/clangd+0x7764c25)
#<!-- -->20 0x00005612a2e7565f clang::Parser::ParseAssignmentExpression(clang::Parser::TypeCastState) (/usr/local/bin/clangd+0x775e65f)
#<!-- -->21 0x00005612a2eb9a6e clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&amp;, clang::Parser::ParsedTemplateInfo const&amp;, clang::Parser::ForRangeInit*) (/usr/local/bin/clangd+0x77a2a6e)
#<!-- -->22 0x00005612a2eb71e4 clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&amp;, clang::DeclaratorContext, clang::ParsedAttributes&amp;, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/usr/local/bin/clangd+0x77a01e4)
#<!-- -->23 0x00005612a2e5b6cf clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;, clang::ParsingDeclSpec&amp;, clang::AccessSpecifier) (/usr/local/bin/clangd+0x77446cf)
#<!-- -->24 0x00005612a2e5b0d1 clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/usr/local/bin/clangd+0x77440d1)
#<!-- -->25 0x00005612a2e5a3b6 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;, clang::ParsingDeclSpec*) (/usr/local/bin/clangd+0x77433b6)
#<!-- -->26 0x00005612a2e588c6 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&lt;clang::DeclGroupRef&gt;&amp;, clang::Sema::ModuleImportState&amp;) (/usr/local/bin/clangd+0x77418c6)
#<!-- -->27 0x00005612a2e5491e clang::ParseAST(clang::Sema&amp;, bool, bool) (/usr/local/bin/clangd+0x773d91e)
#<!-- -->28 0x00005612a2b02cb7 clang::FrontendAction::Execute() (/usr/local/bin/clangd+0x73ebcb7)
#<!-- -->29 0x00005612a05ac887 clang::clangd::ParsedAST::build(llvm::StringRef, clang::clangd::ParseInputs const&amp;, std::unique_ptr&lt;clang::CompilerInvocation, std::default_delete&lt;clang::CompilerInvocation&gt;&gt;, llvm::ArrayRef&lt;clang::clangd::Diag&gt;, std::shared_ptr&lt;clang::clangd::PreambleData const&gt;) (/usr/local/bin/clangd+0x4e95887)
#<!-- -->30 0x00005612a0642c86 clang::clangd::(anonymous namespace)::ASTWorker::generateDiagnostics(std::unique_ptr&lt;clang::CompilerInvocation, std::default_delete&lt;clang::CompilerInvocation&gt;&gt;, clang::clangd::ParseInputs, std::vector&lt;clang::clangd::Diag, std::allocator&lt;clang::clangd::Diag&gt;&gt;) TUScheduler.cpp:0:0
#<!-- -->31 0x00005612a06465da void llvm::detail::UniqueFunctionBase&lt;void&gt;::CallImpl&lt;clang::clangd::(anonymous namespace)::ASTWorker::update(clang::clangd::ParseInputs, clang::clangd::WantDiagnostics, bool)::$_0&gt;(void*) TUScheduler.cpp:0:0
#<!-- -->32 0x00005612a063f150 clang::clangd::(anonymous namespace)::ASTWorker::runTask(llvm::StringRef, llvm::function_ref&lt;void ()&gt;) TUScheduler.cpp:0:0
#<!-- -->33 0x00005612a063ebb8 void llvm::detail::UniqueFunctionBase&lt;void&gt;::CallImpl&lt;clang::clangd::(anonymous namespace)::ASTWorker::create(llvm::StringRef, clang::clangd::GlobalCompilationDatabase const&amp;, clang::clangd::TUScheduler::ASTCache&amp;, clang::clangd::TUScheduler::HeaderIncluderCache&amp;, clang::clangd::AsyncTaskRunner*, clang::clangd::Semaphore&amp;, clang::clangd::TUScheduler::Options const&amp;, clang::clangd::ParsingCallbacks&amp;)::$_0&gt;(void*) TUScheduler.cpp:0:0
#<!-- -->34 0x00005612a07a3c72 void* llvm::thread::ThreadProxy&lt;std::tuple&lt;clang::clangd::AsyncTaskRunner::runAsync(llvm::Twine const&amp;, llvm::unique_function&lt;void ()&gt;)::$_1&gt;&gt;(void*) Threading.cpp:0:0
#<!-- -->35 0x00007f72798aa9eb (/usr/lib/libc.so.6+0x8c9eb)
#<!-- -->36 0x00007f727992e7cc (/usr/lib/libc.so.6+0x1107cc)
Signalled during AST worker action: Update
  Filename: /home/fernando/Codes/cpp/libs/test/main.cpp
  Directory: /home/fernando/Codes/cpp/libs/test/build/Debug
  Command Line: /usr/bin/g++ --driver-mode=g++ -g -std=gnu++20 -o CMakeFiles/test_clangd.dir/main.cpp.o -c -resource-dir=/usr/local/lib/clang/18 -- /home/fernando/Codes/cpp/libs/test/main.cpp
  Version: 33
"