[Regression] clang 18 crashes at `clang::Sema::tryCaptureVariable`: Assertion `idx < size()` failed

[yujincheng08]

Min repro

Godbolt Link https://godbolt.org/z/5EWeoefbM

Additional notes

1. cannot reproduce on clang-17.
2. lambda passed to b must have at least one argument, and cannot have auto return type.
3. x's parameter must be a lambda.
4. x's constraints must contain its parameter v.
5. without capturing variables it compiles.

Stack dump

Stack dump:
Stack dump:
0.	Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/clang++ -gdwarf-4 -g -o /app/output.s -mllvm --x86-asm-syntax=intel -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -std=c++23 -Wno-everything <source>
1.	<eof> parser at end of file
2.	<source>:3:27: instantiating function definition 'b<(lambda at <source>:5:14)>'
3.	<source>:5:14: instantiating function definition 't()::(anonymous class)::operator()<(lambda at <source>:5:14)>'
 #0 0x00000000039ea408 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x39ea408)
 #1 0x00000000039e80dc llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x39e80dc)
 #2 0x00000000039311b8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x000073a683842520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x000073a6838969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #5 0x000073a683842476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #6 0x000073a6838287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #7 0x000073a68382871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
 #8 0x000073a683839e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
 #9 0x00000000068bc93f clang::Sema::tryCaptureVariable(clang::ValueDecl*, clang::SourceLocation, clang::Sema::TryCaptureKind, clang::SourceLocation, bool, clang::QualType&, clang::QualType&, unsigned int const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68bc93f)
#10 0x00000000068ea406 clang::Sema::BuildDeclRefExpr(clang::ValueDecl*, clang::QualType, clang::ExprValueKind, clang::DeclarationNameInfo const&, clang::NestedNameSpecifierLoc, clang::NamedDecl*, clang::SourceLocation, clang::TemplateArgumentListInfo const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68ea406)
#11 0x00000000068eac5f clang::Sema::BuildDeclRefExpr(clang::ValueDecl*, clang::QualType, clang::ExprValueKind, clang::DeclarationNameInfo const&, clang::CXXScopeSpec const*, clang::NamedDecl*, clang::SourceLocation, clang::TemplateArgumentListInfo const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68eac5f)
#12 0x00000000068f1343 clang::Sema::BuildDeclarationNameExpr(clang::CXXScopeSpec const&, clang::DeclarationNameInfo const&, clang::NamedDecl*, clang::NamedDecl*, clang::TemplateArgumentListInfo const*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68f1343)
#13 0x0000000006f52732 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformDeclRefExpr(clang::DeclRefExpr*) SemaTemplateInstantiate.cpp:0:0
#14 0x0000000006f5293b (anonymous namespace)::TemplateInstantiator::TransformDeclRefExpr(clang::DeclRefExpr*) SemaTemplateInstantiate.cpp:0:0
#15 0x0000000006f2c7e4 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#16 0x0000000006f58cc6 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformRequiresExpr(clang::RequiresExpr*) SemaTemplateInstantiate.cpp:0:0
#17 0x0000000006f5abae (anonymous namespace)::TemplateInstantiator::TransformRequiresExpr(clang::RequiresExpr*) SemaTemplateInstantiate.cpp:0:0
#18 0x0000000006f2cbe9 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#19 0x0000000006f63b94 clang::Sema::SubstConstraintExpr(clang::Expr*, clang::MultiLevelTemplateArgumentList const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f63b94)
#20 0x00000000065fbd3b calculateConstraintSatisfaction(clang::Sema&, clang::NamedDecl const*, clang::SourceLocation, clang::MultiLevelTemplateArgumentList const&, clang::Expr const*, clang::ConstraintSatisfaction&)::'lambda'(clang::Expr const*)::operator()(clang::Expr const*) const SemaConcept.cpp:0:0
#21 0x00000000065fd66b clang::ActionResult<clang::Expr*, true> calculateConstraintSatisfaction<calculateConstraintSatisfaction(clang::Sema&, clang::NamedDecl const*, clang::SourceLocation, clang::MultiLevelTemplateArgumentList const&, clang::Expr const*, clang::ConstraintSatisfaction&)::'lambda'(clang::Expr const*)>(clang::Sema&, clang::Expr const*, clang::ConstraintSatisfaction&, calculateConstraintSatisfaction(clang::Sema&, clang::NamedDecl const*, clang::SourceLocation, clang::MultiLevelTemplateArgumentList const&, clang::Expr const*, clang::ConstraintSatisfaction&)::'lambda'(clang::Expr const*)&&) SemaConcept.cpp:0:0
#22 0x00000000065fdfe2 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
#23 0x00000000065fe622 clang::Sema::CheckConstraintSatisfaction(clang::NamedDecl const*, llvm::ArrayRef<clang::Expr const*>, llvm::SmallVectorImpl<clang::Expr*>&, clang::MultiLevelTemplateArgumentList const&, clang::SourceRange, clang::ConstraintSatisfaction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x65fe622)
#24 0x00000000065ff6eb clang::Sema::CheckInstantiatedFunctionTemplateConstraints(clang::SourceLocation, clang::FunctionDecl*, llvm::ArrayRef<clang::TemplateArgument>, clang::ConstraintSatisfaction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x65ff6eb)
#25 0x0000000006eea177 clang::Sema::FinishTemplateArgumentDeduction(clang::FunctionTemplateDecl*, llvm::SmallVectorImpl<clang::DeducedTemplateArgument>&, unsigned int, clang::FunctionDecl*&, clang::sema::TemplateDeductionInfo&, llvm::SmallVectorImpl<clang::Sema::OriginalCallArg> const*, bool, llvm::function_ref<bool ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6eea177)
#26 0x0000000006eeb92a void llvm::function_ref<void ()>::callback_fn<clang::Sema::DeduceTemplateArguments(clang::FunctionTemplateDecl*, clang::TemplateArgumentListInfo*, llvm::ArrayRef<clang::Expr*>, clang::FunctionDecl*&, clang::sema::TemplateDeductionInfo&, bool, bool, clang::QualType, clang::Expr::Classification, llvm::function_ref<bool (llvm::ArrayRef<clang::QualType>)>)::'lambda1'()>(long) SemaTemplateDeduction.cpp:0:0
#27 0x000000000649a671 clang::Sema::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x649a671)
#28 0x0000000006ef6355 clang::Sema::DeduceTemplateArguments(clang::FunctionTemplateDecl*, clang::TemplateArgumentListInfo*, llvm::ArrayRef<clang::Expr*>, clang::FunctionDecl*&, clang::sema::TemplateDeductionInfo&, bool, bool, clang::QualType, clang::Expr::Classification, llvm::function_ref<bool (llvm::ArrayRef<clang::QualType>)>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ef6355)
#29 0x0000000006cccec8 clang::Sema::AddTemplateOverloadCandidate(clang::FunctionTemplateDecl*, clang::DeclAccessPair, clang::TemplateArgumentListInfo*, llvm::ArrayRef<clang::Expr*>, clang::OverloadCandidateSet&, bool, bool, bool, clang::CallExpr::ADLCallKind, clang::OverloadCandidateParamOrder, bool) (.constprop.1) SemaOverload.cpp:0:0
#30 0x0000000006cce6d9 clang::Sema::AddOverloadedCallCandidates(clang::UnresolvedLookupExpr*, llvm::ArrayRef<clang::Expr*>, clang::OverloadCandidateSet&, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cce6d9)
#31 0x0000000006cceaae clang::Sema::buildOverloadedCallSet(clang::Scope*, clang::Expr*, clang::UnresolvedLookupExpr*, llvm::MutableArrayRef<clang::Expr*>, clang::SourceLocation, clang::OverloadCandidateSet*, clang::ActionResult<clang::Expr*, true>*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cceaae)
#32 0x0000000006cd6097 clang::Sema::BuildOverloadedCallExpr(clang::Scope*, clang::Expr*, clang::UnresolvedLookupExpr*, clang::SourceLocation, llvm::MutableArrayRef<clang::Expr*>, clang::SourceLocation, clang::Expr*, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cd6097)
#33 0x0000000006918620 clang::Sema::BuildCallExpr(clang::Scope*, clang::Expr*, clang::SourceLocation, llvm::MutableArrayRef<clang::Expr*>, clang::SourceLocation, clang::Expr*, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6918620)
#34 0x000000000691a68c clang::Sema::ActOnCallExpr(clang::Scope*, clang::Expr*, clang::SourceLocation, llvm::MutableArrayRef<clang::Expr*>, clang::SourceLocation, clang::Expr*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x691a68c)
#35 0x0000000006f389c0 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformCallExpr(clang::CallExpr*) SemaTemplateInstantiate.cpp:0:0
#36 0x0000000006f2c72d clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#37 0x0000000006f6860f clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformStmt(clang::Stmt*, clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::StmtDiscardKind) SemaTemplateInstantiate.cpp:0:0
#38 0x0000000006f693e4 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformCompoundStmt(clang::CompoundStmt*, bool) SemaTemplateInstantiate.cpp:0:0
#39 0x0000000006f70a7a clang::Sema::SubstStmt(clang::Stmt*, clang::MultiLevelTemplateArgumentList const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f70a7a)
#40 0x0000000006fc8408 clang::Sema::InstantiateFunctionDefinition(clang::SourceLocation, clang::FunctionDecl*, bool, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fc8408)
#41 0x0000000006fc661f clang::Sema::PerformPendingInstantiations(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fc661f)
#42 0x0000000006fc7e2c clang::Sema::InstantiateFunctionDefinition(clang::SourceLocation, clang::FunctionDecl*, bool, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fc7e2c)
#43 0x0000000006fc661f clang::Sema::PerformPendingInstantiations(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fc661f)
#44 0x00000000064bd09f clang::Sema::ActOnEndOfTranslationUnitFragment(clang::Sema::TUFragmentKind) (.part.0) Sema.cpp:0:0
#45 0x00000000064bd8ea clang::Sema::ActOnEndOfTranslationUnit() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64bd8ea)
#46 0x0000000006337eca clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6337eca)
#47 0x000000000632b97a clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x632b97a)
#48 0x00000000042bf348 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42bf348)
#49 0x0000000004547b09 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4547b09)
#50 0x00000000044cdd8e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44cdd8e)
#51 0x000000000463000e clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x463000e)
#52 0x0000000000c587dc cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc587dc)
#53 0x0000000000c5195a ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#54 0x00000000043029a9 void llvm::function_ref<void ()>::callback_fn<clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#55 0x0000000003931654 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3931654)
#56 0x0000000004302fbf clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const (.part.0) Job.cpp:0:0
#57 0x00000000042c8bd5 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42c8bd5)
#58 0x00000000042c964d clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42c964d)
#59 0x00000000042d1275 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42d1275)
#60 0x0000000000c55c93 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc55c93)
#61 0x0000000000b30a24 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xb30a24)
#62 0x000073a683829d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#63 0x000073a683829e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#64 0x0000000000c5140e _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc5140e)

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: LoveSy (yujincheng08)

### Min repro Godbolt Link https://godbolt.org/z/5EWeoefbM

Additional notes

1. cannot reproduce on clang-17.
2. lambda passed to b must have at least one argument, and cannot have auto return type.
3. x's parameter must be a lambda.
4. x's constraints must contain its parameter v.

Stack dump

Stack dump:
Stack dump:
0.	Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/clang++ -gdwarf-4 -g -o /app/output.s -mllvm --x86-asm-syntax=intel -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -std=c++23 -Wno-everything <source>
1.	<eof> parser at end of file
2.	<source>:3:27: instantiating function definition 'b<(lambda at <source>:5:14)>'
3.	<source>:5:14: instantiating function definition 't()::(anonymous class)::operator()<(lambda at <source>:5:14)>'
 #<!-- -->0 0x00000000039ea408 llvm::sys::PrintStackTrace(llvm::raw_ostream&amp;, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x39ea408)
 #<!-- -->1 0x00000000039e80dc llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x39e80dc)
 #<!-- -->2 0x00000000039311b8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #<!-- -->3 0x000073a683842520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #<!-- -->4 0x000073a6838969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #<!-- -->5 0x000073a683842476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #<!-- -->6 0x000073a6838287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #<!-- -->7 0x000073a68382871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
 #<!-- -->8 0x000073a683839e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
 #<!-- -->9 0x00000000068bc93f clang::Sema::tryCaptureVariable(clang::ValueDecl*, clang::SourceLocation, clang::Sema::TryCaptureKind, clang::SourceLocation, bool, clang::QualType&amp;, clang::QualType&amp;, unsigned int const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68bc93f)
#<!-- -->10 0x00000000068ea406 clang::Sema::BuildDeclRefExpr(clang::ValueDecl*, clang::QualType, clang::ExprValueKind, clang::DeclarationNameInfo const&amp;, clang::NestedNameSpecifierLoc, clang::NamedDecl*, clang::SourceLocation, clang::TemplateArgumentListInfo const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68ea406)
#<!-- -->11 0x00000000068eac5f clang::Sema::BuildDeclRefExpr(clang::ValueDecl*, clang::QualType, clang::ExprValueKind, clang::DeclarationNameInfo const&amp;, clang::CXXScopeSpec const*, clang::NamedDecl*, clang::SourceLocation, clang::TemplateArgumentListInfo const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68eac5f)
#<!-- -->12 0x00000000068f1343 clang::Sema::BuildDeclarationNameExpr(clang::CXXScopeSpec const&amp;, clang::DeclarationNameInfo const&amp;, clang::NamedDecl*, clang::NamedDecl*, clang::TemplateArgumentListInfo const*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68f1343)
#<!-- -->13 0x0000000006f52732 clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformDeclRefExpr(clang::DeclRefExpr*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->14 0x0000000006f5293b (anonymous namespace)::TemplateInstantiator::TransformDeclRefExpr(clang::DeclRefExpr*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->15 0x0000000006f2c7e4 clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->16 0x0000000006f58cc6 clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformRequiresExpr(clang::RequiresExpr*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->17 0x0000000006f5abae (anonymous namespace)::TemplateInstantiator::TransformRequiresExpr(clang::RequiresExpr*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->18 0x0000000006f2cbe9 clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->19 0x0000000006f63b94 clang::Sema::SubstConstraintExpr(clang::Expr*, clang::MultiLevelTemplateArgumentList const&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f63b94)
#<!-- -->20 0x00000000065fbd3b calculateConstraintSatisfaction(clang::Sema&amp;, clang::NamedDecl const*, clang::SourceLocation, clang::MultiLevelTemplateArgumentList const&amp;, clang::Expr const*, clang::ConstraintSatisfaction&amp;)::'lambda'(clang::Expr const*)::operator()(clang::Expr const*) const SemaConcept.cpp:0:0
#<!-- -->21 0x00000000065fd66b 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;)::'lambda'(clang::Expr const*)&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;)::'lambda'(clang::Expr const*)&amp;&amp;) SemaConcept.cpp:0:0
#<!-- -->22 0x00000000065fdfe2 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
#<!-- -->23 0x00000000065fe622 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;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x65fe622)
#<!-- -->24 0x00000000065ff6eb clang::Sema::CheckInstantiatedFunctionTemplateConstraints(clang::SourceLocation, clang::FunctionDecl*, llvm::ArrayRef&lt;clang::TemplateArgument&gt;, clang::ConstraintSatisfaction&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x65ff6eb)
#<!-- -->25 0x0000000006eea177 clang::Sema::FinishTemplateArgumentDeduction(clang::FunctionTemplateDecl*, llvm::SmallVectorImpl&lt;clang::DeducedTemplateArgument&gt;&amp;, unsigned int, clang::FunctionDecl*&amp;, clang::sema::TemplateDeductionInfo&amp;, llvm::SmallVectorImpl&lt;clang::Sema::OriginalCallArg&gt; const*, bool, llvm::function_ref&lt;bool ()&gt;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6eea177)
#<!-- -->26 0x0000000006eeb92a void llvm::function_ref&lt;void ()&gt;::callback_fn&lt;clang::Sema::DeduceTemplateArguments(clang::FunctionTemplateDecl*, clang::TemplateArgumentListInfo*, llvm::ArrayRef&lt;clang::Expr*&gt;, clang::FunctionDecl*&amp;, clang::sema::TemplateDeductionInfo&amp;, bool, bool, clang::QualType, clang::Expr::Classification, llvm::function_ref&lt;bool (llvm::ArrayRef&lt;clang::QualType&gt;)&gt;)::'lambda1'()&gt;(long) SemaTemplateDeduction.cpp:0:0
#<!-- -->27 0x000000000649a671 clang::Sema::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref&lt;void ()&gt;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x649a671)
#<!-- -->28 0x0000000006ef6355 clang::Sema::DeduceTemplateArguments(clang::FunctionTemplateDecl*, clang::TemplateArgumentListInfo*, llvm::ArrayRef&lt;clang::Expr*&gt;, clang::FunctionDecl*&amp;, clang::sema::TemplateDeductionInfo&amp;, bool, bool, clang::QualType, clang::Expr::Classification, llvm::function_ref&lt;bool (llvm::ArrayRef&lt;clang::QualType&gt;)&gt;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ef6355)
#<!-- -->29 0x0000000006cccec8 clang::Sema::AddTemplateOverloadCandidate(clang::FunctionTemplateDecl*, clang::DeclAccessPair, clang::TemplateArgumentListInfo*, llvm::ArrayRef&lt;clang::Expr*&gt;, clang::OverloadCandidateSet&amp;, bool, bool, bool, clang::CallExpr::ADLCallKind, clang::OverloadCandidateParamOrder, bool) (.constprop.1) SemaOverload.cpp:0:0
#<!-- -->30 0x0000000006cce6d9 clang::Sema::AddOverloadedCallCandidates(clang::UnresolvedLookupExpr*, llvm::ArrayRef&lt;clang::Expr*&gt;, clang::OverloadCandidateSet&amp;, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cce6d9)
#<!-- -->31 0x0000000006cceaae clang::Sema::buildOverloadedCallSet(clang::Scope*, clang::Expr*, clang::UnresolvedLookupExpr*, llvm::MutableArrayRef&lt;clang::Expr*&gt;, clang::SourceLocation, clang::OverloadCandidateSet*, clang::ActionResult&lt;clang::Expr*, true&gt;*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cceaae)
#<!-- -->32 0x0000000006cd6097 clang::Sema::BuildOverloadedCallExpr(clang::Scope*, clang::Expr*, clang::UnresolvedLookupExpr*, clang::SourceLocation, llvm::MutableArrayRef&lt;clang::Expr*&gt;, clang::SourceLocation, clang::Expr*, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cd6097)
#<!-- -->33 0x0000000006918620 clang::Sema::BuildCallExpr(clang::Scope*, clang::Expr*, clang::SourceLocation, llvm::MutableArrayRef&lt;clang::Expr*&gt;, clang::SourceLocation, clang::Expr*, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6918620)
#<!-- -->34 0x000000000691a68c clang::Sema::ActOnCallExpr(clang::Scope*, clang::Expr*, clang::SourceLocation, llvm::MutableArrayRef&lt;clang::Expr*&gt;, clang::SourceLocation, clang::Expr*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x691a68c)
#<!-- -->35 0x0000000006f389c0 clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformCallExpr(clang::CallExpr*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->36 0x0000000006f2c72d clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->37 0x0000000006f6860f clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformStmt(clang::Stmt*, clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::StmtDiscardKind) SemaTemplateInstantiate.cpp:0:0
#<!-- -->38 0x0000000006f693e4 clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformCompoundStmt(clang::CompoundStmt*, bool) SemaTemplateInstantiate.cpp:0:0
#<!-- -->39 0x0000000006f70a7a clang::Sema::SubstStmt(clang::Stmt*, clang::MultiLevelTemplateArgumentList const&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f70a7a)
#<!-- -->40 0x0000000006fc8408 clang::Sema::InstantiateFunctionDefinition(clang::SourceLocation, clang::FunctionDecl*, bool, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fc8408)
#<!-- -->41 0x0000000006fc661f clang::Sema::PerformPendingInstantiations(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fc661f)
#<!-- -->42 0x0000000006fc7e2c clang::Sema::InstantiateFunctionDefinition(clang::SourceLocation, clang::FunctionDecl*, bool, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fc7e2c)
#<!-- -->43 0x0000000006fc661f clang::Sema::PerformPendingInstantiations(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fc661f)
#<!-- -->44 0x00000000064bd09f clang::Sema::ActOnEndOfTranslationUnitFragment(clang::Sema::TUFragmentKind) (.part.0) Sema.cpp:0:0
#<!-- -->45 0x00000000064bd8ea clang::Sema::ActOnEndOfTranslationUnit() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64bd8ea)
#<!-- -->46 0x0000000006337eca clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&lt;clang::DeclGroupRef&gt;&amp;, clang::Sema::ModuleImportState&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6337eca)
#<!-- -->47 0x000000000632b97a clang::ParseAST(clang::Sema&amp;, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x632b97a)
#<!-- -->48 0x00000000042bf348 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42bf348)
#<!-- -->49 0x0000000004547b09 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4547b09)
#<!-- -->50 0x00000000044cdd8e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44cdd8e)
#<!-- -->51 0x000000000463000e clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x463000e)
#<!-- -->52 0x0000000000c587dc cc1_main(llvm::ArrayRef&lt;char const*&gt;, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc587dc)
#<!-- -->53 0x0000000000c5195a ExecuteCC1Tool(llvm::SmallVectorImpl&lt;char const*&gt;&amp;, llvm::ToolContext const&amp;) driver.cpp:0:0
#<!-- -->54 0x00000000043029a9 void llvm::function_ref&lt;void ()&gt;::callback_fn&lt;clang::driver::CC1Command::Execute(llvm::ArrayRef&lt;std::optional&lt;llvm::StringRef&gt;&gt;, std::__cxx11::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt;&gt;*, bool*) const::'lambda'()&gt;(long) Job.cpp:0:0
#<!-- -->55 0x0000000003931654 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref&lt;void ()&gt;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3931654)
#<!-- -->56 0x0000000004302fbf clang::driver::CC1Command::Execute(llvm::ArrayRef&lt;std::optional&lt;llvm::StringRef&gt;&gt;, std::__cxx11::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt;&gt;*, bool*) const (.part.0) Job.cpp:0:0
#<!-- -->57 0x00000000042c8bd5 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&amp;, clang::driver::Command const*&amp;, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42c8bd5)
#<!-- -->58 0x00000000042c964d clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&amp;, llvm::SmallVectorImpl&lt;std::pair&lt;int, clang::driver::Command const*&gt;&gt;&amp;, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42c964d)
#<!-- -->59 0x00000000042d1275 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&amp;, llvm::SmallVectorImpl&lt;std::pair&lt;int, clang::driver::Command const*&gt;&gt;&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42d1275)
#<!-- -->60 0x0000000000c55c93 clang_main(int, char**, llvm::ToolContext const&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc55c93)
#<!-- -->61 0x0000000000b30a24 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xb30a24)
#<!-- -->62 0x000073a683829d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#<!-- -->63 0x000073a683829e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#<!-- -->64 0x0000000000c5140e _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc5140e)

[cor3ntin]

@erichkeane @mizvekov

[zyn0217]

Tested locally and this can be fixed by #93206

[zyn0217]

So the key to the fix is to skip past RequiresExpr bodies because they don't form function scopes; our algorithm in the traversal assumes that the VarDecl being captured is at least defined in one of the DCs we're visiting.