[clang]: clang crashed with Assertion TL.getDecl()->getDepth() <= TemplateDepth && "Nothing should reference a value below the actual template depth, " "depth is likely wrong"` failed in `(anonymous namespace)::ConstraintRefersToContainingTemplateChecker::TransformTemplateTypeParmType` when try to use a generic lambda inside the reqauires clause of a friend function template

[RungeCC]

Reproducer(godbolt:https://godbolt.org/z/19qTP6xzT):

// clang++ -std=c++26 -fsyntax-only -Wall -Werror -Wextra main.cpp
template <class T>
struct H {
    T v;

    template <class U>
    friend auto x(H, U) requires requires {H{[](auto) {  }};}
    {
        return 0;
    }
};

Assertion:

clang++: /root/llvm-project/clang/lib/Sema/SemaTemplate.cpp:1693: clang::QualType {anonymous}::ConstraintRefersToContainingTemplateChecker::TransformTemplateTypeParmType(clang::TypeLocBuilder&, clang::TemplateTypeParmTypeLoc, bool): Assertion `TL.getDecl()->getDepth() <= TemplateDepth && "Nothing should reference a value below the actual template depth, " "depth is likely wrong"' failed.

Stacktrace:

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++26 -Wall -Wextra -Werror -fsyntax-only <source>
1.	<source>:7:5: current parser token '{'
2.	<source>:2:1: parsing struct/union/class body 'H'
 #0 0x0000000003a3d0e8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a3d0e8)
 #1 0x0000000003a3addc llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a3addc)
 #2 0x0000000003985168 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x000073c864842520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x000073c8648969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #5 0x000073c864842476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #6 0x000073c8648287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #7 0x000073c86482871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
 #8 0x000073c864839e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
 #9 0x0000000006e6f103 (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e6f103)
#10 0x0000000006e70a7b (anonymous namespace)::ConstraintRefersToContainingTemplateChecker::TransformTemplateTypeParmType(clang::TypeLocBuilder&, clang::TemplateTypeParmTypeLoc, bool) (.constprop.0) SemaTemplate.cpp:0:0
#11 0x0000000006eed655 clang::TreeTransform<(anonymous namespace)::ConstraintRefersToContainingTemplateChecker>::TransformType(clang::TypeLocBuilder&, clang::TypeLoc) SemaTemplate.cpp:0:0
#12 0x0000000006eee482 clang::TreeTransform<(anonymous namespace)::ConstraintRefersToContainingTemplateChecker>::TransformType(clang::TypeSourceInfo*) SemaTemplate.cpp:0:0
#13 0x0000000006f0dfae clang::TreeTransform<(anonymous namespace)::ConstraintRefersToContainingTemplateChecker>::TransformFunctionTypeParam(clang::ParmVarDecl*, int, std::optional<unsigned int>, bool) (.constprop.0) SemaTemplate.cpp:0:0
#14 0x0000000006f0e7bc clang::TreeTransform<(anonymous namespace)::ConstraintRefersToContainingTemplateChecker>::TransformFunctionTypeParams(clang::SourceLocation, llvm::ArrayRef<clang::ParmVarDecl*>, clang::QualType const*, clang::FunctionType::ExtParameterInfo const*, llvm::SmallVectorImpl<clang::QualType>&, llvm::SmallVectorImpl<clang::ParmVarDecl*>*, clang::Sema::ExtParameterInfoBuilder&, unsigned int*) (.isra.0.constprop.0) SemaTemplate.cpp:0:0
#15 0x0000000006f1011b clang::QualType clang::TreeTransform<(anonymous namespace)::ConstraintRefersToContainingTemplateChecker>::TransformFunctionProtoType<clang::TreeTransform<(anonymous namespace)::ConstraintRefersToContainingTemplateChecker>::TransformLambdaExpr(clang::LambdaExpr*)::'lambda0'(clang::TypeLocBuilder&, clang::FunctionProtoTypeLoc)::operator()(clang::TypeLocBuilder&, clang::FunctionProtoTypeLoc) const::'lambda'(clang::FunctionProtoType::ExceptionSpecInfo&, bool&)>(clang::TypeLocBuilder&, clang::FunctionProtoTypeLoc, clang::CXXRecordDecl*, clang::Qualifiers, clang::TreeTransform<(anonymous namespace)::ConstraintRefersToContainingTemplateChecker>::TransformLambdaExpr(clang::LambdaExpr*)::'lambda0'(clang::TypeLocBuilder&, clang::FunctionProtoTypeLoc)::operator()(clang::TypeLocBuilder&, clang::FunctionProtoTypeLoc) const::'lambda'(clang::FunctionProtoType::ExceptionSpecInfo&, bool&)) (.constprop.0) SemaTemplate.cpp:0:0
#16 0x0000000006edf496 clang::TreeTransform<(anonymous namespace)::ConstraintRefersToContainingTemplateChecker>::TransformLambdaExpr(clang::LambdaExpr*) SemaTemplate.cpp:0:0
#17 0x0000000006ee1f0b clang::TreeTransform<(anonymous namespace)::ConstraintRefersToContainingTemplateChecker>::TransformExprs(clang::Expr* const*, unsigned int, bool, llvm::SmallVectorImpl<clang::Expr*>&, bool*) SemaTemplate.cpp:0:0
#18 0x0000000006ee2f3c clang::TreeTransform<(anonymous namespace)::ConstraintRefersToContainingTemplateChecker>::TransformInitListExpr(clang::InitListExpr*) SemaTemplate.cpp:0:0
#19 0x0000000006ee2188 clang::TreeTransform<(anonymous namespace)::ConstraintRefersToContainingTemplateChecker>::TransformInitializer(clang::Expr*, bool) (.part.0) SemaTemplate.cpp:0:0
#20 0x0000000006ee1e76 clang::TreeTransform<(anonymous namespace)::ConstraintRefersToContainingTemplateChecker>::TransformExprs(clang::Expr* const*, unsigned int, bool, llvm::SmallVectorImpl<clang::Expr*>&, bool*) SemaTemplate.cpp:0:0
#21 0x0000000006efebf2 clang::TreeTransform<(anonymous namespace)::ConstraintRefersToContainingTemplateChecker>::TransformCXXUnresolvedConstructExpr(clang::CXXUnresolvedConstructExpr*) SemaTemplate.cpp:0:0
#22 0x0000000006f0f7d5 clang::TreeTransform<(anonymous namespace)::ConstraintRefersToContainingTemplateChecker>::TransformRequiresExpr(clang::RequiresExpr*) SemaTemplate.cpp:0:0
#23 0x0000000006ee182e clang::Sema::ConstraintExpressionDependsOnEnclosingTemplate(clang::FunctionDecl const*, unsigned int, clang::Expr const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ee182e)
#24 0x00000000066acd8a clang::Sema::FriendConstraintsDependOnEnclosingTemplate(clang::FunctionDecl const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66acd8a)
#25 0x000000000678b714 clang::Sema::ActOnFunctionDeclarator(clang::Scope*, clang::Declarator&, clang::DeclContext*, clang::TypeSourceInfo*, clang::LookupResult&, llvm::MutableArrayRef<clang::TemplateParameterList*>, bool&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x678b714)
#26 0x0000000006824c8f clang::Sema::ActOnFriendFunctionDecl(clang::Scope*, clang::Declarator&, llvm::MutableArrayRef<clang::TemplateParameterList*>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6824c8f)
#27 0x00000000063f7534 clang::Parser::ParseCXXInlineMethodDef(clang::AccessSpecifier, clang::ParsedAttributesView const&, clang::ParsingDeclarator&, clang::Parser::ParsedTemplateInfo const&, clang::VirtSpecifiers const&, clang::SourceLocation) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63f7534)
#28 0x000000000643a48a clang::Parser::ParseCXXClassMemberDeclaration(clang::AccessSpecifier, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x643a48a)
#29 0x00000000064db8d5 clang::Parser::ParseDeclarationAfterTemplate(clang::DeclaratorContext, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject&, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64db8d5)
#30 0x00000000064e8d44 clang::Parser::ParseTemplateDeclarationOrSpecialization(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (.part.0) ParseTemplate.cpp:0:0
#31 0x000000000643ad4d clang::Parser::ParseCXXClassMemberDeclaration(clang::AccessSpecifier, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x643ad4d)
#32 0x000000000643caa5 clang::Parser::ParseCXXClassMemberDeclarationWithPragmas(clang::AccessSpecifier&, clang::ParsedAttributes&, clang::TypeSpecifierType, clang::Decl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x643caa5)
#33 0x000000000643e5e7 clang::Parser::ParseCXXMemberSpecification(clang::SourceLocation, clang::SourceLocation, clang::ParsedAttributes&, unsigned int, clang::Decl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x643e5e7)
#34 0x0000000006440e80 clang::Parser::ParseClassSpecifier(clang::tok::TokenKind, clang::SourceLocation, clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, bool, clang::Parser::DeclSpecContext, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6440e80)
#35 0x00000000064167b5 clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::Parser::LateParsedAttrList*, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64167b5)
#36 0x00000000064db54a clang::Parser::ParseDeclarationAfterTemplate(clang::DeclaratorContext, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject&, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64db54a)
#37 0x00000000064e8d44 clang::Parser::ParseTemplateDeclarationOrSpecialization(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (.part.0) ParseTemplate.cpp:0:0
#38 0x00000000064e905a clang::Parser::ParseDeclarationStartingWithTemplate(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64e905a)
#39 0x000000000641d473 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x641d473)
#40 0x00000000063d9441 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63d9441)
#41 0x00000000063da2ef clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63da2ef)
#42 0x00000000063da7b0 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63da7b0)
#43 0x00000000063cdabc clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63cdabc)
#44 0x00000000045b4099 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x45b4099)
#45 0x0000000004539cae clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4539cae)
#46 0x000000000469c48e clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x469c48e)
#47 0x0000000000c72945 cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc72945)
#48 0x0000000000c6bb6a ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#49 0x0000000004370819 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
#50 0x0000000003985614 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3985614)
#51 0x0000000004370e0f 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
#52 0x00000000043368a5 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43368a5)
#53 0x000000000433730d 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+++0x433730d)
#54 0x000000000433ed15 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x433ed15)
#55 0x0000000000c6fd15 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc6fd15)
#56 0x0000000000b47444 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xb47444)
#57 0x000073c864829d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#58 0x000073c864829e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#59 0x0000000000c6b61e _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc6b61e)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134

• clang on trunk crashed (with assertions), gcc and msvc accept it (it seems to be valid).
• clang does not crash without assertion. However, there's still a seemingly related weird issue (https://godbolt.org/z/6T8efjsnW):

// clang++ -std=c++26 -fsyntax-only -Wall -Werror -Wextra main.cpp
template <class T>
struct H {

    template <class U>
    friend auto x(U) requires requires {
        []([[maybe_unused]]auto x) {  }; }
    {
        return 0;
    }
};

clang complains:

<source>:6:33: error: unused parameter 'x' [-Werror,-Wunused-parameter]
    6 |         []([[maybe_unused]]auto x) {  }; }
      |                                 ^
1 error generated.
Compiler returned: 1

which is confusing since I've already mark x as [[maybe_unused]]. Not sure is this issue actually related.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Runge (RungeCC)

Reproducer(godbolt:https://godbolt.org/z/19qTP6xzT):

// clang++ -std=c++26 -fsyntax-only -Wall -Werror -Wextra main.cpp
template <class T>
struct H {
    T v;

    template <class U>
    friend auto x(H, U) requires requires {H{[](auto) {  }};}
    {
        return 0;
    }
};

Assertion:

clang++: /root/llvm-project/clang/lib/Sema/SemaTemplate.cpp:1693: clang::QualType {anonymous}::ConstraintRefersToContainingTemplateChecker::TransformTemplateTypeParmType(clang::TypeLocBuilder&, clang::TemplateTypeParmTypeLoc, bool): Assertion `TL.getDecl()->getDepth() <= TemplateDepth && "Nothing should reference a value below the actual template depth, " "depth is likely wrong"' failed.

Stacktrace:

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++26 -Wall -Wextra -Werror -fsyntax-only <source>
1.	<source>:7:5: current parser token '{'
2.	<source>:2:1: parsing struct/union/class body 'H'
 #<!-- -->0 0x0000000003a3d0e8 llvm::sys::PrintStackTrace(llvm::raw_ostream&amp;, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a3d0e8)
 #<!-- -->1 0x0000000003a3addc llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a3addc)
 #<!-- -->2 0x0000000003985168 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #<!-- -->3 0x000073c864842520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #<!-- -->4 0x000073c8648969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #<!-- -->5 0x000073c864842476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #<!-- -->6 0x000073c8648287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #<!-- -->7 0x000073c86482871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
 #<!-- -->8 0x000073c864839e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
 #<!-- -->9 0x0000000006e6f103 (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e6f103)
#<!-- -->10 0x0000000006e70a7b (anonymous namespace)::ConstraintRefersToContainingTemplateChecker::TransformTemplateTypeParmType(clang::TypeLocBuilder&amp;, clang::TemplateTypeParmTypeLoc, bool) (.constprop.0) SemaTemplate.cpp:0:0
#<!-- -->11 0x0000000006eed655 clang::TreeTransform&lt;(anonymous namespace)::ConstraintRefersToContainingTemplateChecker&gt;::TransformType(clang::TypeLocBuilder&amp;, clang::TypeLoc) SemaTemplate.cpp:0:0
#<!-- -->12 0x0000000006eee482 clang::TreeTransform&lt;(anonymous namespace)::ConstraintRefersToContainingTemplateChecker&gt;::TransformType(clang::TypeSourceInfo*) SemaTemplate.cpp:0:0
#<!-- -->13 0x0000000006f0dfae clang::TreeTransform&lt;(anonymous namespace)::ConstraintRefersToContainingTemplateChecker&gt;::TransformFunctionTypeParam(clang::ParmVarDecl*, int, std::optional&lt;unsigned int&gt;, bool) (.constprop.0) SemaTemplate.cpp:0:0
#<!-- -->14 0x0000000006f0e7bc clang::TreeTransform&lt;(anonymous namespace)::ConstraintRefersToContainingTemplateChecker&gt;::TransformFunctionTypeParams(clang::SourceLocation, llvm::ArrayRef&lt;clang::ParmVarDecl*&gt;, clang::QualType const*, clang::FunctionType::ExtParameterInfo const*, llvm::SmallVectorImpl&lt;clang::QualType&gt;&amp;, llvm::SmallVectorImpl&lt;clang::ParmVarDecl*&gt;*, clang::Sema::ExtParameterInfoBuilder&amp;, unsigned int*) (.isra.0.constprop.0) SemaTemplate.cpp:0:0
#<!-- -->15 0x0000000006f1011b clang::QualType clang::TreeTransform&lt;(anonymous namespace)::ConstraintRefersToContainingTemplateChecker&gt;::TransformFunctionProtoType&lt;clang::TreeTransform&lt;(anonymous namespace)::ConstraintRefersToContainingTemplateChecker&gt;::TransformLambdaExpr(clang::LambdaExpr*)::'lambda0'(clang::TypeLocBuilder&amp;, clang::FunctionProtoTypeLoc)::operator()(clang::TypeLocBuilder&amp;, clang::FunctionProtoTypeLoc) const::'lambda'(clang::FunctionProtoType::ExceptionSpecInfo&amp;, bool&amp;)&gt;(clang::TypeLocBuilder&amp;, clang::FunctionProtoTypeLoc, clang::CXXRecordDecl*, clang::Qualifiers, clang::TreeTransform&lt;(anonymous namespace)::ConstraintRefersToContainingTemplateChecker&gt;::TransformLambdaExpr(clang::LambdaExpr*)::'lambda0'(clang::TypeLocBuilder&amp;, clang::FunctionProtoTypeLoc)::operator()(clang::TypeLocBuilder&amp;, clang::FunctionProtoTypeLoc) const::'lambda'(clang::FunctionProtoType::ExceptionSpecInfo&amp;, bool&amp;)) (.constprop.0) SemaTemplate.cpp:0:0
#<!-- -->16 0x0000000006edf496 clang::TreeTransform&lt;(anonymous namespace)::ConstraintRefersToContainingTemplateChecker&gt;::TransformLambdaExpr(clang::LambdaExpr*) SemaTemplate.cpp:0:0
#<!-- -->17 0x0000000006ee1f0b clang::TreeTransform&lt;(anonymous namespace)::ConstraintRefersToContainingTemplateChecker&gt;::TransformExprs(clang::Expr* const*, unsigned int, bool, llvm::SmallVectorImpl&lt;clang::Expr*&gt;&amp;, bool*) SemaTemplate.cpp:0:0
#<!-- -->18 0x0000000006ee2f3c clang::TreeTransform&lt;(anonymous namespace)::ConstraintRefersToContainingTemplateChecker&gt;::TransformInitListExpr(clang::InitListExpr*) SemaTemplate.cpp:0:0
#<!-- -->19 0x0000000006ee2188 clang::TreeTransform&lt;(anonymous namespace)::ConstraintRefersToContainingTemplateChecker&gt;::TransformInitializer(clang::Expr*, bool) (.part.0) SemaTemplate.cpp:0:0
#<!-- -->20 0x0000000006ee1e76 clang::TreeTransform&lt;(anonymous namespace)::ConstraintRefersToContainingTemplateChecker&gt;::TransformExprs(clang::Expr* const*, unsigned int, bool, llvm::SmallVectorImpl&lt;clang::Expr*&gt;&amp;, bool*) SemaTemplate.cpp:0:0
#<!-- -->21 0x0000000006efebf2 clang::TreeTransform&lt;(anonymous namespace)::ConstraintRefersToContainingTemplateChecker&gt;::TransformCXXUnresolvedConstructExpr(clang::CXXUnresolvedConstructExpr*) SemaTemplate.cpp:0:0
#<!-- -->22 0x0000000006f0f7d5 clang::TreeTransform&lt;(anonymous namespace)::ConstraintRefersToContainingTemplateChecker&gt;::TransformRequiresExpr(clang::RequiresExpr*) SemaTemplate.cpp:0:0
#<!-- -->23 0x0000000006ee182e clang::Sema::ConstraintExpressionDependsOnEnclosingTemplate(clang::FunctionDecl const*, unsigned int, clang::Expr const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ee182e)
#<!-- -->24 0x00000000066acd8a clang::Sema::FriendConstraintsDependOnEnclosingTemplate(clang::FunctionDecl const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66acd8a)
#<!-- -->25 0x000000000678b714 clang::Sema::ActOnFunctionDeclarator(clang::Scope*, clang::Declarator&amp;, clang::DeclContext*, clang::TypeSourceInfo*, clang::LookupResult&amp;, llvm::MutableArrayRef&lt;clang::TemplateParameterList*&gt;, bool&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x678b714)
#<!-- -->26 0x0000000006824c8f clang::Sema::ActOnFriendFunctionDecl(clang::Scope*, clang::Declarator&amp;, llvm::MutableArrayRef&lt;clang::TemplateParameterList*&gt;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6824c8f)
#<!-- -->27 0x00000000063f7534 clang::Parser::ParseCXXInlineMethodDef(clang::AccessSpecifier, clang::ParsedAttributesView const&amp;, clang::ParsingDeclarator&amp;, clang::Parser::ParsedTemplateInfo const&amp;, clang::VirtSpecifiers const&amp;, clang::SourceLocation) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63f7534)
#<!-- -->28 0x000000000643a48a clang::Parser::ParseCXXClassMemberDeclaration(clang::AccessSpecifier, clang::ParsedAttributes&amp;, clang::Parser::ParsedTemplateInfo&amp;, clang::ParsingDeclRAIIObject*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x643a48a)
#<!-- -->29 0x00000000064db8d5 clang::Parser::ParseDeclarationAfterTemplate(clang::DeclaratorContext, clang::Parser::ParsedTemplateInfo&amp;, clang::ParsingDeclRAIIObject&amp;, clang::SourceLocation&amp;, clang::ParsedAttributes&amp;, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64db8d5)
#<!-- -->30 0x00000000064e8d44 clang::Parser::ParseTemplateDeclarationOrSpecialization(clang::DeclaratorContext, clang::SourceLocation&amp;, clang::ParsedAttributes&amp;, clang::AccessSpecifier) (.part.0) ParseTemplate.cpp:0:0
#<!-- -->31 0x000000000643ad4d clang::Parser::ParseCXXClassMemberDeclaration(clang::AccessSpecifier, clang::ParsedAttributes&amp;, clang::Parser::ParsedTemplateInfo&amp;, clang::ParsingDeclRAIIObject*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x643ad4d)
#<!-- -->32 0x000000000643caa5 clang::Parser::ParseCXXClassMemberDeclarationWithPragmas(clang::AccessSpecifier&amp;, clang::ParsedAttributes&amp;, clang::TypeSpecifierType, clang::Decl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x643caa5)
#<!-- -->33 0x000000000643e5e7 clang::Parser::ParseCXXMemberSpecification(clang::SourceLocation, clang::SourceLocation, clang::ParsedAttributes&amp;, unsigned int, clang::Decl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x643e5e7)
#<!-- -->34 0x0000000006440e80 clang::Parser::ParseClassSpecifier(clang::tok::TokenKind, clang::SourceLocation, clang::DeclSpec&amp;, clang::Parser::ParsedTemplateInfo&amp;, clang::AccessSpecifier, bool, clang::Parser::DeclSpecContext, clang::ParsedAttributes&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6440e80)
#<!-- -->35 0x00000000064167b5 clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&amp;, clang::Parser::ParsedTemplateInfo&amp;, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::Parser::LateParsedAttrList*, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64167b5)
#<!-- -->36 0x00000000064db54a clang::Parser::ParseDeclarationAfterTemplate(clang::DeclaratorContext, clang::Parser::ParsedTemplateInfo&amp;, clang::ParsingDeclRAIIObject&amp;, clang::SourceLocation&amp;, clang::ParsedAttributes&amp;, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64db54a)
#<!-- -->37 0x00000000064e8d44 clang::Parser::ParseTemplateDeclarationOrSpecialization(clang::DeclaratorContext, clang::SourceLocation&amp;, clang::ParsedAttributes&amp;, clang::AccessSpecifier) (.part.0) ParseTemplate.cpp:0:0
#<!-- -->38 0x00000000064e905a clang::Parser::ParseDeclarationStartingWithTemplate(clang::DeclaratorContext, clang::SourceLocation&amp;, clang::ParsedAttributes&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64e905a)
#<!-- -->39 0x000000000641d473 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&amp;, clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x641d473)
#<!-- -->40 0x00000000063d9441 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63d9441)
#<!-- -->41 0x00000000063da2ef clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&lt;clang::DeclGroupRef&gt;&amp;, clang::Sema::ModuleImportState&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63da2ef)
#<!-- -->42 0x00000000063da7b0 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr&lt;clang::DeclGroupRef&gt;&amp;, clang::Sema::ModuleImportState&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63da7b0)
#<!-- -->43 0x00000000063cdabc clang::ParseAST(clang::Sema&amp;, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63cdabc)
#<!-- -->44 0x00000000045b4099 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x45b4099)
#<!-- -->45 0x0000000004539cae clang::CompilerInstance::ExecuteAction(clang::FrontendAction&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4539cae)
#<!-- -->46 0x000000000469c48e clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x469c48e)
#<!-- -->47 0x0000000000c72945 cc1_main(llvm::ArrayRef&lt;char const*&gt;, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc72945)
#<!-- -->48 0x0000000000c6bb6a ExecuteCC1Tool(llvm::SmallVectorImpl&lt;char const*&gt;&amp;, llvm::ToolContext const&amp;) driver.cpp:0:0
#<!-- -->49 0x0000000004370819 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
#<!-- -->50 0x0000000003985614 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref&lt;void ()&gt;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3985614)
#<!-- -->51 0x0000000004370e0f 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
#<!-- -->52 0x00000000043368a5 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&amp;, clang::driver::Command const*&amp;, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43368a5)
#<!-- -->53 0x000000000433730d 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+++0x433730d)
#<!-- -->54 0x000000000433ed15 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+++0x433ed15)
#<!-- -->55 0x0000000000c6fd15 clang_main(int, char**, llvm::ToolContext const&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc6fd15)
#<!-- -->56 0x0000000000b47444 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xb47444)
#<!-- -->57 0x000073c864829d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#<!-- -->58 0x000073c864829e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#<!-- -->59 0x0000000000c6b61e _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc6b61e)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134

• clang on trunk crashed, gcc and msvc accept it (it seems to be valid).

[RungeCC]

Inject class name seems to be unrelated(https://godbolt.org/z/3KbaPM7c6):

template <class T>
struct H {
    T v;

    template <class U>
    friend auto x(U) requires requires {
        [](auto) {  }; }
    {
        return 0;
    }
};

I will change the title of this issue.

[Backl1ght]

template <class T>
struct H {
    T v;

    template <class U>
    friend auto x(U) requires requires {
        [](auto) {  }; }
    {
        return 0;
    }
};

The direct cause of the assertion failure is that template depth of auto inside the lambda parameter list is evaluated to be 2 while template depth of the friend function x is evaluated to be 1.

I think template depth of the friend function x should be 2, and friend function declarations might be handled wrongly in TemplateInstArgsHelpers::HandleFunction, but I haven't figure out an elegant way to fix it.

[Backl1ght]

After some more debugging, I find out that the assumption "Nothing should reference a value below the actual template depth" is incorrect since we can have a generic lambda inside require clause of friend function template. For example,

struct S {
  template <typename U>
  friend void f() requires requires { []<typename V>(V){}; } {
    return;
  }
};