Regression: Clang crash exit code 139 OR rejects-valid when decltype-ing the result of a templated lambda.

[13steinj]

Found on compiler explorer while playing with https://godbolt.org/z/d8a87z3YK.
When the variable is not templated but the lambda is, instead of segfault, appears to be a rejects-valid (determines that a non-lambda is a lambda): https://godbolt.org/z/fa13nn1qE.
Source of boost-ext/mp for the case of asking "what use would such code have": https://github.com/boost-ext/mp/blob/76149014e223a3371895d9b23d9dbd1a05f6af0b/mp.

---

Compiler explorer currently uses 28cea99 for "trunk". 18.1.0 succeeds.
Minimal reproduction to get a rejects-valid (seemingly confusing the type of the NTTP in the lambda to be the type of the lambda itself)?

template<auto Pred = []<const char c> {
    (void) static_cast<char>(c);
}> // needs to be dependent to take effect
using broken = decltype(Pred.template operator()<'\0'>());

broken<>* boom;

Minimal reproduction to get a crash dump:

template<typename = void>
static constexpr auto innocuous = []<const char m> {
    return m;
};

template<auto Pred = innocuous<>>
using broken = decltype(Pred.template operator()<'\0'>());

broken<>* boom;

It happens doing the static cast as well here; I just felt that simply returning was "more minimal." In both cases, making the NTTP const felt "more minimal" but I don't believe it's necessary, in case clang devs feel the opposite.

Crash Dump

PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.	Program arguments: /opt/compiler-explorer/clang-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 -Wall -Wextra -Werror -pedantic -pedantic-errors <source>
1.	<source>:9:1: at annotation token
2.	<source>:2:35: instantiating function definition '(anonymous class)::operator()<'\x00'>'
 #0 0x00000000036be7c8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x36be7c8)
 #1 0x00000000036bc914 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x36bc914)
 #2 0x0000000003605828 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x00007ee49c642520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x00000000067de6b6 clang::Sema::BuildExpressionFromDeclTemplateArgument(clang::TemplateArgument const&, clang::QualType, clang::SourceLocation) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x67de6b6)
 #5 0x00000000067deb63 clang::Sema::BuildExpressionFromNonTypeTemplateArgument(clang::TemplateArgument const&, clang::SourceLocation) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x67deb63)
 #6 0x000000000698330b (anonymous namespace)::TemplateInstantiator::transformNonTypeTemplateParmRef(clang::Decl*, clang::NonTypeTemplateParmDecl const*, clang::SourceLocation, clang::TemplateArgument, std::optional<unsigned int>) SemaTemplateInstantiate.cpp:0:0
 #7 0x0000000006971ebc clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
 #8 0x00000000069ab55e clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformCXXStaticCastExpr(clang::CXXStaticCastExpr*) SemaTemplateInstantiate.cpp:0:0
 #9 0x000000000697118a clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#10 0x00000000069a17ee clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformCStyleCastExpr(clang::CStyleCastExpr*) SemaTemplateInstantiate.cpp:0:0
#11 0x000000000697114e clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#12 0x00000000069a3cb1 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformStmt(clang::Stmt*, clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::StmtDiscardKind) SemaTemplateInstantiate.cpp:0:0
#13 0x00000000069a4cab clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformCompoundStmt(clang::CompoundStmt*, bool) SemaTemplateInstantiate.cpp:0:0
#14 0x00000000069aad91 clang::Sema::SubstStmt(clang::Stmt*, clang::MultiLevelTemplateArgumentList const&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x69aad91)
#15 0x00000000069f6c96 clang::Sema::InstantiateFunctionDefinition(clang::SourceLocation, clang::FunctionDecl*, bool, bool, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x69f6c96)
#16 0x0000000005f559f1 clang::Sema::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5f559f1)
#17 0x00000000068ec1a1 clang::Sema::DeduceReturnType(clang::FunctionDecl*, clang::SourceLocation, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x68ec1a1)
#18 0x000000000634e667 clang::Sema::DiagnoseUseOfDecl(clang::NamedDecl*, llvm::ArrayRef<clang::SourceLocation>, clang::ObjCInterfaceDecl const*, bool, bool, clang::ObjCInterfaceDecl*, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x634e667)
#19 0x000000000673d0ca clang::Sema::BuildCallToMemberFunction(clang::Scope*, clang::Expr*, clang::SourceLocation, llvm::MutableArrayRef<clang::Expr*>, clang::SourceLocation, clang::Expr*, bool, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x673d0ca)
#20 0x00000000063c2074 clang::Sema::BuildCallExpr(clang::Scope*, clang::Expr*, clang::SourceLocation, llvm::MutableArrayRef<clang::Expr*>, clang::SourceLocation, clang::Expr*, bool, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x63c2074)
#21 0x00000000063c885c clang::Sema::ActOnCallExpr(clang::Scope*, clang::Expr*, clang::SourceLocation, llvm::MutableArrayRef<clang::Expr*>, clang::SourceLocation, clang::Expr*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x63c885c)
#22 0x000000000697ae17 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformCallExpr(clang::CallExpr*) SemaTemplateInstantiate.cpp:0:0
#23 0x000000000697113a clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#24 0x000000000697d05a clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformType(clang::TypeLocBuilder&, clang::TypeLoc) SemaTemplateInstantiate.cpp:0:0
#25 0x0000000006981a3b clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformType(clang::TypeSourceInfo*) SemaTemplateInstantiate.cpp:0:0
#26 0x0000000006983b61 clang::Sema::SubstType(clang::QualType, clang::MultiLevelTemplateArgumentList const&, clang::SourceLocation, clang::DeclarationName) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x6983b61)
#27 0x00000000068147c2 clang::Sema::CheckTemplateIdType(clang::TemplateName, clang::SourceLocation, clang::TemplateArgumentListInfo&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x68147c2)
#28 0x00000000068173d7 clang::Sema::ActOnTemplateIdType(clang::Scope*, clang::CXXScopeSpec&, clang::SourceLocation, clang::OpaquePtr<clang::TemplateName>, clang::IdentifierInfo const*, clang::SourceLocation, clang::SourceLocation, llvm::MutableArrayRef<clang::ParsedTemplateArgument>, clang::SourceLocation, bool, bool, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x68173d7)
#29 0x0000000005f00c67 clang::Parser::AnnotateTemplateIdTokenAsType(clang::CXXScopeSpec&, clang::ImplicitTypenameContext, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5f00c67)
#30 0x0000000005e2a994 clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo const&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::Parser::LateParsedAttrList*, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5e2a994)
#31 0x0000000005deb047 clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5deb047)
#32 0x0000000005debfc3 clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5debfc3)
#33 0x0000000005df4672 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5df4672)
#34 0x0000000005df61e7 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5df61e7)
#35 0x0000000005de51da clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5de51da)
#36 0x0000000003f24df5 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3f24df5)
#37 0x00000000041bc611 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-trunk/bin/clang+++0x41bc611)
#38 0x0000000004141ddb clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x4141ddb)
#39 0x000000000429fd73 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x429fd73)
#40 0x0000000000c5468c cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0xc5468c)
#41 0x0000000000c4e63d ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#42 0x0000000003f6dcd9 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
#43 0x0000000003605c53 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3605c53)
#44 0x0000000003f6def9 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
#45 0x0000000003f34447 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3f34447)
#46 0x0000000003f34dfd clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&, bool) const (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3f34dfd)
#47 0x0000000003f3ec1c clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3f3ec1c)
#48 0x0000000000c51af1 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0xc51af1)
#49 0x0000000000b34794 main (/opt/compiler-explorer/clang-trunk/bin/clang+++0xb34794)
#50 0x00007ee49c629d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#51 0x00007ee49c629e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#52 0x0000000000c4e19e _start (/opt/compiler-explorer/clang-trunk/bin/clang+++0xc4e19e)
clang++: error: clang frontend command failed with exit code 139 (use -v to see invocation)
Compiler returned: 139

In case it gives a hint, -Wunused-variable (no crash, no rejects valid) in this variation in "trunk" but not 18.1.0:

template<auto Pred = []<const char m> {
    return m;
}>
using broken = decltype(Pred.template operator()<'\0'>());

broken<>* boom;

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (13steinj)

Found on compiler explorer while playing with https://godbolt.org/z/d8a87z3YK. When the variable is not templated but the lambda is, instead of segfault, appears to be a rejects-valid (determines that a non-lambda is a lambda): https://godbolt.org/z/fa13nn1qE. Source of boost-ext/mp for the case of asking "what use would such code have": https://github.com/boost-ext/mp/blob/76149014e223a3371895d9b23d9dbd1a05f6af0b/mp.

---

Compiler explorer currently uses 28cea99 for "trunk". 18.1.0 succeeds.
Minimal reproduction to get a rejects-valid (seemingly confusing the type of the NTTP in the lambda to be the type of the lambda itself)?

template<auto Pred = []<const char c> {
    (void) static_cast<char>(c);
}> // needs to be dependent to take effect
using broken = decltype(Pred.template operator()<'\0'>());

broken<>* boom;

Minimal reproduction to get a crash dump:

template<typename = void>
static constexpr auto innocuous = []<const char m> {
    return m;
};

template<auto Pred = innocuous<>>
using broken = decltype(Pred.template operator()<'\0'>());

broken<>* boom;

It happens doing the static cast as well here; I just felt that simply returning was "more minimal." In both cases, making the NTTP const felt "more minimal" but I don't believe it's necessary, in case clang devs feel the opposite.

<details>
<summary>Crash Dump</summary>

PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.	Program arguments: /opt/compiler-explorer/clang-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 -Wall -Wextra -Werror -pedantic -pedantic-errors <source>
1.	<source>:9:1: at annotation token
2.	<source>:2:35: instantiating function definition '(anonymous class)::operator()<'\x00'>'
 #<!-- -->0 0x00000000036be7c8 llvm::sys::PrintStackTrace(llvm::raw_ostream&amp;, int) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x36be7c8)
 #<!-- -->1 0x00000000036bc914 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x36bc914)
 #<!-- -->2 0x0000000003605828 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #<!-- -->3 0x00007ee49c642520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #<!-- -->4 0x00000000067de6b6 clang::Sema::BuildExpressionFromDeclTemplateArgument(clang::TemplateArgument const&amp;, clang::QualType, clang::SourceLocation) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x67de6b6)
 #<!-- -->5 0x00000000067deb63 clang::Sema::BuildExpressionFromNonTypeTemplateArgument(clang::TemplateArgument const&amp;, clang::SourceLocation) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x67deb63)
 #<!-- -->6 0x000000000698330b (anonymous namespace)::TemplateInstantiator::transformNonTypeTemplateParmRef(clang::Decl*, clang::NonTypeTemplateParmDecl const*, clang::SourceLocation, clang::TemplateArgument, std::optional&lt;unsigned int&gt;) SemaTemplateInstantiate.cpp:0:0
 #<!-- -->7 0x0000000006971ebc clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
 #<!-- -->8 0x00000000069ab55e clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformCXXStaticCastExpr(clang::CXXStaticCastExpr*) SemaTemplateInstantiate.cpp:0:0
 #<!-- -->9 0x000000000697118a clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->10 0x00000000069a17ee clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformCStyleCastExpr(clang::CStyleCastExpr*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->11 0x000000000697114e clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->12 0x00000000069a3cb1 clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformStmt(clang::Stmt*, clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::StmtDiscardKind) SemaTemplateInstantiate.cpp:0:0
#<!-- -->13 0x00000000069a4cab clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformCompoundStmt(clang::CompoundStmt*, bool) SemaTemplateInstantiate.cpp:0:0
#<!-- -->14 0x00000000069aad91 clang::Sema::SubstStmt(clang::Stmt*, clang::MultiLevelTemplateArgumentList const&amp;) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x69aad91)
#<!-- -->15 0x00000000069f6c96 clang::Sema::InstantiateFunctionDefinition(clang::SourceLocation, clang::FunctionDecl*, bool, bool, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x69f6c96)
#<!-- -->16 0x0000000005f559f1 clang::Sema::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref&lt;void ()&gt;) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5f559f1)
#<!-- -->17 0x00000000068ec1a1 clang::Sema::DeduceReturnType(clang::FunctionDecl*, clang::SourceLocation, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x68ec1a1)
#<!-- -->18 0x000000000634e667 clang::Sema::DiagnoseUseOfDecl(clang::NamedDecl*, llvm::ArrayRef&lt;clang::SourceLocation&gt;, clang::ObjCInterfaceDecl const*, bool, bool, clang::ObjCInterfaceDecl*, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x634e667)
#<!-- -->19 0x000000000673d0ca clang::Sema::BuildCallToMemberFunction(clang::Scope*, clang::Expr*, clang::SourceLocation, llvm::MutableArrayRef&lt;clang::Expr*&gt;, clang::SourceLocation, clang::Expr*, bool, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x673d0ca)
#<!-- -->20 0x00000000063c2074 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-trunk/bin/clang+++0x63c2074)
#<!-- -->21 0x00000000063c885c clang::Sema::ActOnCallExpr(clang::Scope*, clang::Expr*, clang::SourceLocation, llvm::MutableArrayRef&lt;clang::Expr*&gt;, clang::SourceLocation, clang::Expr*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x63c885c)
#<!-- -->22 0x000000000697ae17 clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformCallExpr(clang::CallExpr*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->23 0x000000000697113a clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->24 0x000000000697d05a clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformType(clang::TypeLocBuilder&amp;, clang::TypeLoc) SemaTemplateInstantiate.cpp:0:0
#<!-- -->25 0x0000000006981a3b clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformType(clang::TypeSourceInfo*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->26 0x0000000006983b61 clang::Sema::SubstType(clang::QualType, clang::MultiLevelTemplateArgumentList const&amp;, clang::SourceLocation, clang::DeclarationName) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x6983b61)
#<!-- -->27 0x00000000068147c2 clang::Sema::CheckTemplateIdType(clang::TemplateName, clang::SourceLocation, clang::TemplateArgumentListInfo&amp;) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x68147c2)
#<!-- -->28 0x00000000068173d7 clang::Sema::ActOnTemplateIdType(clang::Scope*, clang::CXXScopeSpec&amp;, clang::SourceLocation, clang::OpaquePtr&lt;clang::TemplateName&gt;, clang::IdentifierInfo const*, clang::SourceLocation, clang::SourceLocation, llvm::MutableArrayRef&lt;clang::ParsedTemplateArgument&gt;, clang::SourceLocation, bool, bool, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x68173d7)
#<!-- -->29 0x0000000005f00c67 clang::Parser::AnnotateTemplateIdTokenAsType(clang::CXXScopeSpec&amp;, clang::ImplicitTypenameContext, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5f00c67)
#<!-- -->30 0x0000000005e2a994 clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&amp;, clang::Parser::ParsedTemplateInfo const&amp;, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::Parser::LateParsedAttrList*, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5e2a994)
#<!-- -->31 0x0000000005deb047 clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;, clang::ParsingDeclSpec&amp;, clang::AccessSpecifier) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5deb047)
#<!-- -->32 0x0000000005debfc3 clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5debfc3)
#<!-- -->33 0x0000000005df4672 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5df4672)
#<!-- -->34 0x0000000005df61e7 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&lt;clang::DeclGroupRef&gt;&amp;, clang::Sema::ModuleImportState&amp;) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5df61e7)
#<!-- -->35 0x0000000005de51da clang::ParseAST(clang::Sema&amp;, bool, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5de51da)
#<!-- -->36 0x0000000003f24df5 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3f24df5)
#<!-- -->37 0x00000000041bc611 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-trunk/bin/clang+++0x41bc611)
#<!-- -->38 0x0000000004141ddb clang::CompilerInstance::ExecuteAction(clang::FrontendAction&amp;) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x4141ddb)
#<!-- -->39 0x000000000429fd73 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x429fd73)
#<!-- -->40 0x0000000000c5468c cc1_main(llvm::ArrayRef&lt;char const*&gt;, char const*, void*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0xc5468c)
#<!-- -->41 0x0000000000c4e63d ExecuteCC1Tool(llvm::SmallVectorImpl&lt;char const*&gt;&amp;, llvm::ToolContext const&amp;) driver.cpp:0:0
#<!-- -->42 0x0000000003f6dcd9 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
#<!-- -->43 0x0000000003605c53 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref&lt;void ()&gt;) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3605c53)
#<!-- -->44 0x0000000003f6def9 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
#<!-- -->45 0x0000000003f34447 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&amp;, clang::driver::Command const*&amp;, bool) const (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3f34447)
#<!-- -->46 0x0000000003f34dfd 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-trunk/bin/clang+++0x3f34dfd)
#<!-- -->47 0x0000000003f3ec1c 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-trunk/bin/clang+++0x3f3ec1c)
#<!-- -->48 0x0000000000c51af1 clang_main(int, char**, llvm::ToolContext const&amp;) (/opt/compiler-explorer/clang-trunk/bin/clang+++0xc51af1)
#<!-- -->49 0x0000000000b34794 main (/opt/compiler-explorer/clang-trunk/bin/clang+++0xb34794)
#<!-- -->50 0x00007ee49c629d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#<!-- -->51 0x00007ee49c629e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#<!-- -->52 0x0000000000c4e19e _start (/opt/compiler-explorer/clang-trunk/bin/clang+++0xc4e19e)
clang++: error: clang frontend command failed with exit code 139 (use -v to see invocation)
Compiler returned: 139

</details>

In case it gives a hint, -Wunused-variable (no crash, no rejects valid) in this variation in "trunk" but not 18.1.0:

template<auto Pred = []<const char m> {
    return m;
}>
using broken = decltype(Pred.template operator()<'\0'>());

broken<>* boom;

[zyn0217]

Thanks for the report! This is a regression from my recent patch #82310. However, I've come up with a fix and will submit the patch in a moment.

[13steinj]

tyvm for the quick response + triage!

Also, apologies if my crash dump auto-referenced a slew of GH issues before! Something with the way GH parses markdown made it so that without a blank line it wasn't treated as a code-block. I edited to add the line but don't know if references get removed if edited.