Assertion in clang/lib/Sema/SemaTemplateVariadic.cpp failed: `!Unexpanded.empty() && "Unable to find unexpanded parameter packs"'

[llvmbot]

Bugzilla Link	21826
Version	trunk
OS	All
Blocks	#23431
Attachments	test case
Reporter	LLVM Bugzilla Contributor
CC	@Quuxplusone,@davidstone,@DougGregor,@zygoloid,@rnk

Extended Description

Fuzzing discovered that the attached test case, when given as input to

clang -fno-crash-diagnostics -std=c++11 -xc++ -c -emit-llvm

causes this assertion failure:

unexpanded-parm-pack.cpp:1:24: error: expected ']'
template <int... N> a[N
^
unexpanded-parm-pack.cpp:1:22: note: to match this '['
template <int... N> a[N
^
unexpanded-parm-pack.cpp:1:21: error: C++ requires a type specifier for all declarations
template <int... N> a[N
^
clang-3.6: tools/clang/lib/Sema/SemaTemplateVariadic.cpp:281: bool clang::Sema::DiagnoseUnexpandedParameterPack(clang::SourceLocation, clang::TypeSourceInfo *, clang::Sema::UnexpandedParameterPackContext): Assertion `!Unexpanded.empty() && "Unable to find unexpanded parameter packs"' failed.

[rnk]

still repros

[Quuxplusone]

Still repros. Here's another test case that fails with the same assertion:

template <class... Ts>
struct S {
typedef Ts A;
A a;
};

[davidstone]

Here is a valid code reproduction

template <class... Ts>
void f(Ts...) {
	(..., [] {
		using Us = Ts;
		Us{};
	});
}

See it live: https://godbolt.org/z/b6YWGnWz7

[davidstone]

Full backtrace

clang++: /root/llvm-project/clang/lib/Sema/SemaTemplateVariadic.cpp:407: bool clang::Sema::DiagnoseUnexpandedParameterPack(clang::Expr*, clang::Sema::UnexpandedParameterPackContext): Assertion `!Unexpanded.empty() && "Unable to find unexpanded parameter packs"' failed.
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.	Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/clang++ -g -o /app/output.s -mllvm --x86-asm-syntax=intel -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -isystem/opt/compiler-explorer/libs/boost_1_71_0 -std=c++20 <source>
1.	<source>:6:2: current parser token '}'
2.	<source>:2:15: parsing function body 'f'
3.	<source>:2:15: in compound statement ('{}')
4.	<source>:3:8: lambda expression parsing
5.	<source>:3:11: in compound statement ('{}')
 #&#8203;0 0x00005604ab383f6c llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3744f6c)
 #&#8203;1 0x00005604ab381e24 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3742e24)
 #&#8203;2 0x00005604ab3820a5 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x37430a5)
 #&#8203;3 0x00005604ab2dc868 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #&#8203;4 0x00007fe428e373c0 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x153c0)
 #&#8203;5 0x00007fe42890718b raise (/lib/x86_64-linux-gnu/libc.so.6+0x4618b)
 #&#8203;6 0x00007fe4288e6859 abort (/lib/x86_64-linux-gnu/libc.so.6+0x25859)
 #&#8203;7 0x00007fe4288e6729 (/lib/x86_64-linux-gnu/libc.so.6+0x25729)
 #&#8203;8 0x00007fe4288f7f36 (/lib/x86_64-linux-gnu/libc.so.6+0x36f36)
 #&#8203;9 0x00005604ad982ed2 (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5d43ed2)
#&#8203;10 0x00005604ad5d2b86 clang::Sema::ActOnFinishFullExpr(clang::Expr*, clang::SourceLocation, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5993b86)
#&#8203;11 0x00005604ad7cc8f4 clang::Sema::ActOnExprStmt(clang::ActionResult<clang::Expr*, true>, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5b8d8f4)
#&#8203;12 0x00005604ad1f86c3 clang::Parser::ParseExprStatement(clang::Parser::ParsedStmtContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55b96c3)
#&#8203;13 0x00005604ad1f53e4 clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::ParsedAttributesWithRange&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55b63e4)
#&#8203;14 0x00005604ad1f569c clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55b669c)
#&#8203;15 0x00005604ad1f2ad1 clang::Parser::ParseCompoundStatementBody(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55b3ad1)
#&#8203;16 0x00005604ad1b42fa clang::Parser::ParseLambdaExpressionAfterIntroducer(clang::LambdaIntroducer&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55752fa)
#&#8203;17 0x00005604ad1b4873 clang::Parser::ParseLambdaExpression() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5575873)
#&#8203;18 0x00005604ad19f438 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&, clang::Parser::TypeCastState, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5560438)
#&#8203;19 0x00005604ad1a08a6 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, clang::Parser::TypeCastState, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55618a6)
#&#8203;20 0x00005604ad1a18b8 clang::Parser::ParseAssignmentExpression(clang::Parser::TypeCastState) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55628b8)
#&#8203;21 0x00005604ad1a25e9 clang::Parser::ParseExpression(clang::Parser::TypeCastState) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55635e9)
#&#8203;22 0x00005604ad1a26e8 clang::Parser::ParseFoldExpression(clang::ActionResult<clang::Expr*, true>, clang::BalancedDelimiterTracker&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55636e8)
#&#8203;23 0x00005604ad1a6692 clang::Parser::ParseParenExpression(clang::Parser::ParenParseOption&, bool, bool, clang::OpaquePtr<clang::QualType>&, clang::SourceLocation&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5567692)
#&#8203;24 0x00005604ad19e9a6 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&, clang::Parser::TypeCastState, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x555f9a6)
#&#8203;25 0x00005604ad1a08a6 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, clang::Parser::TypeCastState, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55618a6)
#&#8203;26 0x00005604ad1a18b8 clang::Parser::ParseAssignmentExpression(clang::Parser::TypeCastState) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55628b8)
#&#8203;27 0x00005604ad1a25e9 clang::Parser::ParseExpression(clang::Parser::TypeCastState) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55635e9)
#&#8203;28 0x00005604ad1f864d clang::Parser::ParseExprStatement(clang::Parser::ParsedStmtContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55b964d)
#&#8203;29 0x00005604ad1f53e4 clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::ParsedAttributesWithRange&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55b63e4)
#&#8203;30 0x00005604ad1f569c clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55b669c)
#&#8203;31 0x00005604ad1f2ad1 clang::Parser::ParseCompoundStatementBody(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55b3ad1)
#&#8203;32 0x00005604ad1faf6b clang::Parser::ParseFunctionStatementBody(clang::Decl*, clang::Parser::ParseScope&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55bbf6b)
#&#8203;33 0x00005604ad15f644 clang::Parser::ParseFunctionDefinition(clang::ParsingDeclarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::LateParsedAttrList*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5520644)
#&#8203;34 0x00005604ad209509 clang::Parser::ParseSingleDeclarationAfterTemplate(clang::DeclaratorContext, clang::Parser::ParsedTemplateInfo const&, clang::ParsingDeclRAIIObject&, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55ca509)
#&#8203;35 0x00005604ad2131b3 clang::Parser::ParseTemplateDeclarationOrSpecialization(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55d41b3)
#&#8203;36 0x00005604ad213589 clang::Parser::ParseDeclarationStartingWithTemplate(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x55d4589)
#&#8203;37 0x00005604ad181e46 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributesWithRange&, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5542e46)
#&#8203;38 0x00005604ad15c12d clang::Parser::ParseExternalDeclaration(clang::ParsedAttributesWithRange&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x551d12d)
#&#8203;39 0x00005604ad15cd49 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x551dd49)
#&#8203;40 0x00005604ad15d0b5 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x551e0b5)
#&#8203;41 0x00005604ad153243 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5514243)
#&#8203;42 0x00005604ac2f0518 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x46b1518)
#&#8203;43 0x00005604abc70971 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4031971)
#&#8203;44 0x00005604abc0c6f2 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3fcd6f2)
#&#8203;45 0x00005604abd3bdaa clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x40fcdaa)
#&#8203;46 0x00005604a8fe3fdc cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x13a4fdc)
#&#8203;47 0x00005604a8fdf319 ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&) driver.cpp:0:0
#&#8203;48 0x00005604abac4255 void llvm::function_ref<void ()>::callback_fn<clang::driver::CC1Command::Execute(llvm::ArrayRef<llvm::Optional<llvm::StringRef> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#&#8203;49 0x00005604ab2dc943 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x369d943)
#&#8203;50 0x00005604abac4bf4 clang::driver::CC1Command::Execute(llvm::ArrayRef<llvm::Optional<llvm::StringRef> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*, bool*) const (.part.161) Job.cpp:0:0
#&#8203;51 0x00005604aba9ceda clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3e5deda)
#&#8203;52 0x00005604aba9db7f clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*> >&) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3e5eb7f)
#&#8203;53 0x00005604abaa651a clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*> >&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3e6751a)
#&#8203;54 0x00005604a8f0d235 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x12ce235)
#&#8203;55 0x00007fe4288e80b3 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b3)
#&#8203;56 0x00005604a8fdee8a _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x139fe8a)
clang-13: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134

[kcc]

mentioned in issue #23431

[Quuxplusone]

mentioned in issue llvm/llvm-bugzilla-archive#28471

[mordante]

I just ran into the same issue. The godbolt link provided still reproduces the issue.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[oToToT]

Closing as this seems to have already been fixed. (I checked the godbolt link above)