CTAD for alias template crashes.

[hokein]

This was spotted in a code review comment : #125478 (comment)

https://godbolt.org/z/b84rzhjjK

template<typename T>
struct A { A(T){} };

template<typename T>
using Proxy = T;

template<typename T>
using C = Proxy< A<T> >;

int main() {
    C test{ 42 };
} 

Assertion:

clang++: /root/llvm-project/clang/lib/Sema/SemaTemplateDeductionGuide.cpp:980:
std::pair<clang::TemplateDecl*, llvm::ArrayRef<clang::TemplateArgument> > {anonymous}::getRHSTemplateDeclAndArgs(clang::Sema&, clang::TypeAliasTemplateDecl*):
Assertion `false && "unhandled RHS type of the alias"' failed.

Backtrace:

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-assertions-trunk/bin/clang++ -gdwarf-4 -g -o /app/output.s -mllvm --x86-asm-syntax=intel -fno-verbose-asm -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -std=c++20 <source>
1.	<source>:13:17: current parser token ';'
2.	<source>:12:1: parsing function body 'main'
3.	<source>:12:1: in compound statement ('{}')
 #0 0x0000000003e4c608 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3e4c608)
 #1 0x0000000003e4a2c4 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3e4a2c4)
 #2 0x0000000003d96798 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x00007ce095842520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x00007ce0958969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #5 0x00007ce095842476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #6 0x00007ce0958287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #7 0x00007ce09582871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
 #8 0x00007ce095839e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
 #9 0x0000000006ef4b3c (anonymous namespace)::getRHSTemplateDeclAndArgs(clang::Sema&, clang::TypeAliasTemplateDecl*) (.isra.0) SemaTemplateDeductionGuide.cpp:0:0
#10 0x0000000006f3c374 clang::Sema::DeclareImplicitDeductionGuides(clang::TemplateDecl*, clang::SourceLocation) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f3c374)
#11 0x0000000006b45237 LookupDirect(clang::Sema&, clang::LookupResult&, clang::DeclContext const*) SemaLookup.cpp:0:0
#12 0x0000000006b30532 clang::Sema::LookupQualifiedName(clang::LookupResult&, clang::DeclContext*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b30532)
#13 0x0000000006f3c472 clang::Sema::DeclareImplicitDeductionGuides(clang::TemplateDecl*, clang::SourceLocation) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f3c472)
#14 0x0000000006b45237 LookupDirect(clang::Sema&, clang::LookupResult&, clang::DeclContext const*) SemaLookup.cpp:0:0
#15 0x0000000006b30532 clang::Sema::LookupQualifiedName(clang::LookupResult&, clang::DeclContext*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b30532)
#16 0x0000000006b0ef8e clang::Sema::DeduceTemplateSpecializationFromInitializer(clang::TypeSourceInfo*, clang::InitializedEntity const&, clang::InitializationKind const&, llvm::MutableArrayRef<clang::Expr*>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b0ef8e)
#17 0x000000000679c65d clang::Sema::deduceVarTypeFromInitializer(clang::VarDecl*, clang::DeclarationName, clang::QualType, clang::TypeSourceInfo*, clang::SourceRange, bool, clang::Expr*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x679c65d)
#18 0x00000000067a1427 clang::Sema::DeduceVariableDeclarationType(clang::VarDecl*, bool, clang::Expr*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x67a1427)
#19 0x00000000067bd7a2 clang::Sema::AddInitializerToDecl(clang::Decl*, clang::Expr*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x67bd7a2)
#20 0x00000000064530c5 clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64530c5)
#21 0x0000000006462989 clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6462989)
#22 0x000000000646b387 clang::Parser::ParseSimpleDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, bool, clang::Parser::ForRangeInit*, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x646b387)
#23 0x000000000646b849 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x646b849)
#24 0x00000000065199c2 clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::ParsedAttributes&, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x65199c2)
#25 0x000000000651a6f0 clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x651a6f0)
#26 0x000000000651b5b5 clang::Parser::ParseCompoundStatementBody(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x651b5b5)
#27 0x000000000651cdba clang::Parser::ParseFunctionStatementBody(clang::Decl*, clang::Parser::ParseScope&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x651cdba)
#28 0x000000000642e623 clang::Parser::ParseFunctionDefinition(clang::ParsingDeclarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::LateParsedAttrList*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x642e623)
#29 0x000000000646336d clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x646336d)
#30 0x000000000642236e clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x642236e)
#31 0x0000000006422b29 clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6422b29)
#32 0x000000000642a2d3 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x642a2d3)
#33 0x000000000642b1cd clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x642b1cd)
#34 0x000000000641d66a clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x641d66a)
#35 0x00000000047dfc18 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x47dfc18)
#36 0x0000000004aa4315 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4aa4315)
#37 0x0000000004a27b0e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4a27b0e)
#38 0x0000000004b9246e clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b9246e)
#39 0x0000000000d3d21f cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd3d21f)
#40 0x0000000000d34cba ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#41 0x00000000048268c9 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
#42 0x0000000003d96c44 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3d96c44)
#43 0x0000000004826ebf 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
#44 0x00000000047e996d clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x47e996d)
#45 0x00000000047ea9ee 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+++0x47ea9ee)
#46 0x00000000047f28d5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x47f28d5)
#47 0x0000000000d3a033 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd3a033)
#48 0x0000000000c02644 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc02644)
#49 0x00007ce095829d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#50 0x00007ce095829e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#51 0x0000000000d34765 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd34765)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Haojian Wu (hokein)

This was spotted in a code review comment : https://github.com//pull/125478#discussion_r1940367085

https://godbolt.org/z/b84rzhjjK

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Haojian Wu (hokein)

This was spotted in a code review comment : https://github.com//pull/125478#discussion_r1940367085

https://godbolt.org/z/b84rzhjjK

[shafik]

Added assertion and backtrace, so it is easier for folks to find this if they have a duplicate.

[hokein]

Per the C++ standard https://eel.is/c++draft/over.match.class.deduct#3, my understanding is that this code is invalid.

• The simple-template-id is Proxy<A<T>>.
• The alias Proxy doesn't have any generated deduction guides, thus no deduction guides for C are generated.
• As a result, template argument deduction fails (which aligns with the behavior of Clang without assert and MSVC, https://godbolt.org/z/j1jzP418e).

Regarding the assertion issue, BuildDeductionGuideForTypeAlias is invoked twice -- once for C and once for Proxy. The issue arises due to the following:

1. When generating deduction guides for the alias template C, Clang looks at its right-hand side (Proxy<A<T>>) and attempts to find deduction guides for Proxy.
2. Since Proxy is also an alias template, Clang recursively tries to generate deduction guides for it.
3. This leads to an assertion failure because using Proxy = T; does not conform to the expected alias template form (using A = B<T>).

The assertion assumes that alias templates always follow the using A = B<T> form, which is not always true. I think the fix is to remove the assertion.