[clang] Assertion `(Qualifier || TPLs.empty()) && "template parameter lists without qualifier"' failed.

Although it does not reproduce on Godbolt, the following reproducer triggers an assertion failure with the clang I built today.

Reproducer:

template <typename> struct S {
  template <typename typename> inline auto foo = 42;
};

int bar = S<int>::foo<int, int>;

Backtrace:

 #0 0x0000756ef89d3e0a llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) 
 #1 0x0000756ef89d0c6e llvm::sys::RunSignalHandlers() 
 #2 0x0000756ef89d298e llvm::sys::CleanupOnSignal(unsigned long) 
 #3 0x0000756ef88e6878 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #4 0x0000756ef8042520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #5 0x0000756ef80969fc __pthread_kill_implementation ./nptl/pthread_kill.c:44:76
 #6 0x0000756ef80969fc __pthread_kill_internal ./nptl/pthread_kill.c:78:10
 #7 0x0000756ef80969fc pthread_kill ./nptl/pthread_kill.c:89:10
 #8 0x0000756ef8042476 gsignal ./signal/../sysdeps/posix/raise.c:27:6
 #9 0x0000756ef80287f3 abort ./stdlib/abort.c:81:7
#10 0x0000756ef802871b _nl_load_domain ./intl/loadmsgcat.c:1177:9
#11 0x0000756ef8039e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#12 0x0000756ef5597ab2 clang::Sema::getTemplateInstantiationArgs(clang::Decl const*, std::optional<llvm::ArrayRef<clang::TemplateArgument>>, clang::OptionalUnsigned<unsigned int>, bool) 
#13 0x0000756ef566923a clang::Sema::InstantiateVariableDefinition(clang::SourceLocation, clang::VarDecl*, bool, bool, bool) 
#14 0x0000756ef566a792 clang::Sema::PerformPendingInstantiations(bool, bool) 
#15 0x0000756ef4d12968 clang::Sema::ActOnEndOfTranslationUnitFragment(clang::TUFragmentKind) 
#16 0x0000756ef4d132d6 clang::Sema::ActOnEndOfTranslationUnit() 
#17 0x0000756ef83a3be4 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) 
#18 0x0000756ef82a8b0e clang::ParseAST(clang::Sema&, bool, bool) 
#19 0x0000756efb360de6 clang::FrontendAction::Execute() 
#20 0x0000756efb2ce328 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&)
#21 0x0000756efeecf8c5 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) 
#22 0x000063fba3c77e86 cc1_main(llvm::ArrayRef<char const*>, char const*, void*)
#23 0x000063fba3c73f36 ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>) driver.cpp:0:0
#24 0x000063fba3c76359 int llvm::function_ref<int (llvm::SmallVectorImpl<char const*>&)>::callback_fn<clang_main(int, char**, llvm::ToolContext const&)::$_2>(long, llvm::SmallVectorImpl<char const*>&) driver.cpp:0:0
#25 0x0000756efb6bfe19 void llvm::function_ref<void ()>::callback_fn<clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const::$_1>(long) Job.cpp:0:0
#26 0x0000756ef88e658b llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) 
#27 0x0000756efb6bf113 clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const 
#28 0x0000756efb670a63 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const 
#29 0x0000756efb670d57 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&, bool) const 
#30 0x0000756efb699ce8 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) 
#31 0x000063fba3c73551 clang_main(int, char**, llvm::ToolContext const&) 
#32 0x000063fba3c8360d main 
#33 0x0000756ef8029d90 __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:58:16
#34 0x0000756ef8029e40 call_init ./csu/../csu/libc-start.c:128:20
#35 0x0000756ef8029e40 __libc_start_main ./csu/../csu/libc-start.c:379:5
#36 0x000063fba3c718c5 _start 
clang++: error: clang frontend command failed due to signal (use -v to see invocation)
clang version 23.0.0git (https://github.com/llvm/llvm-project.git 06f4b027b2df1febf3cd496573b8739074e8aa29)
Target: x86_64-unknown-linux-gnu
Thread model: posix

Assertion failure:

llvm-project/clang/lib/Sema/SemaTemplateInstantiate.cpp:251: clang::MultiLevelTemplateArgumentList clang::Sema::getTemplateInstantiationArgs(const clang::Decl *, std::optional<ArrayRef<TemplateArgument>>, clang::UnsignedOrNone, bool): Assertion `(Qualifier || TPLs.empty()) && "template parameter lists without qualifier"' failed.

Start by running the provided C++ reproducer with Clang and reading getTemplateInstantiationArgs in clang/lib/Sema/SemaTemplateInstantiate.cpp around the reported assertion. Trace the variable-template instantiation path and add regression coverage for this case; done means the reproducer no longer triggers the assertion.