[clang] Assertion `PrevClassTemplate && "Member specialization without a primary template?"' failed.

similar (but already closed) issue: https://github.com/llvm/llvm-project/issues/201490 https://github.com/llvm/llvm-project/issues/206866

Reproducer:
https://godbolt.org/z/8Y91b18bE
```cpp
class C {
class P {};

template class FT {
template class BT1 {};
};

template <> template class FT::BT2 {};
};
```

Backtrace:
```console
clang++: /root/llvm-project/llvm/tools/clang/lib/Sema/SemaTemplate.cpp:2272: clang::DeclResult clang::Sema::CheckClassTemplate(clang::Scope*, unsigned int, clang::TagUseKind, clang::SourceLocation, clang::CXXScopeSpec&, clang::IdentifierInfo*, clang::SourceLocation, const clang::ParsedAttributesView&, clang::TemplateParameterList*, clang::AccessSpecifier, clang::SourceLocation, clang::SourceLocation, unsigned int, clang::TemplateParameterList**, bool, clang::SkipBodyInfo*): Assertion `PrevClassTemplate && "Member specialization without a primary template?"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and dumped files.
Stack dump:
0. Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/clang++ -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
1. :8:63: current parser token '{'
2. :1:1: parsing struct/union/class body 'C'
#0 0x000000000439e6c8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x439e6c8)
#1 0x000000000439b584 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x439b584)
#2 0x000000000439bbac llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x439bbac)
#3 0x00000000042dabf8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#4 0x00007432f6445330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#5 0x00007432f649eb2c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9eb2c)
#6 0x00007432f644527e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4527e)
#7 0x00007432f64288ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
#8 0x00007432f642881b (/lib/x86_64-linux-gnu/libc.so.6+0x2881b)
#9 0x00007432f643b517 (/lib/x86_64-linux-gnu/libc.so.6+0x3b517)
#10 0x00000000076f12fb clang::Sema::CheckClassTemplate(clang::Scope*, unsigned int, clang::TagUseKind, clang::SourceLocation, clang::CXXScopeSpec&, clang::IdentifierInfo*, clang::SourceLocation, clang::ParsedAttributesView const&, clang::TemplateParameterList*, clang::AccessSpecifier, clang::SourceLocation, clang::SourceLocation, unsigned int, clang::TemplateParameterList**, bool, clang::SkipBodyInfo*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x76f12fb)
#11 0x000000000705fac8 clang::Sema::ActOnTag(clang::Scope*, unsigned int, clang::TagUseKind, clang::SourceLocation, clang::CXXScopeSpec&, clang::IdentifierInfo*, clang::SourceLocation, clang::ParsedAttributesView const&, clang::AccessSpecifier, clang::SourceLocation, llvm::MutableArrayRef, bool&, bool&, clang::SourceLocation, bool, clang::ActionResult, false>, bool, bool, clang::OffsetOfKind, clang::SkipBodyInfo*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x705fac8)
#12 0x0000000006cfbc1a 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+++0x6cfbc1a)
#13 0x0000000006ccbefc clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::LateParsedAttrList*, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ccbefc)
#14 0x0000000006cf7366 clang::Parser::ParseCXXClassMemberDeclaration(clang::AccessSpecifier, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cf7366)
#15 0x0000000006da2973 clang::Parser::ParseDeclarationAfterTemplate(clang::DeclaratorContext, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject&, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6da2973)
#16 0x0000000006daec73 clang::Parser::ParseTemplateDeclarationOrSpecialization(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (.part.0) ParseTemplate.cpp:0:0
#17 0x0000000006cf77cd clang::Parser::ParseCXXClassMemberDeclaration(clang::AccessSpecifier, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cf77cd)
#18 0x0000000006cf987f clang::Parser::ParseCXXClassMemberDeclarationWithPragmas(clang::AccessSpecifier&, clang::ParsedAttributes&, clang::TypeSpecifierType, clang::Decl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cf987f)
#19 0x0000000006cf9dff clang::Parser::ParseCXXMemberSpecification(clang::SourceLocation, clang::SourceLocation, clang::ParsedAttributes&, unsigned int, clang::Decl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cf9dff)
#20 0x0000000006cfc97b 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+++0x6cfc97b)
#21 0x0000000006ccbefc clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::LateParsedAttrList*, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ccbefc)
#22 0x0000000006c89177 clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c89177)
#23 0x0000000006c89bff clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c89bff)
#24 0x0000000006c96421 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c96421)
#25 0x0000000006c97335 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c97335)
#26 0x0000000006c97720 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c97720)
#27 0x0000000006c741f3 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c741f3)
#28 0x0000000004e992c8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4e992c8)
#29 0x00000000051c4098 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x51c4098)
#30 0x0000000005151dad clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5151dad)
#31 0x000000000529c7cd clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x529c7cd)
#32 0x0000000000db774c cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdb774c)
#33 0x0000000000dae35a ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#34 0x0000000000dae4dd int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#35 0x0000000004f104b9 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#36 0x00000000042db034 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42db034)
#37 0x0000000004f11025 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#38 0x0000000004ec9ad2 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ec9ad2)
#39 0x0000000004ece307 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ece307)
#40 0x0000000004ed87f5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ed87f5)
#41 0x0000000000db3c3a clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdb3c3a)
#42 0x0000000000ccff2a main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xccff2a)
#43 0x00007432f642a1ca (/lib/x86_64-linux-gnu/libc.so.6+0x2a1ca)
#44 0x00007432f642a28b __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28b)
#45 0x0000000000dadd05 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdadd05)
clang++: error: clang frontend command failed due to signal (use -v to see invocation)
Compiler returned: 134
```

Start by compiling the linked reproducer with an assertions-enabled clang and inspect llvm/tools/clang/lib/Sema/SemaTemplate.cpp, especially Sema::CheckClassTemplate at the reported assertion. Compare the behavior with the related issues; done means this input no longer aborts in the frontend and receives appropriate handling.

Written by the indexing model from the issue text.