Clang crashes while diagnosing invalid in-class member initializer

Clang crashes during diagnostic formatting when compiling invalid code (in-class member initializer with function pointer to non-existing symbol / wrong namespace).
Instead of emitting a diagnostic, clang crashes in Diagnostic::FormatDiagnostic.

Version:
clang version 21.1.8
Target: x86_64-w64-windows-gnu
Thread model: posix

Command:
clang++ -fsyntax-only context.hpp

Stackstrace

```sh
wusik@LAPTOP-4PA9J0V4:app3d clang++ -fsyntax-only modules/acul/include/acul/detail/simd/context.hpp
modules/acul/include/acul/detail/simd/context.hpp:13:9: error: unknown type name 'PFN_fill_line_buffer'; did you mean
'fs::detail::PFN_fill_line_buffer'?
13 | PFN_fill_line_buffer fill_line_buffer = &nosimd::fill_line_buffer;
| ^~~~~~~~~~~~~~~~~~~~
| fs::detail::PFN_fill_line_buffer
modules/acul/include/acul/detail/simd/../../io/fs/detail/file_simd_fn.hpp:23:11: note: 'fs::detail::PFN_fill_line_buffer' declared here
23 | using PFN_fill_line_buffer = void (*)(const char *data, size_t size, string_view_pool &dst);
| ^
modules/acul/include/acul/detail/simd/context.hpp:13:50: error: no member named 'fill_line_buffer' in namespace 'acul::detail::nosimd';
did you mean simply 'fill_line_buffer'?
13 | PFN_fill_line_buffer fill_line_buffer = &nosimd::fill_line_buffer;
| ^~~~~~~~~~~~~~~~~~~~~~~~
| fill_line_buffer
modules/acul/include/acul/detail/simd/context.hpp:13:30: note: 'fill_line_buffer' declared here
13 | PFN_fill_line_buffer fill_line_buffer = &nosimd::fill_line_buffer;
| ^
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: C:\\msys64\\clang64\\bin\\clang++.exe -fsyntax-only modules/acul/include/acul/detail/simd/context.hpp
1. parser at end of file
2. modules/acul/include/acul/detail/simd/context.hpp:7:1: parsing namespace 'acul'
3. modules/acul/include/acul/detail/simd/context.hpp:9:23: parsing struct/union/class body 'acul::detail::simd_context'
Exception Code: 0xC0000005
#0 0x00007ffc8e930581 clang::TemplateSpecializationType const* clang::Type::getAs() const (C:\msys64\clang64\bin\libclang-cpp.dll+0x1f0581)
#1 0x00007ffc8f2c4d7f clang::FormatUTFCodeUnitAsCodepoint(unsigned int, clang::QualType) (C:\msys64\clang64\bin\libclang-cpp.dll+0xb84d7f)
#2 0x00007ffc8f08d0d4 clang::FormatASTNodeDiagnosticArgument(clang::DiagnosticsEngine::ArgumentKind, long long, llvm::StringRef, llvm::StringRef, llvm::ArrayRef>, llvm::SmallVectorImpl&, void*, llvm::ArrayRef) (C:\msys64\clang64\bin\libclang-cpp.dll+0x94d0d4)
#3 0x00007ffc8f0a8296 clang::Diagnostic::FormatDiagnostic(char const*, char const*, llvm::SmallVectorImpl&) const (C:\msys64\clang64\bin\libclang-cpp.dll+0x968296)
#4 0x00007ffc8f0a72f4 clang::Diagnostic::FormatDiagnostic(llvm::SmallVectorImpl&) const (C:\msys64\clang64\bin\libclang-cpp.dll+0x9672f4)
#5 0x00007ffc8f0a6c73 clang::TextDiagnosticPrinter::HandleDiagnostic(clang::DiagnosticsEngine::Level, clang::Diagnostic const&) (C:\msys64\clang64\bin\libclang-cpp.dll+0x966c73)
#6 0x00007ffc8e86cee1 clang::DiagnosticsEngine::EmitDiagnostic(clang::DiagnosticBuilder const&, bool) (C:\msys64\clang64\bin\libclang-cpp.dll+0x12cee1)
#7 0x00007ffc8e9ddbb5 clang::Sema::EmitDiagnostic(unsigned int, clang::DiagnosticBuilder const&) (C:\msys64\clang64\bin\libclang-cpp.dll+0x29dbb5)
#8 0x00007ffc8e9d97cc clang::SemaBase::ImmediateDiagBuilder::~ImmediateDiagBuilder() (C:\msys64\clang64\bin\libclang-cpp.dll+0x2997cc)
#9 0x00007ffc8e9d76fc clang::SemaBase::SemaDiagnosticBuilder::~SemaDiagnosticBuilder() (C:\msys64\clang64\bin\libclang-cpp.dll+0x2976fc)
#10 0x00007ffc8f090862 clang::InitializationSequence::Diagnose(clang::Sema&, clang::InitializedEntity const&, clang::InitializationKind const&, llvm::ArrayRef) (C:\msys64\clang64\bin\libclang-cpp.dll+0x950862)
#11 0x00007ffc8ecc809e clang::InitializationSequence::Perform(clang::Sema&, clang::InitializedEntity const&, clang::InitializationKind const&, llvm::MutableArrayRef, clang::QualType*) (C:\msys64\clang64\bin\libclang-cpp.dll+0x58809e)
#12 0x00007ffc8ed1168c clang::Sema::ConvertMemberDefaultInitExpression(clang::FieldDecl*, clang::Expr*, clang::SourceLocation) (C:\msys64\clang64\bin\libclang-cpp.dll+0x5d168c)
#13 0x00007ffc8ed1148d clang::Sema::ActOnFinishCXXInClassMemberInitializer(clang::Decl*, clang::SourceLocation, clang::ActionResult) (C:\msys64\clang64\bin\libclang-cpp.dll+0x5d148d)
#14 0x00007ffc8ed10fa5 clang::Parser::ParseLexedMemberInitializer(clang::Parser::LateParsedMemberInitializer&) (C:\msys64\clang64\bin\libclang-cpp.dll+0x5d0fa5)
#15 0x00007ffc8e7ccdea clang::Parser::ParseLexedMemberInitializers(clang::Parser::ParsingClass&) (C:\msys64\clang64\bin\libclang-cpp.dll+0x8cdea)
#16 0x00007ffc8e7c95d8 clang::Parser::ParseCXXMemberSpecification(clang::SourceLocation, clang::SourceLocation, clang::ParsedAttributes&, unsigned int, clang::Decl*) (C:\msys64\clang64\bin\libclang-cpp.dll+0x895d8)
#17 0x00007ffc8ed956be clang::Parser::ParseClassSpecifier(clang::tok::TokenKind, clang::SourceLocation, clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, bool, clang::Parser::DeclSpecContext, clang::ParsedAttributes&) (C:\msys64\clang64\bin\libclang-cpp.dll+0x6556be)
#18 0x00007ffc8e88e503 clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::Parser::LateParsedAttrList*, clang::ImplicitTypenameContext) (C:\msys64\clang64\bin\libclang-cpp.dll+0x14e503)
#19 0x00007ffc8ed1fa6e clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (C:\msys64\clang64\bin\libclang-cpp.dll+0x5dfa6e)
#20 0x00007ffc8ed1db45 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (C:\msys64\clang64\bin\libclang-cpp.dll+0x5ddb45)
#21 0x00007ffc8ed1d641 clang::Parser::ParseInnerNamespace(llvm::SmallVector const&, unsigned int, clang::SourceLocation&, clang::ParsedAttributes&, clang::BalancedDelimiterTracker&) (C:\msys64\clang64\bin\libclang-cpp.dll+0x5dd641)
#22 0x00007ffc8ed1d761 clang::Parser::ParseInnerNamespace(llvm::SmallVector const&, unsigned int, clang::SourceLocation&, clang::ParsedAttributes&, clang::BalancedDelimiterTracker&) (C:\msys64\clang64\bin\libclang-cpp.dll+0x5dd761)
#23 0x00007ffc8ed1c085 clang::Parser::ParseNamespace(clang::DeclaratorContext, clang::SourceLocation&, clang::SourceLocation) (C:\msys64\clang64\bin\libclang-cpp.dll+0x5dc085)
#24 0x00007ffc8ed1eb8a clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (C:\msys64\clang64\bin\libclang-cpp.dll+0x5deb8a)
#25 0x00007ffc8ed1da34 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (C:\msys64\clang64\bin\libclang-cpp.dll+0x5dda34)
#26 0x00007ffc8ef58451 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (C:\msys64\clang64\bin\libclang-cpp.dll+0x818451)
#27 0x00007ffc8ef58114 clang::ParseAST(clang::Sema&, bool, bool) (C:\msys64\clang64\bin\libclang-cpp.dll+0x818114)
#28 0x00007ffc8ef80e2f clang::FrontendAction::Execute() (C:\msys64\clang64\bin\libclang-cpp.dll+0x840e2f)
#29 0x00007ffc8ef8099b clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (C:\msys64\clang64\bin\libclang-cpp.dll+0x84099b)
#30 0x00007ffc8ef80422 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (C:\msys64\clang64\bin\libclang-cpp.dll+0x840422)
#31 0x00007ff65b52442d cc1_main(llvm::ArrayRef, char const*, void*) (C:\msys64\clang64\bin\clang++.exe+0x442d)
#32 0x00007ff65b523c06 llvm::raw_ostream::operator<<(llvm::StringRef) (C:\msys64\clang64\bin\clang++.exe+0x3c06)
#33 0x00007ffc9027e7ed void std::__1::vector, std::__1::allocator>>, std::__1::allocator, std::__1::allocator>>>>::__assign_with_size[abi:nn210108], std::__1::allocator>>*, std::__1::optional, std::__1::allocator>>*>(std::__1::optional, std::__1::allocator>>*, std::__1::optional, std::__1::allocator>>*, long long) (C:\msys64\clang64\bin\libclang-cpp.dll+0x1b3e7ed)
#34 0x00007ffbea85bf05 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (C:\msys64\clang64\bin\libLLVM-21.dll+0xf1bf05)
#35 0x00007ffc8f0a43b7 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__1::basic_string, std::__1::allocator>*, bool*) const (C:\msys64\clang64\bin\libclang-cpp.dll+0x9643b7)
#36 0x00007ffc8f05e153 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (C:\msys64\clang64\bin\libclang-cpp.dll+0x91e153)
#37 0x00007ffc8f05ddd0 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (C:\msys64\clang64\bin\libclang-cpp.dll+0x91ddd0)
#38 0x00007ffc8f05dafb clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (C:\msys64\clang64\bin\libclang-cpp.dll+0x91dafb)
#39 0x00007ff65b521bfc clang_main(int, char**, llvm::ToolContext const&) (C:\msys64\clang64\bin\clang++.exe+0x1bfc)
#40 0x00007ff65b521075 main (C:\msys64\clang64\bin\clang++.exe+0x1075)
#41 0x00007ff65b52a96e WinMainCRTStartup (C:\msys64\clang64\bin\clang++.exe+0xa96e)
#42 0x00007ff65b52ac56 mainCRTStartup (C:\msys64\clang64\bin\clang++.exe+0xac56)
#43 0x00007ffd3adbe8d7 (C:\WINDOWS\System32\KERNEL32.DLL+0x2e8d7)
#44 0x00007ffd3be8c53c (C:\WINDOWS\SYSTEM32\ntdll.dll+0x8c53c)
clang++: error: clang frontend command failed due to signal (use -v to see invocation)
clang version 21.1.8
Target: x86_64-w64-windows-gnu
Thread model: posix
InstalledDir: C:/msys64/clang64/bin
clang++: note: diagnostic msg:
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang++: note: diagnostic msg: C:/Users/wusik/AppData/Local/Temp/context-0f0fe5.hh
clang++: note: diagnostic msg: C:/Users/wusik/AppData/Local/Temp/context-0f0fe5.sh
clang++: note: diagnostic msg:

********************
```

[context-0f0fe5.sh](https://github.com/user-attachments/files/24912895/context-0f0fe5.sh)

After the crash, clang automatically generated two files:
- context-0f0fe5.sh (run script with full compiler invocation)
- context-0f0fe5.hh (preprocessed source)

I am attaching context-0f0fe5.sh, since it contains the full invocation and flags used to reproduce the issue.
The preprocessed source file (context-0f0fe5.hh) is large (~300k lines, expanded includes and system headers), so I did not attach it by default, but I can provide it if needed.
If you need any further information from my side, please let me know.