As reported in https://bugs.freebsd.org/240764, clang 9.0.0 (and recent trunk) give an assertion when compiling parts of VirtualBox:

Assertion failed: (Chunk.Kind == DeclaratorChunk::Function), function VisitFunctionTypeLoc, file /home/dim/src/llvm/trunk/tools/clang/lib/Sema/SemaType.cpp, line 5655.

#0  thr_kill () at thr_kill.S:3
#1  0x0000000804e2d934 in __raise (s=6) at /usr/src/lib/libc/gen/raise.c:52
#2  0x0000000804da0d99 in abort () at /usr/src/lib/libc/stdlib/abort.c:67
#3  0x0000000804e1d5c1 in __assert (func=<optimized out>, file=<optimized out>, line=<optimized out>, failedexpr=<optimized out>) at /usr/src/lib/libc/gen/assert.c:51
#4  0x00000000044abdd9 in ?? ()
#5  0x0000000004493e29 in ?? ()
#6  0x000000000448935b in clang::Sema::GetTypeForDeclarator(clang::Declarator&, clang::Scope*) ()
#7  0x0000000003ec8630 in clang::Sema::HandleDeclarator(clang::Scope*, clang::Declarator&, llvm::MutableArrayRef<clang::TemplateParameterList*>) ()
#8  0x0000000003ec823b in clang::Sema::ActOnDeclarator(clang::Scope*, clang::Declarator&) ()
#9  0x0000000003cb9112 in clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) ()
#10 0x0000000003cb7e59 in clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::SourceLocation*, clang::Parser::ForRangeInit*) ()
#11 0x0000000003cb302c in clang::Parser::ParseSimpleDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::Parser::ParsedAttributesWithRange&, bool, clang::Parser::ForRangeInit*, clang::SourceLocation*) ()
#12 0x0000000003cb2a33 in clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::Parser::ParsedAttributesWithRange&, clang::SourceLocation*) ()
#13 0x0000000003c85944 in clang::Parser::ParseExternalDeclaration(clang::Parser::ParsedAttributesWithRange&, clang::ParsingDeclSpec*) ()
#14 0x0000000003c8401e in clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, bool) ()
#15 0x0000000003c7ed17 in clang::ParseAST(clang::Sema&, bool, bool) ()
#16 0x00000000033d9413 in clang::FrontendAction::Execute() ()
#17 0x0000000003352348 in clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) ()
#18 0x000000000347e8a5 in clang::ExecuteCompilerInvocation(clang::CompilerInstance*) ()
#19 0x00000000016696bc in cc1_main(llvm::ArrayRef<char const*>, char const*, void*) ()
#20 0x00000000016670d7 in main ()

This regressed with https://reviews.llvm.org/rL360109 (Recommit r359859 "[Attribute/Diagnostics] Print macro if definition is an attribute declaration").

Minimized test case:

// clang -cc1 -triple x86_64-- -S tstVMStructRC-min.cpp
#define a __attribute__((__cdecl__, __regparm__(0)))
int(a b)();