Using the latest commit as of writing (https://github.com/llvm/llvm-project/commit/3fa2d37eb3f8acddcfde749ca822f2cc7d900cbb), clang-tidy fails to process the following preprocessed source:

	# 1 "main.cpp"
	# 1 "usr/include/sys/cdefs.h" 1
	# 1 "usr/include/android/api-level.h" 1
	# 47 "usr/include/android/api-level.h"
	extern "C" {
	# 170 "usr/include/android/api-level.h"
	#if 1
	# 1 "usr/include/bits/get_device_api_level_inlines.h" 1
	# 30 "usr/include/bits/get_device_api_level_inlines.h"
	# 32 "usr/include/bits/get_device_api_level_inlines.h"
	# 33 "usr/include/bits/get_device_api_level_inlines.h"
	# 34 "usr/include/bits/get_device_api_level_inlines.h"
	# 51 "usr/include/bits/get_device_api_level_inlines.h"
	#endif
	# 177 "usr/include/android/api-level.h" 2
	# 191 "usr/include/android/api-level.h"
	}
	# 363 "usr/include/sys/cdefs.h" 2
	# 1 "main.cpp" 2

The following command line was used:

	clang-tidy main.cpp -- -std=c++20

In Release, clang-tidy outputs this error:

	1 error generated.
	Error while processing main.cpp.
	main.cpp:19:16: error: invalid line marker flag '2': cannot pop empty include stack [clang-diagnostic-error]
	# 1 "main.cpp" 2
	               ^
	Found compiler error(s).

In Debug, clang-tidy crashes with this assert:

	clang-tidy: /tmp/llvm-project/clang/lib/Basic/SourceManager.cpp:210: void clang::LineTableInfo::AddLineNote(clang::FileID, unsigned int, unsigned int, int, unsigned int, clang::SrcMgr::CharacteristicKind): Assertion `(Entries.empty() || Entries.back().FileOffset < Offset) && "Adding line entries out of order!"' failed.
	PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace.
	Stack dump:
	0.      Program arguments: ./clang-tidy /tmp/main.cpp -- -std=c++20
	1.      main.cpp:0:3: current parser token '1'
	 #0 0x000055cffd08ed58 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /tmp/llvm-project/llvm/lib/Support/Unix/Signals.inc:563:0
	 #1 0x000055cffd08ee0f PrintStackTraceSignalHandler(void*) /tmp/llvm-project/llvm/lib/Support/Unix/Signals.inc:630:0
	 #2 0x000055cffd08caf3 llvm::sys::RunSignalHandlers() /tmp/llvm-project/llvm/lib/Support/Signals.cpp:71:0
	 #3 0x000055cffd08e6da SignalHandler(int) /tmp/llvm-project/llvm/lib/Support/Unix/Signals.inc:405:0
	 #4 0x00007fb179480980 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x12980)
	 #5 0x00007fb178335fb7 gsignal /build/glibc-S7xCS9/glibc-2.27/signal/../sysdeps/unix/sysv/linux/raise.c:51:0
	 #6 0x00007fb178337921 abort /build/glibc-S7xCS9/glibc-2.27/stdlib/abort.c:81:0
	 #7 0x00007fb17832748a __assert_fail_base /build/glibc-S7xCS9/glibc-2.27/assert/assert.c:89:0
	 #8 0x00007fb178327502 (/lib/x86_64-linux-gnu/libc.so.6+0x30502)
	 #9 0x000055cffcb5f09d clang::LineTableInfo::AddLineNote(clang::FileID, unsigned int, unsigned int, int, unsigned int, clang::SrcMgr::CharacteristicKind) /tmp/llvm-project/clang/lib/Basic/SourceManager.cpp:209:0
	#10 0x000055cffcb5f46d clang::SourceManager::AddLineNote(clang::SourceLocation, unsigned int, int, bool, bool, clang::SrcMgr::CharacteristicKind) /tmp/llvm-project/clang/lib/Basic/SourceManager.cpp:290:0
	#11 0x000055cffca8a137 clang::Preprocessor::HandleDigitDirective(clang::Token&) /tmp/llvm-project/clang/lib/Lex/PPDirectives.cpp:1363:0
	#12 0x000055cffca88b1a clang::Preprocessor::HandleDirective(clang::Token&) /tmp/llvm-project/clang/lib/Lex/PPDirectives.cpp:977:0
	#13 0x000055cffca4c554 clang::Lexer::LexTokenInternal(clang::Token&, bool) /tmp/llvm-project/clang/lib/Lex/Lexer.cpp:3975:0
	#14 0x000055cffca498d8 clang::Lexer::Lex(clang::Token&) /tmp/llvm-project/clang/lib/Lex/Lexer.cpp:3188:0
	#15 0x000055cffcacc1fa clang::Preprocessor::Lex(clang::Token&) /tmp/llvm-project/clang/lib/Lex/Preprocessor.cpp:898:0
	#16 0x000055cffaded894 clang::Parser::ConsumeToken() /tmp/llvm-project/clang/include/clang/Parse/Parser.h:483:0
	#17 0x000055cffade2f55 clang::Parser::Initialize() /tmp/llvm-project/clang/lib/Parse/Parser.cpp:562:0
	#18 0x000055cffaddedb6 clang::ParseAST(clang::Sema&, bool, bool) /tmp/llvm-project/clang/lib/Parse/ParseAST.cpp:156:0
	#19 0x000055cffab19495 clang::ASTFrontendAction::ExecuteAction() /tmp/llvm-project/clang/lib/Frontend/FrontendAction.cpp:1056:0
	#20 0x000055cffab18d5e clang::FrontendAction::Execute() /tmp/llvm-project/clang/lib/Frontend/FrontendAction.cpp:953:0
	#21 0x000055cffaa7ab38 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) /tmp/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:991:0
	#22 0x000055cff9ed747d clang::tooling::FrontendActionFactory::runInvocation(std::shared_ptr<clang::CompilerInvocation>, clang::FileManager*, std::shared_ptr<clang::PCHContainerOperations>, clang::DiagnosticConsumer*) /tmp/llvm-project/clang/lib/Tooling/Tooling.cpp:396:0
	#23 0x000055cff9e5f9a3 runInvocation /tmp/llvm-project/clang-tools-extra/clang-tidy/ClangTidy.cpp:556:0
	#24 0x000055cff9e5f9a3 clang::tidy::runClangTidy(clang::tidy::ClangTidyContext&, clang::tooling::CompilationDatabase const&, llvm::ArrayRef<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, llvm::IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem>, bool, llvm::StringRef)::ActionFactory::runInvocation(std::shared_ptr<clang::CompilerInvocation>, clang::FileManager*, std::shared_ptr<clang::PCHContainerOperations>, clang::DiagnosticConsumer*) (./clang-tidy+0x11e19a3)
	#25 0x000055cff9ed72ad clang::tooling::ToolInvocation::runInvocation(char const*, clang::driver::Compilation*, std::shared_ptr<clang::CompilerInvocation>, std::shared_ptr<clang::PCHContainerOperations>) /tmp/llvm-project/clang/lib/Tooling/Tooling.cpp:372:0
	#26 0x000055cff9ed70c4 clang::tooling::ToolInvocation::run() /tmp/llvm-project/clang/lib/Tooling/Tooling.cpp:357:0
	#27 0x000055cff9ed884d clang::tooling::ClangTool::run(clang::tooling::ToolAction*) /tmp/llvm-project/clang/lib/Tooling/Tooling.cpp:549:0
	#28 0x000055cff9e5fda1 clang::tidy::runClangTidy(clang::tidy::ClangTidyContext&, clang::tooling::CompilationDatabase const&, llvm::ArrayRef<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, llvm::IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem>, bool, llvm::StringRef) /tmp/llvm-project/clang-tools-extra/clang-tidy/ClangTidy.cpp:577:0
	#29 0x000055cff927ce27 clang::tidy::clangTidyMain(int, char const**) /tmp/llvm-project/clang-tools-extra/clang-tidy/tool/ClangTidyMain.cpp:486:0
	#30 0x000055cff927b1ea main /tmp/llvm-project/clang-tools-extra/clang-tidy/tool/ClangTidyToolMain.cpp:21:0
	#31 0x00007fb178318bf7 __libc_start_main /build/glibc-S7xCS9/glibc-2.27/csu/../csu/libc-start.c:344:0
	#32 0x000055cff927b0ea _start (./clang-tidy+0x5fd0ea)
	Aborted

This only occurs with -std=c++20. It does not occur with -std=c++17.

If I disable the use of ExpandModularHeadersPPCallbacks here (https://github.com/llvm/llvm-project/blob/3fa2d37eb3f8acddcfde749ca822f2cc7d900cbb/clang-tools-extra/clang-tidy/ClangTidy.cpp#L414-L419) then clang-tidy succeeds even in C++20 mode.