When building clang (7 or 8) with the newly shipped gcc-9 compiler, the build failed during validation (see for instance https://koji.fedoraproject.org/koji/taskinfo?taskID=32504081).

I've been reproducing the error on a ppc64le machine, installing gcc-9 from source through

$ svn export svn://gcc.gnu.org/svn/gcc/branches/redhat/gcc-9-branch@268371
$ cd gcc-9-branch/
$ mkdir _build && cd _build
$ ../configure --prefix=$HOME/gcc-install
$ make
$ make install

then compiling llvm with the generated gcc

$ export LD_LIBRARY_PATH=$HOME/gcc-install/lib:$HOME/gcc-install/lib64
$ git clone https://github.com/llvm/llvm-project.git
$ git checkout release/8.x
$ mkdir _build && cd _build
$ cmake ../llvm -DCMAKE_BUILD_TYPE=RelWithDebInfo -DLLVM_ENABLE_PROJECTS=clang -DCMAKE_C_COMPILER=$HOME/gcc-install/bin/gcc -DCMAKE_CXX_COMPILER=$HOME/gcc-install/bin/g++
$ make
$ make check-clang

Fails on a single test case, but not always (!)

Running valgrind on the faulty test brings me the following log:

$ valgrind --track-origins=yes /home/sergesanspaille/llvm-project/_build/bin/c-index-test -test-annotate-tokens=/home/sergesanspaille/llvm-project/clang/test/Index/annotate-deep-statements.cpp:1:1:1000:1 /home/sergesanspaille/llvm-project/clang/test/Index/annotate-deep-statements.cpp
[...]
=4645== Thread 3:
==4645== Conditional jump or move depends on uninitialised value(s)
==4645==    at 0x4B855F4: reset (unique_ptr.h:398)
==4645==    by 0x4B855F4: clang::DiagnosticsEngine::setClient(clang::DiagnosticConsumer*, bool) (Diagnostic.cpp:102)
==4645==    by 0x4BE9B1B: ~CaptureDroppedDiagnostics (ASTUnit.cpp:661)
==4645==    by 0x4BE9B1B: clang::ASTUnit::LoadFromCommandLine(char const**, char const**, std::shared_ptr<clang::PCHContainerOperations>, llvm::IntrusiveRefCntPtr<clang::DiagnosticsEngine>, llvm::StringRef, bool, bool, llvm::ArrayRef<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, llvm::MemoryBuffer*> >, bool, unsigned int, clang::TranslationUnitKind, bool, bool, bool, clang::SkipFunctionBodiesScope, bool, bool, bool, llvm::Optional<llvm::StringRef>, std::unique_ptr<clang::ASTUnit, std::default_delete<clang::ASTUnit> >*, llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>) (ASTUnit.cpp:1722)
==4645==    by 0x47BDECB: clang_parseTranslationUnit_Impl(void*, char const*, char const* const*, int, llvm::ArrayRef<CXUnsavedFile>, unsigned int, CXTranslationUnitImpl**) (CIndex.cpp:3497)
==4645==    by 0x47BEA13: operator() (CIndex.cpp:3570)
==4645==    by 0x47BEA13: void llvm::function_ref<void ()>::callback_fn<clang_parseTranslationUnit2FullArgv::{lambda()#1}>(long) (STLExtras.h:126)
==4645==    by 0x6EFC5BF: operator() (STLExtras.h:143)
==4645==    by 0x6EFC5BF: llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (CrashRecoveryContext.cpp:359)
==4645==    by 0x6EFC69B: RunSafelyOnThread_Dispatch(void*) (CrashRecoveryContext.cpp:402)
==4645==    by 0x6F874B7: ExecuteOnThread_Dispatch(void*) (Threading.inc:53)
==4645==    by 0x40D8B93: start_thread (in /usr/lib64/libpthread-2.17.so)
==4645==    by 0xB7985F3: clone (in /usr/lib64/libc-2.17.so)
==4645==  Uninitialised value was created by a heap allocation
==4645==    at 0x40846F4: operator new(unsigned long) (vg_replace_malloc.c:334)
==4645==    by 0x4BF2907: clang::CompilerInstance::createDiagnostics(clang::DiagnosticOptions*, clang::DiagnosticConsumer*, bool, clang::CodeGenOptions const*) (CompilerInstance.cpp:282)
==4645==    by 0x47BD6FF: clang_parseTranslationUnit_Impl(void*, char const*, char const* const*, int, llvm::ArrayRef<CXUnsavedFile>, unsigned int, CXTranslationUnitImpl**) (CIndex.cpp:3409)
==4645==    by 0x47BEA13: operator() (CIndex.cpp:3570)
==4645==    by 0x47BEA13: void llvm::function_ref<void ()>::callback_fn<clang_parseTranslationUnit2FullArgv::{lambda()#1}>(long) (STLExtras.h:126)
==4645==    by 0x6EFC5BF: operator() (STLExtras.h:143)
==4645==    by 0x6EFC5BF: llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (CrashRecoveryContext.cpp:359)
==4645==    by 0x6EFC69B: RunSafelyOnThread_Dispatch(void*) (CrashRecoveryContext.cpp:402)
==4645==    by 0x6F874B7: ExecuteOnThread_Dispatch(void*) (Threading.inc:53)
==4645==    by 0x40D8B93: start_thread (in /usr/lib64/libpthread-2.17.so)
==4645==    by 0xB7985F3: clone (in /usr/lib64/libc-2.17.so)
==4645== 


Modifying clang/include/clang/Frontend/TextDiagnosticPrinter.h from

class TextDiagnosticPrinter : public DiagnosticConsumer {
  raw_ostream &OS;
  IntrusiveRefCntPtr<DiagnosticOptions> DiagOpts;

  /// Handle to the currently active text diagnostic emitter.
  std::unique_ptr<TextDiagnostic> TextDiag;

  /// A string to prefix to error messages.
  std::string Prefix;

  unsigned OwnsOutputStream : 1;
/*...*/


to

class TextDiagnosticPrinter : public DiagnosticConsumer {
  raw_ostream &OS;
  IntrusiveRefCntPtr<DiagnosticOptions> DiagOpts;

  /// Handle to the currently active text diagnostic emitter.
  std::unique_ptr<TextDiagnostic> TextDiag;

  /// A string to prefix to error messages.
  std::string Prefix;

  bool OwnsOutputStream;
/*...*/

removes the valgrind warning. Compiling with -O1 or -O0 also removes the warning. Would it be possible for gcc-9 to somehow miscompile bitfields?