e.g. https://build.chromium.org/p/chromium.fyi/builders/ClangToTMacASan/builds/8565/steps/compile/logs/stdio :

FAILED: obj/third_party/boringssl/boringssl_crypto_tests/gtest_main.o 
export DEVELOPER_DIR=/b/c/b/ClangToTMacASan/src/build/mac_files/Xcode.app;  ../../third_party/llvm-build/Release+Asserts/bin/clang++ -MMD -MF obj/third_party/boringssl/boringssl_crypto_tests/gtest_main.o.d -DV8_DEPRECATION_WARNINGS -DNO_TCMALLOC -DMEMORY_TOOL_REPLACES_ALLOCATOR -DMEMORY_SANITIZER_INITIAL_SIZE -DADDRESS_SANITIZER -DUSE_EXTERNAL_POPUP_MENU=1 -DDISABLE_NACL -DFULL_SAFE_BROWSING -DSAFE_BROWSING_CSD -DSAFE_BROWSING_DB_LOCAL -DCHROMIUM_BUILD -DENABLE_MEDIA_ROUTER=1 -DFIELDTRIAL_TESTING_ENABLED -DCR_CLANG_REVISION=293068 -DCR_XCODE_VERSION=0511 -D__ASSERT_MACROS_DEFINE_VERSIONS_WITHOUT_UNDERSCORE=0 -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DBORINGSSL_IMPLEMENTATION -DBORINGSSL_NO_STATIC_INITIALIZER -DOPENSSL_SMALL -D_XOPEN_SOURCE=700 -DGTEST_HAS_POSIX_RE=0 -DGTEST_LANG_CXX11=1 -DUNIT_TEST -I../.. -Igen -I../../testing/gtest/include -I../../third_party/boringssl/src/include -fno-strict-aliasing -fstack-protector -fcolor-diagnostics -arch x86_64 -O2 -gdwarf-2 -fno-standalone-debug -isysroot /b/c/b/ClangToTMacASan/src/build/mac_files/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk -mmacosx-version-min=10.9 -gline-tables-only -gcolumn-info -fno-omit-frame-pointer -fsanitize=address -mllvm -asan-globals=0 -fsanitize-blacklist=../../tools/memory/asan/blacklist.txt -fvisibility=hidden -Wheader-hygiene -Wstring-conversion -Wtautological-overlap-compare -Werror -Wall -Wno-unused-variable -Wpartial-availability -Wno-missing-field-initializers -Wno-unused-parameter -Wno-c++11-narrowing -Wno-covered-switch-default -Wno-deprecated-register -Wno-unneeded-internal-declaration -Wno-inconsistent-missing-override -Wno-shift-negative-value -Wno-undefined-var-template -Wno-nonportable-include-path -Wno-address-of-packed-member -Wno-unused-lambda-capture -Wno-user-defined-warnings -fno-threadsafe-statics -fvisibility-inlines-hidden -std=c++11 -stdlib=libc++ -fno-rtti -fno-exceptions -c ../../third_party/boringssl/src/crypto/test/gtest_main.cc -o obj/third_party/boringssl/boringssl_crypto_tests/gtest_main.o
In file included from ../../third_party/boringssl/src/crypto/test/gtest_main.cc:15:
In file included from ../../testing/gtest/include/gtest/gtest.h:55:
In file included from /b/c/b/ClangToTMacASan/src/third_party/llvm-build/Release+Asserts/bin/../include/c++/v1/ostream:138:
In file included from /b/c/b/ClangToTMacASan/src/third_party/llvm-build/Release+Asserts/bin/../include/c++/v1/ios:216:
In file included from /b/c/b/ClangToTMacASan/src/third_party/llvm-build/Release+Asserts/bin/../include/c++/v1/__locale:18:
In file included from /b/c/b/ClangToTMacASan/src/third_party/llvm-build/Release+Asserts/bin/../include/c++/v1/mutex:189:
In file included from /b/c/b/ClangToTMacASan/src/third_party/llvm-build/Release+Asserts/bin/../include/c++/v1/__mutex_base:17:
/b/c/b/ClangToTMacASan/src/third_party/llvm-build/Release+Asserts/bin/../include/c++/v1/__threading_support:154:1: error: unknown type name 'mach_port_t'
mach_port_t __libcpp_thread_get_port();
^
/b/c/b/ClangToTMacASan/src/third_party/llvm-build/Release+Asserts/bin/../include/c++/v1/__threading_support:301:1: error: unknown type name 'mach_port_t'
mach_port_t __libcpp_thread_get_port() {
^
/b/c/b/ClangToTMacASan/src/third_party/llvm-build/Release+Asserts/bin/../include/c++/v1/__threading_support:302:12: error: use of undeclared identifier 'pthread_mach_thread_np'
    return pthread_mach_thread_np(pthread_self());
           ^
3 errors generated.


This is from r290889 (Jan 3! :-/).

Asked on http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20170123/183693.html