For example: https://build.chromium.org/p/chromium.fyi/builders/ClangToTMac/builds/13702

[46/41483] OBJCXX obj/base/base/precompile.h-mm.gch
FAILED: obj/base/base/precompile.h-mm.gch 
export DEVELOPER_DIR=/b/c/b/ClangToTMac/src/build/mac_files/Xcode.app;  ../../third_party/llvm-build/Release+Asserts/bin/clang++ -MMD -MF obj/base/base/precompile.h-mm.gch.d -DSYSTEM_NATIVE_UTF8 -DV8_DEPRECATION_WARNINGS -DNO_TCMALLOC -DFULL_SAFE_BROWSING -DSAFE_BROWSING_CSD -DSAFE_BROWSING_DB_LOCAL -DCHROMIUM_BUILD -DENABLE_MEDIA_ROUTER=1 -DFIELDTRIAL_TESTING_ENABLED -DCR_CLANG_REVISION=\"300068\" -DCR_XCODE_VERSION=0511 -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D_FORTIFY_SOURCE=2 -DCOMPONENT_BUILD -D__ASSERT_MACROS_DEFINE_VERSIONS_WITHOUT_UNDERSCORE=0 -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DBASE_IMPLEMENTATION -I../.. -Igen -fno-strict-aliasing -fstack-protector -Wno-builtin-macro-redefined -D__DATE__= -D__TIME__= -D__TIMESTAMP__= -fcolor-diagnostics -arch x86_64 -Wall -Werror -Wextra -Wpartial-availability -Wno-missing-field-initializers -Wno-unused-parameter -Wno-c++11-narrowing -Wno-covered-switch-default -Wno-unneeded-internal-declaration -Wno-inconsistent-missing-override -Wno-undefined-var-template -Wno-nonportable-include-path -Wno-address-of-packed-member -Wno-unused-lambda-capture -Wno-user-defined-warnings -fno-omit-frame-pointer -g1 -isysroot /b/c/b/ClangToTMac/src/build/mac_files/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk -mmacosx-version-min=10.9 -fvisibility=hidden -Wheader-hygiene -Wstring-conversion -Wtautological-overlap-compare -Wno-char-subscripts -Wexit-time-destructors -Wexit-time-destructors -O2 -fvisibility-inlines-hidden -std=c++11 -stdlib=libc++ -fobjc-call-cxx-cdtors -Wobjc-missing-property-synthesis -fno-rtti -fno-exceptions -x objective-c++-header -c ../../build/precompile.h -o obj/base/base/precompile.h-mm.gch
Assertion failed: (!hasUninstantiatedDefaultArg() && "Default argument is not yet instantiated!"), function getDefaultArg, file /b/c/b/ClangToTMac/src/third_party/llvm/tools/clang/lib/AST/Decl.cpp, line 2424.
0  clang-5.0                0x000000010d18a128 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1  clang-5.0                0x000000010d18a826 llvm::sys::PrintStackTraceOnErrorSignal(llvm::StringRef, bool) + 838
2  libsystem_platform.dylib 0x00007fffde723bba _sigtramp + 26
3  libsystem_platform.dylib 000000000000000000 _sigtramp + 562938976
4  libsystem_c.dylib        0x00007fffde5aa420 abort + 129
5  libsystem_c.dylib        0x00007fffde571893 basename_r + 0
6  clang-5.0                0x000000010ec3dcdd clang::ParmVarDecl::getDefaultArg() + 109
7  clang-5.0                0x000000010ed3b639 clang::ODRHash::AddType(clang::Type const*) + 409
8  clang-5.0                0x000000010ed3bb42 clang::ODRHash::AddType(clang::Type const*) + 1698
9  clang-5.0                0x000000010ed3b3cf clang::ODRHash::AddCXXRecordDecl(clang::CXXRecordDecl const*) + 463
10 clang-5.0                0x000000010ec62616 clang::CXXRecordDecl::getODRHash() const + 294
11 clang-5.0                0x000000010e19d259 clang::ASTRecordWriter::AddCXXDefinitionData(clang::CXXRecordDecl const*) + 1753
12 clang-5.0                0x000000010e1b80fa clang::ASTDeclWriter::VisitCXXRecordDecl(clang::CXXRecordDecl*) + 314
13 clang-5.0                0x000000010e1b9580 clang::ASTDeclWriter::VisitClassTemplateSpecializationDecl(clang::ClassTemplateSpecializationDecl*) + 128
14 clang-5.0                0x000000010e1aeba6 clang::ASTDeclWriter::Visit(clang::Decl*) + 22
15 clang-5.0                0x000000010e1c254b clang::ASTWriter::WriteDecl(clang::ASTContext&, clang::Decl*) + 459
16 clang-5.0                0x000000010e199d3a clang::ASTWriter::WriteASTCore(clang::Sema&, llvm::StringRef, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, clang::Module*) + 12154
17 clang-5.0                0x000000010e196c3c clang::ASTWriter::WriteAST(clang::Sema&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, clang::Module*, llvm::StringRef, bool) + 652
18 clang-5.0                0x000000010e1d90ab clang::PCHGenerator::HandleTranslationUnit(clang::ASTContext&) + 251
19 clang-5.0                0x000000010d8aa44c clang::MultiplexConsumer::HandleTranslationUnit(clang::ASTContext&) + 44
20 clang-5.0                0x000000010dfebeb5 clang::ParseAST(clang::Sema&, bool, bool) + 469
21 clang-5.0                0x000000010d8686bc clang::FrontendAction::Execute() + 76
22 clang-5.0                0x000000010d825ce1 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) + 1217
23 clang-5.0                0x000000010d8c9cb2 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) + 4914
24 clang-5.0                0x000000010b9e0f0c cc1_main(llvm::ArrayRef<char const*>, char const*, void*) + 1388
25 clang-5.0                0x000000010b9df5d3 main + 11875
26 libdyld.dylib            0x00007fffde516255 start + 1
27 libdyld.dylib            0x000000000000008f start + 565091899
Stack dump:
0.	Program arguments: /b/c/b/ClangToTMac/src/third_party/llvm-build/Release+Asserts/bin/clang-5.0 -cc1 -triple x86_64-apple-macosx10.9.0 -Wdeprecated-objc-isa-usage -Werror=deprecated-objc-isa-usage -emit-pch -disable-free -main-file-name precompile.h -mrelocation-model pic -pic-level 2 -mthread-model posix -mdisable-fp-elim -relaxed-aliasing -masm-verbose -munwind-tables -target-cpu core2 -target-linker-version 274.1 -dwarf-column-info -debug-info-kind=line-tables-only -dwarf-version=2 -debugger-tuning=lldb -coverage-notes-file /b/c/b/ClangToTMac/src/out/Release/obj/base/base/precompile.h-mm.gcno -resource-dir /b/c/b/ClangToTMac/src/third_party/llvm-build/Release+Asserts/lib/clang/5.0.0 -dependency-file obj/base/base/precompile.h-mm.gch.d -MT obj/base/base/precompile.h-mm.gch -module-file-deps -isysroot /b/c/b/ClangToTMac/src/build/mac_files/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk -D SYSTEM_NATIVE_UTF8 -D V8_DEPRECATION_WARNINGS -D NO_TCMALLOC -D FULL_SAFE_BROWSING -D SAFE_BROWSING_CSD -D SAFE_BROWSING_DB_LOCAL -D CHROMIUM_BUILD -D ENABLE_MEDIA_ROUTER=1 -D FIELDTRIAL_TESTING_ENABLED -D CR_CLANG_REVISION="300068" -D CR_XCODE_VERSION=0511 -D __STDC_CONSTANT_MACROS -D __STDC_FORMAT_MACROS -D _FORTIFY_SOURCE=2 -D COMPONENT_BUILD -D __ASSERT_MACROS_DEFINE_VERSIONS_WITHOUT_UNDERSCORE=0 -D NDEBUG -D NVALGRIND -D DYNAMIC_ANNOTATIONS_ENABLED=0 -D BASE_IMPLEMENTATION -I ../.. -I gen -D __DATE__= -D __TIME__= -D __TIMESTAMP__= -stdlib=libc++ -O2 -Wno-builtin-macro-redefined -Wall -Werror -Wextra -Wpartial-availability -Wno-missing-field-initializers -Wno-unused-parameter -Wno-c++11-narrowing -Wno-covered-switch-default -Wno-unneeded-internal-declaration -Wno-inconsistent-missing-override -Wno-undefined-var-template -Wno-nonportable-include-path -Wno-address-of-packed-member -Wno-unused-lambda-capture -Wno-user-defined-warnings -Wheader-hygiene -Wstring-conversion -Wtautological-overlap-compare -Wno-char-subscripts -Wexit-time-destructors -Wexit-time-destructors -Wobjc-missing-property-synthesis -std=c++11 -fdeprecated-macro -fdebug-compilation-dir /b/c/b/ClangToTMac/src/out/Release -ferror-limit 19 -fmessage-length 0 -fvisibility hidden -fvisibility-inlines-hidden -stack-protector 1 -fblocks -fno-rtti -fobjc-runtime=macosx-10.9.0 -fencode-extended-block-signature -fobjc-exceptions -fexceptions -fmax-type-align=16 -fdiagnostics-show-option -fcolor-diagnostics -vectorize-loops -vectorize-slp -o obj/base/base/precompile.h-mm.gch -x objective-c++-header ../../build/precompile.h 
1.	<eof> parser at end of file
clang-5.0: error: unable to execute command: Abort trap: 6
clang-5.0: error: clang frontend command failed due to signal (use -v to see invocation)
clang version 5.0.0 (trunk 300068)
Target: x86_64-apple-darwin16.3.0
Thread model: posix
InstalledDir: /b/c/b/ClangToTMac/src/out/Release/../../third_party/llvm-build/Release+Asserts/bin
clang-5.0: note: diagnostic msg: PLEASE submit a bug report to http://llvm.org/bugs/ and include the crash backtrace, preprocessed source, and associated run script.
clang-5.0: note: diagnostic msg: 
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang-5.0: note: diagnostic msg: /var/folders/p4/py6nlvcx4fs5s3fg51_pzqmc0000gm/T/precompile-03b88b.h
clang-5.0: note: diagnostic msg: /var/folders/p4/py6nlvcx4fs5s3fg51_pzqmc0000gm/T/precompile-03b88b.sh
clang-5.0: note: diagnostic msg: Crash backtrace is located in
clang-5.0: note: diagnostic msg: /Users/chrome-bot/Library/Logs/DiagnosticReports/clang-5.0_<YYYY-MM-DD-HHMMSS>_<hostname>.crash
clang-5.0: note: diagnostic msg: (choose the .crash file that corresponds to your crash)
clang-5.0: note: diagnostic msg: 

********************


Last good build:
https://build.chromium.org/p/chromium.fyi/builders/ClangToTMac/builds/13675
Clang 299979

First bad:
https://build.chromium.org/p/chromium.fyi/builders/ClangToTMac/builds/13676
Clang 300006


Since ODRHash is on the stack, I'm guessing:

------------------------------------------------------------------------
r299989 | rtrieu | 2017-04-11 14:31:00 -0700 (Tue, 11 Apr 2017) | 9 lines

[ODRHash] Improve handling of hash values

Calculating the hash in Sema::ActOnTagFinishDefinition could happen before
all sub-Decls were parsed or processed, which would produce the wrong hash
value.  Change to calculating the hash on the first use and storing the value
instead.  Also, avoid using the macros that were only for Boolean fields and
use an explicit checker during the DefintionData merge.  No functional change,
but was this blocking other ODRHash patches.

------------------------------------------------------------------------

or

------------------------------------------------------------------------
r300001 | rtrieu | 2017-04-11 15:32:03 -0700 (Tue, 11 Apr 2017) | 11 lines

Revert r298824 & r298816, recommit r298742 & r298754

r299989 fixes the underlying issue by waiting long enough to late parsed
arguments to be processed before doing an calculating the hash.

r298742
[ODRHash] Add error messages for mismatched parameters in methods.

r298754
[ODRHash] Add support for array and decayed types.

------------------------------------------------------------------------