There is a build bot that doesn't support 'constexpr'.
FAILED: C:\PROGRA~2\MICROS~1.0\VC\bin\amd64\cl.exe /nologo /TP /DWIN32 /D_WINDOWS /W4 -wd4141 -wd4146 -wd4180 -wd4244 -wd4258 -wd4267 -wd4291 -wd4345 -wd4351 -wd4355 -wd4456 -wd4457 -wd4458 -wd4459 -wd4503 -wd4624 -wd4722 -wd4800 -wd4100 -wd4127 -wd4512 -wd4505 -wd4610 -wd4510 -wd4702 -wd4245 -wd4706 -wd4310 -wd4701 -wd4703 -wd4389 -wd4611 -wd4805 -wd4204 -wd4577 -wd4091 -wd4592 -wd4319 -wd4324 -w14062 -we4238 /Zc:inline /Oi /Zc:rvalueCast /MD /O2 /Ob2 -Itools\clang\tools\extra\clang-tidy\misc -ID:\buildslave\clang-x64-ninja-win7\llvm\tools\clang\tools\extra\clang-tidy\misc -ID:\buildslave\clang-x64-ninja-win7\llvm\tools\clang\include -Itools\clang\include -Iinclude -ID:\buildslave\clang-x64-ninja-win7\llvm\include -UNDEBUG /EHs-c- /GR- /showIncludes -DCLANG_ENABLE_ARCMT -DCLANG_ENABLE_OBJC_REWRITER -DCLANG_ENABLE_STATIC_ANALYZER -DGTEST_HAS_RTTI=0 -D_CRT_NONSTDC_NO_DEPRECATE -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_SECURE_NO_DEPRECATE -D_CRT_SECURE_NO_WARNINGS -D_DEBUG_POINTER_IMPL="" -D_GNU_SOURCE -D_HAS_EXCEPTIONS=0 -D_SCL_SECURE_NO_DEPRECATE -D_SCL_SECURE_NO_WARNINGS -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS /Fotools\clang\tools\extra\clang-tidy\misc\CMakeFiles\clangTidyMiscModule.dir\SuspiciousStringCompareCheck.cpp.obj /Fdtools\clang\tools\extra\clang-tidy\misc\CMakeFiles\clangTidyMiscModule.dir\ /FS -c D:\buildslave\clang-x64-ninja-win7\llvm\tools\clang\tools\extra\clang-tidy\misc\SuspiciousStringCompareCheck.cpp D:\buildslave\clang-x64-ninja-win7\llvm\tools\clang\tools\extra\clang-tidy\misc\SuspiciousStringCompareCheck.cpp(25) : error C2144: syntax error : 'char' should be preceded by ';' D:\buildslave\clang-x64-ninja-win7\llvm\tools\clang\tools\extra\clang-tidy\misc\SuspiciousStringCompareCheck.cpp(25) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int ninja: build stopped: subcommand failed. program finished with exit code 1