Third try for clang-win

appveyor.yml

@@ -14,6 +14,10 @@ branches:
 
 environment:
   matrix:
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      TOOLSET: clang-win
+      CLANG_WIN: 1
+      USER_CONFIG: "using clang-win : : clang-cl.exe : <compatibility>vc14 ;"
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2013
       TOOLSET: msvc-9.0
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2013
@@ -77,4 +81,5 @@ build: off
 test_script:
   - PATH=%ADDPATH%%PATH%
   - if not "%CXXSTD%" == "" set CXXSTD=cxxstd=%CXXSTD%
+  - if not "%USER_CONFIG%" == "" echo %USER_CONFIG% > %HOMEDRIVE%%HOMEPATH%\user-config.jam
   - b2 -j3 libs/system/test toolset=%TOOLSET% variant=debug,release %CXXSTD%

include/boost/system/error_code.hpp

@@ -48,6 +48,12 @@
 #  undef BOOST_SYSTEM_HAS_CONSTEXPR
 #endif
 
+#if defined(__clang__) && defined(_MSC_VER) && defined(_CPPLIB_VER)
+// Clang on Windows with MSVC headers, the constructor of std::error_category
+// is not constexpr at least up to VS2017 15.7.x (_MSVC_STL_UPDATE 201803)
+#  undef BOOST_SYSTEM_HAS_CONSTEXPR
+#endif
+
 #if defined(BOOST_SYSTEM_HAS_CONSTEXPR)
 # define BOOST_SYSTEM_CONSTEXPR constexpr
 #else

test/Jamfile.v2

@@ -76,11 +76,12 @@ project
       return $(result) ;
    }
 
-if [ os.environ UBSAN ]
+if [ os.environ UBSAN ] || [ os.environ CLANG_WIN ]
 {
    rule system-run ( sources + )
    {
       # The 03/11 tests are ODR violations, no point running them under -fsanitize=undefined
+      # Clang on Windows uses MSVC headers, which don't parse under 03/11
       return [ system-run- $(sources) ] ;
    }
 }