[libc++] Explain XFAILs with std::uncaught_exceptions test
diff --git a/libcxx/test/std/language.support/support.exception/uncaught/uncaught_exceptions.pass.cpp b/libcxx/test/std/language.support/support.exception/uncaught/uncaught_exceptions.pass.cpp
index c498b57..966e8c6 100644
--- a/libcxx/test/std/language.support/support.exception/uncaught/uncaught_exceptions.pass.cpp
+++ b/libcxx/test/std/language.support/support.exception/uncaught/uncaught_exceptions.pass.cpp
@@ -9,13 +9,17 @@
// UNSUPPORTED: libcpp-no-exceptions
// XFAIL: libcpp-no-exceptions
-// XFAIL: macosx10.7
-// XFAIL: macosx10.8
-// XFAIL: macosx10.9
-// XFAIL: macosx10.10
-// XFAIL: macosx10.11
-// XFAIL: with_system_cxx_lib=macosx10.12
+// std::uncaught_exceptions() was introduced in the dylib on Mac OS 10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+// XFAIL: with_system_cxx_lib=macosx10.8
+// XFAIL: with_system_cxx_lib=macosx10.7
+
+// However, std::uncaught_exceptions() gives the wrong answer in Mac OS 10.12
+// and 10.13, where it only gives 0 or 1. This was fixed later.
// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
// test uncaught_exceptions
