Index: libcxx/docs/ReleaseNotes.rst
===================================================================
--- libcxx/docs/ReleaseNotes.rst
+++ libcxx/docs/ReleaseNotes.rst
@@ -41,3 +41,10 @@
 API Changes
 -----------
 - Building libc++ for Mac OSX 10.6 is not supported anymore.
+- Starting with LLVM 8.0.0, users that wish to link together translation units
+  built with different versions of libc++’s headers into the same final linked
+  image MUST define the _LIBCPP_HIDE_FROM_ABI_PER_TU macro to 1 when building
+  those translation units. Not defining _LIBCPP_HIDE_FROM_ABI_PER_TU to 1 and
+  linking translation units built with different versions of libc++’s headers
+  together may lead to ODR violations and ABI issues. On the flipside, code
+  size improvements should be expected for everyone not defining the macro.
Index: libcxx/include/__config
===================================================================
--- libcxx/include/__config
+++ libcxx/include/__config
@@ -787,6 +787,15 @@
 #  define _LIBCPP_INTERNAL_LINKAGE _LIBCPP_ALWAYS_INLINE
 #endif
 
+#if __has_attribute(exclude_from_explicit_instantiation)
+#  define _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION __attribute__ ((__exclude_from_explicit_instantiation__))
+#else
+   // Try to approximate the effect of exclude_from_explicit_instantiation
+   // (which is that entities are not assumed to be provided by explicit
+   // template instantitations in the dylib) by always inlining those entities.
+#  define _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION _LIBCPP_ALWAYS_INLINE
+#endif
+
 #ifndef _LIBCPP_HIDE_FROM_ABI_PER_TU
 #  ifndef _LIBCPP_HIDE_FROM_ABI_PER_TU_BY_DEFAULT
 #    define _LIBCPP_HIDE_FROM_ABI_PER_TU 0
@@ -799,7 +808,7 @@
 #  if _LIBCPP_HIDE_FROM_ABI_PER_TU
 #    define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_INTERNAL_LINKAGE
 #  else
-#    define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_ALWAYS_INLINE
+#    define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION
 #  endif
 #endif