Index: Makefile.inc1
===================================================================
--- Makefile.inc1
+++ Makefile.inc1
@@ -452,7 +452,7 @@
 #
 
 BOOTSTRAPPING?=	0
-# Keep these in sync
+# Keep these in sync -- see below for special case exception
 MINIMUM_SUPPORTED_OSREL?= 900044
 MINIMUM_SUPPORTED_REL?= 9.1
 
@@ -1556,10 +1556,24 @@
 .endif
 
 legacy: .PHONY
+# Temporary special case for automatically detecting the clang compiler issue
+# Note: 9.x didn't have FreeBSD_version bumps often enough, so you may need to
+# set BOOTSTRAPPING to 0 if you're stable/9 tree post-dates r286035 but is before
+# the version bump in r296219 (from July 29, 2015 -> Feb 29, 2016).
+.if ${BOOTSTRAPPING} != 0 && ${WANT_COIMPILER_TYPE} == "clang" && ${COMPILER_TYPE} == "clang" && 
+	${COMPILER_VERSION} <  30601
+.if ${BOOTSTRAPPING} > 10000000 && ${BOOTSTRAPPING} < 1002501
+	@echo "ERROR: Source upgrades from stable/10 prior to r286033 are not supported."; false
+.elif ${BOOTSTRAPPING} >  9000000 && ${BOOTSTRAPPING} < 903509
+	@echo "ERROR: Source upgrades from stable/9 prior to r286035 are not supported."; false
+.endif
+.else
 .if ${BOOTSTRAPPING} < ${MINIMUM_SUPPORTED_OSREL} && ${BOOTSTRAPPING} != 0
 	@echo "ERROR: Source upgrades from versions prior to ${MINIMUM_SUPPORTED_REL} are not supported."; \
 	false
 .endif
+.endif
+
 .for _tool in tools/build ${_elftoolchain_libs}
 	${_+_}@${ECHODIR} "===> ${_tool} (obj,includes,all,install)"; \
 	    cd ${.CURDIR}/${_tool}; \
Index: UPDATING
===================================================================
--- UPDATING
+++ UPDATING
@@ -29,21 +29,26 @@
 	machines to maximize performance.  (To completely disable malloc
 	debugging, define MALLOC_PRODUCTION in /etc/make.conf, or to merely
 	disable the most expensive debugging functionality run
-	"ln -s 'abort:false,junk:false' /etc/malloc.
+	"ln -s 'abort:false,junk:false' /etc/malloc.conf".)
 
 
 ****************************** SPECIAL WARNING: ******************************
 
 	Due to a bug in some versions of clang that's very hard to workaround in
 	the upgrade process, to upgrade to -current you must first upgrade
-	either stable/9 after r286035 or stable/10 after r286033 or current
-	after r286007 (including stable/11 and 11.0-RELEASE). These revisions
-	post-date 10.3-RELEASE and 9.3-RELEASE, so you'll need to take the
-	unusual step of upgrading to the tip of the stable branch before moving
-	to 11 or -current via a source upgrade.  stable/11 and 11.0-RELEASE
-	post-date the fix so you can move from them to -current. This differs
-	from the historical situation where one coule upgrade from anywhere on
-	the last couple of stable branches, so be careful.
+	either stable/9 after r286035 or stable/10 after r286033 (including
+	10.3-RELEASE) or current after r286007 (including stable/11 and
+	11.0-RELEASE). These revisions post-date the 10.2 and 9.3 releases, so
+	you'll need to take the unusual step of upgrading to the tip of the
+	stable branch before moving to 11 or -current via a source upgrade.
+	stable/11 and 11.0-RELEASE post-date the fix so you can move from them
+	to -current. This differs from the historical situation where one could
+	upgrade from anywhere on the last couple of stable branches, so be
+	careful.
+
+	If you're running a hybrid system on 9.x or 10.x with an updated clang
+	compiler or are using an supported external toolchain, the build system
+	will allow the upgrade. Otherwise it will print a reminder.
 
 ****************************** SPECIAL WARNING: ******************************