Index: test/tsan/libcxx/lit.local.cfg
===================================================================
--- test/tsan/libcxx/lit.local.cfg
+++ test/tsan/libcxx/lit.local.cfg
@@ -5,6 +5,8 @@
 
 root = getRoot(config)
 
-if not root.has_libcxx:
+# Only run if we an instrumented libcxx.  On Darwin, run always (we have
+# interceptors to support the system-provided libcxx).
+if not root.has_libcxx and root.host_os != 'Darwin':
   config.unsupported = True