Index: lib/Driver/ToolChains/Gnu.cpp
===================================================================
--- lib/Driver/ToolChains/Gnu.cpp
+++ lib/Driver/ToolChains/Gnu.cpp
@@ -1617,7 +1617,8 @@
     }
 
     // Then look for gcc installed alongside clang.
-    Prefixes.push_back(D.InstalledDir + "/..");
+    if (!D.InstalledDir.empty())
+      Prefixes.push_back(D.InstalledDir + "/..");
 
     // Then look for distribution supplied gcc installations.
     if (D.SysRoot.empty()) {