Index: cfe/trunk/include/clang/Basic/DiagnosticDriverKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticDriverKinds.td
+++ cfe/trunk/include/clang/Basic/DiagnosticDriverKinds.td
@@ -247,6 +247,10 @@
 def warn_drv_invoking_fallback : Warning<"falling back to %0">,
   InGroup<Fallback>;
 
+def warn_slash_u_filename : Warning<"'/U%0' treated as the '/U' option">,
+  InGroup<DiagGroup<"slash-u-filename">>;
+def note_use_dashdash : Note<"Use '--' to treat subsequent arguments as filenames">;
+
 def err_drv_ropi_rwpi_incompatible_with_pic : Error<
   "embedded and GOT-based position independence are incompatible">;
 def err_drv_ropi_incompatible_with_cxx : Error<
Index: cfe/trunk/lib/Driver/Driver.cpp
===================================================================
--- cfe/trunk/lib/Driver/Driver.cpp
+++ cfe/trunk/lib/Driver/Driver.cpp
@@ -1583,6 +1583,14 @@
         Diag(clang::diag::err_drv_unknown_language) << A->getValue();
         InputType = types::TY_Object;
       }
+    } else if (A->getOption().getID() == options::OPT__SLASH_U) {
+      assert(A->getNumValues() == 1 && "The /U option has one value.");
+      StringRef Val = A->getValue(0);
+      if (Val.find_first_of("/\\") != StringRef::npos) {
+        // Warn about e.g. "/Users/me/myfile.c".
+        Diag(diag::warn_slash_u_filename) << Val;
+        Diag(diag::note_use_dashdash);
+      }
     }
   }
   if (CCCIsCPP() && Inputs.empty()) {
Index: cfe/trunk/test/Driver/cl-options.c
===================================================================
--- cfe/trunk/test/Driver/cl-options.c
+++ cfe/trunk/test/Driver/cl-options.c
@@ -442,6 +442,12 @@
 // Xclang: "-cc1"
 // Xclang: "hellocc1"
 
+// Files under /Users are often confused with the /U flag. (This could happen
+// for other flags too, but this is the one people run into.)
+// RUN: %clang_cl /c /Users/me/myfile.c -### 2>&1 | FileCheck -check-prefix=SlashU %s
+// SlashU: warning: '/Users/me/myfile.c' treated as the '/U' option
+// SlashU: note: Use '--' to treat subsequent arguments as filenames
+
 // RTTI is on by default. /GR- controls -fno-rtti-data.
 // RUN: %clang_cl /c /GR- -### -- %s 2>&1 | FileCheck -check-prefix=NoRTTI %s
 // NoRTTI: "-fno-rtti-data"