Index: include/clang/Driver/CLCompatOptions.td
===================================================================
--- include/clang/Driver/CLCompatOptions.td
+++ include/clang/Driver/CLCompatOptions.td
@@ -98,6 +98,7 @@
 def _SLASH_Tp : CLJoinedOrSeparate<"Tp">, HelpText<"Specify a C++ source file">,
   MetaVarName<"<filename>">;
 def _SLASH_TP : CLFlag<"TP">, HelpText<"Treat all source files as C++">;
+def _SLASH_Za : CLFlag<"Za">;
 
 
 // Ignored:
Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -3842,10 +3842,11 @@
       llvm_unreachable("Unexpected option ID.");
   }
 
-  // This provides POSIX compatibility (maps 'open' to '_open'), which most users
-  // want.  MSVC has a switch to turn off this autolinking, but it's not
-  // implemented in clang yet.
-  CmdArgs.push_back("--dependent-lib=oldnames");
+  if (!Args.hasArg(options::OPT__SLASH_Za)) {
+    // This provides POSIX compatibility (maps 'open' to '_open'),
+    // which most users want.
+    CmdArgs.push_back("--dependent-lib=oldnames");
+  }
 }
 
 void ClangAs::ConstructJob(Compilation &C, const JobAction &JA,
Index: test/Driver/cl-Za.c
===================================================================
--- /dev/null
+++ test/Driver/cl-Za.c
@@ -0,0 +1,11 @@
+// Don't attempt slash switches on msys bash.
+// REQUIRES: shell-preserves-root
+
+// Note: %s must be preceded by --, otherwise it may be interpreted as a
+// command-line option, e.g. on Mac where %s is commonly under /Users.
+
+// RUN: %clang_cl -### -- %s 2>&1 | FileCheck -check-prefix=DEFAULT %s
+// DEFAULT: "--dependent-lib=oldnames"
+
+// RUN: %clang_cl /Za -### -- %s 2>&1 | FileCheck -check-prefix=Za %s
+// Za-NOT: "--dependent-lib=oldnames"