diff --git a/include/clang/Basic/LangOptions.def b/include/clang/Basic/LangOptions.def
index 4f107e5..d2dff89 100644
--- a/include/clang/Basic/LangOptions.def
+++ b/include/clang/Basic/LangOptions.def
@@ -123,6 +123,7 @@
 BENIGN_LANGOPT(InlineVisibilityHidden , 1, 0, "hidden default visibility for inline C++ methods")
 BENIGN_LANGOPT(ParseUnknownAnytype, 1, 0, "__unknown_anytype")
 BENIGN_LANGOPT(DebuggerSupport , 1, 0, "debugger support")
+BENIGN_LANGOPT(DebuggerCastResultToId, 1, 0, "for 'po' in the debugger, cast the result to id if it is of unknown type")
 BENIGN_LANGOPT(AddressSanitizer , 1, 0, "AddressSanitizer enabled")
 
 BENIGN_LANGOPT(SpellChecking , 1, 1, "spell-checking")
diff --git a/include/clang/Driver/CC1Options.td b/include/clang/Driver/CC1Options.td
index fd3e32c..557e73b 100644
--- a/include/clang/Driver/CC1Options.td
+++ b/include/clang/Driver/CC1Options.td
@@ -651,6 +651,8 @@
   HelpText<"Enable parser support for the __unknown_anytype type; for testing purposes only">;
 def fdebugger_support : Flag<"-fdebugger-support">,
   HelpText<"Enable special debugger support behavior">;
+def fdebugger_cast_result_to_id : Flag<"-fdebugger-cast-result-to-id">,
+  HelpText<"Enable casting unknown expression results to id">;
 def fdeprecated_macro : Flag<"-fdeprecated-macro">,
   HelpText<"Defines the __DEPRECATED macro">;
 def fno_deprecated_macro : Flag<"-fno-deprecated-macro">,
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 23fe830..7fd2247 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -826,6 +826,8 @@
     Res.push_back("-funknown-anytype");
   if (Opts.DebuggerSupport)
     Res.push_back("-fdebugger-support");
+  if (Opts.DebuggerCastResultToId)
+    Res.push_back("-fdebugger-cast-result-to-id");
   if (Opts.DelayedTemplateParsing)
     Res.push_back("-fdelayed-template-parsing");
   if (Opts.Deprecated)
@@ -1885,6 +1887,7 @@
   Opts.FakeAddressSpaceMap = Args.hasArg(OPT_ffake_address_space_map);
   Opts.ParseUnknownAnytype = Args.hasArg(OPT_funknown_anytype);
   Opts.DebuggerSupport = Args.hasArg(OPT_fdebugger_support);
+  Opts.DebuggerCastResultToId = Args.hasArg(OPT_fdebugger_cast_result_to_id);
   Opts.AddressSanitizer = Args.hasArg(OPT_faddress_sanitizer);
   Opts.ApplePragmaPack = Args.hasArg(OPT_fapple_pragma_pack);
   Opts.CurrentModule = Args.getLastArgValue(OPT_fmodule_name);
diff --git a/lib/Sema/SemaExprCXX.cpp b/lib/Sema/SemaExprCXX.cpp
index b35ea80..d0e40ba 100644
--- a/lib/Sema/SemaExprCXX.cpp
+++ b/lib/Sema/SemaExprCXX.cpp
@@ -4767,7 +4767,7 @@
     return ExprError();
 
   // Top-level message sends default to 'id' when we're in a debugger.
-  if (getLangOptions().DebuggerSupport &&
+  if (getLangOptions().DebuggerCastResultToId &&
       FullExpr.get()->getType() == Context.UnknownAnyTy &&
       isa<ObjCMessageExpr>(FullExpr.get())) {
     FullExpr = forceUnknownAnyToType(FullExpr.take(), Context.getObjCIdType());
diff --git a/test/SemaObjC/debugger-cast-result-to-id.m b/test/SemaObjC/debugger-cast-result-to-id.m
new file mode 100644
index 0000000..86f4c3a
--- /dev/null
+++ b/test/SemaObjC/debugger-cast-result-to-id.m
@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -funknown-anytype -fsyntax-only -fdebugger-support -fdebugger-cast-result-to-id -verify %s
+
+extern __unknown_anytype test0;
+extern __unknown_anytype test1();
+
+void test_unknown_anytype_receiver() {
+  (void)(int)[[test0 unknownMethod] otherUnknownMethod];;
+  (void)(id)[[test1() unknownMethod] otherUnknownMethod];
+}
+// RUN: %clang_cc1 -funknown-anytype -fsyntax-only -fdebugger-support -fdebugger-cast-result-to-id -verify %s
+
+extern __unknown_anytype test0;
+extern __unknown_anytype test1();
+
+void test_unknown_anytype_receiver() {
+  (void)(int)[[test0 unknownMethod] otherUnknownMethod];;
+  (void)(id)[[test1() unknownMethod] otherUnknownMethod];
+}
+// RUN: %clang_cc1 -funknown-anytype -fsyntax-only -fdebugger-support -fdebugger-cast-result-to-id -verify %s
+
+extern __unknown_anytype test0;
+extern __unknown_anytype test1();
+
+void test_unknown_anytype_receiver() {
+  (void)(int)[[test0 unknownMethod] otherUnknownMethod];;
+  (void)(id)[[test1() unknownMethod] otherUnknownMethod];
+}
diff --git a/test/SemaObjC/unknown-anytype.m b/test/SemaObjC/unknown-anytype.m
index 89e45e4..38e058c 100644
--- a/test/SemaObjC/unknown-anytype.m
+++ b/test/SemaObjC/unknown-anytype.m
@@ -17,9 +17,9 @@
   int *ip = [test0 getIntPtr];
   float *fp = [test1() getFloatPtr];
   double *dp = [test1() getSomePtr]; // okay: picks first method found
-  [[test0 unknownMethod] otherUnknownMethod]; 
+  [[test0 unknownMethod] otherUnknownMethod]; // expected-error{{no known method '-otherUnknownMethod'; cast the message send to the method's return type}}
   (void)(int)[[test0 unknownMethod] otherUnknownMethod];;
-  [[test1() unknownMethod] otherUnknownMethod];
+  [[test1() unknownMethod] otherUnknownMethod]; // expected-error{{no known method '-otherUnknownMethod'; cast the message send to the method's return type}}
   (void)(id)[[test1() unknownMethod] otherUnknownMethod];
 
   if ([[test0 unknownMethod] otherUnknownMethod]) { // expected-error{{no known method '-otherUnknownMethod'; cast the message send to the method's return type}}