Index: cfe/trunk/include/clang/Sema/Sema.h
===================================================================
--- cfe/trunk/include/clang/Sema/Sema.h
+++ cfe/trunk/include/clang/Sema/Sema.h
@@ -1625,7 +1625,7 @@
   void ActOnParamUnparsedDefaultArgument(Decl *param,
                                          SourceLocation EqualLoc,
                                          SourceLocation ArgLoc);
-  void ActOnParamDefaultArgumentError(Decl *param);
+  void ActOnParamDefaultArgumentError(Decl *param, SourceLocation EqualLoc);
   bool SetParamDefaultArgument(ParmVarDecl *Param, Expr *DefaultArg,
                                SourceLocation EqualLoc);
 
Index: cfe/trunk/lib/Parse/ParseCXXInlineMethods.cpp
===================================================================
--- cfe/trunk/lib/Parse/ParseCXXInlineMethods.cpp
+++ cfe/trunk/lib/Parse/ParseCXXInlineMethods.cpp
@@ -337,7 +337,8 @@
       } else
         DefArgResult = ParseAssignmentExpression();
       if (DefArgResult.isInvalid())
-        Actions.ActOnParamDefaultArgumentError(LM.DefaultArgs[I].Param);
+        Actions.ActOnParamDefaultArgumentError(LM.DefaultArgs[I].Param,
+                                               EqualLoc);
       else {
         if (!TryConsumeToken(tok::cxx_defaultarg_end)) {
           // The last two tokens are the terminator and the saved value of
Index: cfe/trunk/lib/Parse/ParseDecl.cpp
===================================================================
--- cfe/trunk/lib/Parse/ParseDecl.cpp
+++ cfe/trunk/lib/Parse/ParseDecl.cpp
@@ -5436,7 +5436,7 @@
           if (!ConsumeAndStoreInitializer(*DefArgToks, CIK_DefaultArgument)) {
             delete DefArgToks;
             DefArgToks = nullptr;
-            Actions.ActOnParamDefaultArgumentError(Param);
+            Actions.ActOnParamDefaultArgumentError(Param, EqualLoc);
           } else {
             // Mark the end of the default argument so that we know when to
             // stop when we parse it later on.
@@ -5465,7 +5465,7 @@
           } else
             DefArgResult = ParseAssignmentExpression();
           if (DefArgResult.isInvalid()) {
-            Actions.ActOnParamDefaultArgumentError(Param);
+            Actions.ActOnParamDefaultArgumentError(Param, EqualLoc);
             SkipUntil(tok::comma, tok::r_paren, StopAtSemi | StopBeforeMatch);
           } else {
             // Inform the actions module about the default argument
Index: cfe/trunk/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaDeclCXX.cpp
+++ cfe/trunk/lib/Sema/SemaDeclCXX.cpp
@@ -344,13 +344,16 @@
 
 /// ActOnParamDefaultArgumentError - Parsing or semantic analysis of
 /// the default argument for the parameter param failed.
-void Sema::ActOnParamDefaultArgumentError(Decl *param) {
+void Sema::ActOnParamDefaultArgumentError(Decl *param,
+                                          SourceLocation EqualLoc) {
   if (!param)
     return;
 
   ParmVarDecl *Param = cast<ParmVarDecl>(param);
   Param->setInvalidDecl();
   UnparsedDefaultArgLocs.erase(Param);
+  Param->setDefaultArg(new(Context)
+                       OpaqueValueExpr(EqualLoc, Param->getType(), VK_RValue));
 }
 
 /// CheckExtraCXXDefaultArguments - Check for any extra default
Index: cfe/trunk/test/SemaCXX/default1.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/default1.cpp
+++ cfe/trunk/test/SemaCXX/default1.cpp
@@ -62,3 +62,6 @@
     j(2, 3); // expected-error{{too many arguments to function call, expected at most single argument 'f', have 2}}
   }
 }
+
+int pr20055_f(int x = 0, int y = UNDEFINED); // expected-error{{use of undeclared identifier}}
+int pr20055_v = pr20055_f(0);