Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -7074,6 +7074,7 @@
     // single void argument.
     // We let through "const void" here because Sema::GetTypeForDeclarator
     // already checks for that case.
+    ParmVarDecl *Prev = nullptr;
     if (FTIHasNonVoidParameters(FTI) && FTI.Params[0].Param) {
       for (unsigned i = 0, e = FTI.NumParams; i != e; ++i) {
         ParmVarDecl *Param = cast<ParmVarDecl>(FTI.Params[i].Param);
@@ -7081,8 +7082,17 @@
         Param->setDeclContext(NewFD);
         Params.push_back(Param);
 
-        if (Param->isInvalidDecl())
+        if (Param->isInvalidDecl()) {
           NewFD->setInvalidDecl();
+          if (!Param->hasDefaultArg() && Prev && Prev->hasDefaultArg()) {
+            for (unsigned j = i; j != 0; --j) {
+              ParmVarDecl *Param = cast<ParmVarDecl>(FTI.Params[j - 1].Param);
+              if (!Param->hasDefaultArg()) break;
+              Param->setDefaultArg(nullptr);
+            }
+          }
+        }
+        Prev = Param;
       }
     }
 
Index: test/SemaCXX/default1.cpp
===================================================================
--- test/SemaCXX/default1.cpp
+++ test/SemaCXX/default1.cpp
@@ -62,3 +62,7 @@
     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}} \
+                                             // expected-note{{candidate function not viable}}
+int pr20055_v = pr20055_f(0); // expected-error{{no matching function for call}}