diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -1849,7 +1849,7 @@
 
 NamedDecl *NamedDecl::getUnderlyingDeclImpl() {
   NamedDecl *ND = this;
-  while (auto *UD = dyn_cast<UsingShadowDecl>(ND))
+  if (auto *UD = dyn_cast<UsingShadowDecl>(ND))
     ND = UD->getTargetDecl();
 
   if (auto *AD = dyn_cast<ObjCCompatibleAliasDecl>(ND))
diff --git a/clang/lib/AST/DeclCXX.cpp b/clang/lib/AST/DeclCXX.cpp
--- a/clang/lib/AST/DeclCXX.cpp
+++ b/clang/lib/AST/DeclCXX.cpp
@@ -2968,8 +2968,10 @@
                                  BaseUsingDecl *Introducer, NamedDecl *Target)
     : NamedDecl(K, DC, Loc, Name), redeclarable_base(C),
       UsingOrNextShadow(Introducer) {
-  if (Target)
+  if (Target) {
+    assert(!isa<UsingShadowDecl>(Target));
     setTargetDecl(Target);
+  }
   setImplicit();
 }