Index: lib/CodeGen/CGExpr.cpp
===================================================================
--- lib/CodeGen/CGExpr.cpp
+++ lib/CodeGen/CGExpr.cpp
@@ -3181,6 +3181,10 @@
   }
 
   llvm::Value *Callee = EmitScalarExpr(E->getCallee());
+
+  if (isa<llvm::UndefValue>(Callee))
+    return RValue::get(Callee);
+
   return EmitCall(E->getCallee()->getType(), Callee, E, ReturnValue,
                   TargetDecl);
 }
Index: lib/CodeGen/CodeGenModule.cpp
===================================================================
--- lib/CodeGen/CodeGenModule.cpp
+++ lib/CodeGen/CodeGenModule.cpp
@@ -1666,6 +1666,21 @@
     Ty = getTypes().ConvertType(cast<ValueDecl>(GD.getDecl())->getType());
   
   StringRef MangledName = getMangledName(GD);
+
+  // Check that there are no declarations with the same mangled name. This check
+  // is valid only if one of declarations is a C++ method.
+  GlobalDecl OldGD;
+  if (lookupRepresentativeDecl(MangledName, OldGD) &&
+      (isa<CXXMethodDecl>(GD.getDecl()) ||
+       isa<CXXMethodDecl>(OldGD.getDecl())) &&
+      GD.getCanonicalDecl().getDecl() != OldGD.getCanonicalDecl().getDecl()) {
+    getDiags().Report(GD.getDecl()->getLocation(),
+                      diag::err_duplicate_mangled_name);
+    getDiags().Report(OldGD.getDecl()->getLocation(),
+                      diag::note_previous_definition);
+    return llvm::UndefValue::get(VoidPtrTy);
+  }
+
   return GetOrCreateLLVMFunction(MangledName, Ty, GD, ForVTable, DontDefer);
 }
 
@@ -2401,6 +2416,9 @@
     llvm::Constant *C =
         GetAddrOfFunction(GD, Ty, /*ForVTable=*/false, /*DontDefer*/ true);
 
+    if (isa<llvm::UndefValue>(C))
+      return;
+
     // Strip off a bitcast if we got one back.
     if (auto *CE = dyn_cast<llvm::ConstantExpr>(C)) {
       assert(CE->getOpcode() == llvm::Instruction::BitCast);
Index: test/CodeGenCXX/duplicate-mangled-name.cpp
===================================================================
--- test/CodeGenCXX/duplicate-mangled-name.cpp
+++ test/CodeGenCXX/duplicate-mangled-name.cpp
@@ -1,6 +1,8 @@
-// RUN: %clang_cc1 -triple %itanium_abi_triple -emit-llvm-only %s -verify
+// RUN: %clang_cc1 -triple %itanium_abi_triple -emit-llvm-only -DTEST1 %s -verify
+// RUN: %clang_cc1 -triple %itanium_abi_triple -emit-llvm-only -DTEST2 %s -verify
 
 // rdar://15522601
+#ifdef TEST1
 class MyClass {
  static void meth();
 };
@@ -8,3 +10,22 @@
 extern "C" {
   void _ZN7MyClass4methEv() { } // expected-error {{definition with same mangled name as another definition}}
 }
+
+#elif TEST2
+
+extern "C" void _ZN1TD1Ev(); // expected-error {{definition with same mangled name as another definition}}
+struct T {
+    ~T() {} // expected-note {{previous}}
+};
+
+int main() {
+  _ZN1TD1Ev();
+  T t;
+}
+
+#else
+
+#error Unknown test mode
+
+#endif
+