Index: lib/CodeGen/CodeGenModule.cpp
===================================================================
--- lib/CodeGen/CodeGenModule.cpp
+++ lib/CodeGen/CodeGenModule.cpp
@@ -1370,6 +1370,31 @@
                               /*DontDefer=*/false);
       return;
     }
+
+    // Compute the function info and LLVM type.
+    const CGFunctionInfo &FI = getTypes().arrangeGlobalDeclaration(GD);
+    llvm::FunctionType *Ty = getTypes().GetFunctionType(FI);
+    llvm::GlobalValue *GV = nullptr;
+
+    // Get or create the prototype for the function.
+    llvm::Constant *C =
+        GetAddrOfFunction(GD, Ty, /*ForVTable=*/false, /*DontDefer*/true);
+
+    // Strip off a bitcast if we got one back.
+    if (auto *CE = dyn_cast<llvm::ConstantExpr>(C)) {
+      assert(CE->getOpcode() == llvm::Instruction::BitCast);
+      GV = cast<llvm::GlobalValue>(CE->getOperand(0));
+    } else {
+      GV = cast<llvm::GlobalValue>(C);
+    }
+
+    if (!GV->isDeclaration()) {
+      getDiags().Report(FD->getLocation(), diag::err_duplicate_mangled_name);
+      GlobalDecl OldGD = Manglings.lookup(GV->getName());
+      if (auto *Prev = OldGD.getDecl())
+        getDiags().Report(Prev->getLocation(), diag::note_previous_definition);
+      return;
+    }
   } else {
     const auto *VD = cast<VarDecl>(Global);
     assert(VD->isFileVarDecl() && "Cannot emit local var decl as global.");
@@ -2412,14 +2437,6 @@
     }
   }
 
-  if (!GV->isDeclaration()) {
-    getDiags().Report(D->getLocation(), diag::err_duplicate_mangled_name);
-    GlobalDecl OldGD = Manglings.lookup(GV->getName());
-    if (auto *Prev = OldGD.getDecl())
-      getDiags().Report(Prev->getLocation(), diag::note_previous_definition);
-    return;
-  }
-
   if (GV->getType()->getElementType() != Ty) {
     // If the types mismatch then we have to rewrite the definition.
     assert(GV->isDeclaration() && "Shouldn't replace non-declaration");
Index: test/CodeGenCXX/duplicate-mangled-name2.cpp
===================================================================
--- test/CodeGenCXX/duplicate-mangled-name2.cpp
+++ test/CodeGenCXX/duplicate-mangled-name2.cpp
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -triple %itanium_abi_triple -emit-llvm-only %s -verify
+// RUN: %clang_cc1 -triple %itanium_abi_triple -femit-all-decls -emit-llvm-only %s -verify
+
+void foo(void) __asm("_ZN1SC2Ev");
+void foo(void) { } // expected-note {{previous}}
+
+struct S {
+  S() {} // expected-error {{definition with same mangled name as another definition}}
+} s;
\ No newline at end of file