Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -5522,6 +5522,19 @@
   return true;
 }
 
+/// \brief Returns true if given declaration is TU-scoped and externally
+/// visible.
+static bool isDeclTUScopedExternallyVisible(const Decl *D) {
+  if (auto *FD = dyn_cast<FunctionDecl>(D))
+    return (FD->getDeclContext()->isTranslationUnit() || FD->isExternC()) &&
+           FD->hasExternalFormalLinkage();
+  else if (auto *VD = dyn_cast<VarDecl>(D))
+    return (VD->getDeclContext()->isTranslationUnit() || VD->isExternC()) &&
+           VD->hasExternalFormalLinkage();
+
+  llvm_unreachable("Unknown type of decl!");
+}
+
 NamedDecl *
 Sema::ActOnVariableDeclarator(Scope *S, Declarator &D, DeclContext *DC,
                               TypeSourceInfo *TInfo, LookupResult &Previous,
@@ -5946,7 +5959,8 @@
 
     NewVD->addAttr(::new (Context) AsmLabelAttr(SE->getStrTokenLoc(0),
                                                 Context, Label, 0));
-  } else if (!ExtnameUndeclaredIdentifiers.empty()) {
+  } else if (!ExtnameUndeclaredIdentifiers.empty() &&
+             isDeclTUScopedExternallyVisible(NewVD)) {
     llvm::DenseMap<IdentifierInfo*,AsmLabelAttr*>::iterator I =
       ExtnameUndeclaredIdentifiers.find(NewVD->getIdentifier());
     if (I != ExtnameUndeclaredIdentifiers.end()) {
@@ -7468,7 +7482,8 @@
     StringLiteral *SE = cast<StringLiteral>(E);
     NewFD->addAttr(::new (Context) AsmLabelAttr(SE->getStrTokenLoc(0), Context,
                                                 SE->getString(), 0));
-  } else if (!ExtnameUndeclaredIdentifiers.empty()) {
+  } else if (!ExtnameUndeclaredIdentifiers.empty() &&
+             isDeclTUScopedExternallyVisible(NewFD)) {
     llvm::DenseMap<IdentifierInfo*,AsmLabelAttr*>::iterator I =
       ExtnameUndeclaredIdentifiers.find(NewFD->getIdentifier());
     if (I != ExtnameUndeclaredIdentifiers.end()) {
Index: test/CodeGen/redefine_extname.c
===================================================================
--- test/CodeGen/redefine_extname.c
+++ test/CodeGen/redefine_extname.c
@@ -13,3 +13,14 @@
 // CHECK:   call i32 @real()
 // Check that this also works with variables names
 // CHECK:   load i32, i32* @alias
+
+// This is a case when redefenition is deferred *and* we have a local of the
+// same name. PR23923.
+#pragma redefine_extname foo bar
+int f() {
+  int foo = 0;
+  return foo;
+}
+extern int foo() { return 1; }
+// CHECK: define i32 @bar()
+
Index: test/CodeGenCXX/redefine_extname.cpp
===================================================================
--- test/CodeGenCXX/redefine_extname.cpp
+++ test/CodeGenCXX/redefine_extname.cpp
@@ -8,7 +8,7 @@
   int statvfs64(struct statvfs64 *);
 }
 
-void foo() {
+void some_func() {
   struct statvfs64 st;
   statvfs64(&st);
 // Check that even if there is a structure with redefined name before the
@@ -16,3 +16,15 @@
 // CHECK:  call i32 @statvfs(%struct.statvfs64* %st)
 }
 
+// This is a case when redefenition is deferred *and* we have a local of the
+// same name. PR23923.
+#pragma redefine_extname foo bar
+int f() {
+  int foo = 0;
+  return foo;
+}
+extern "C" {
+  int foo() { return 1; }
+// CHECK: define i32 @bar()
+}
+