Index: Sema/SemaDecl.cpp
===================================================================
--- Sema/SemaDecl.cpp
+++ Sema/SemaDecl.cpp
@@ -4645,12 +4645,20 @@
     unsigned DiagID;
     if (Record->isUnion()) {
       // C++ [class.union]p6:
+      // C++17 [class.union.anon]p2:
       //   Anonymous unions declared in a named namespace or in the
       //   global namespace shall be declared static.
+
+      // We need to get to the parent scope (extern {} is not a scope).
+      DeclContext *OwnerScope = Owner;
+      while (OwnerScope->getDeclKind() == Decl::LinkageSpec) {
+        OwnerScope = OwnerScope->getParent();
+      }
+
       if (DS.getStorageClassSpec() != DeclSpec::SCS_static &&
-          (isa<TranslationUnitDecl>(Owner) ||
-           (isa<NamespaceDecl>(Owner) &&
-            cast<NamespaceDecl>(Owner)->getDeclName()))) {
+          (isa<TranslationUnitDecl>(OwnerScope) ||
+           (isa<NamespaceDecl>(OwnerScope) &&
+            cast<NamespaceDecl>(OwnerScope)->getDeclName()))) {
         Diag(Record->getLocation(), diag::err_anonymous_union_not_static)
           << FixItHint::CreateInsertion(Record->getLocation(), "static ");
 
Index: SemaCXX/anonymous-union.cpp
===================================================================
--- SemaCXX/anonymous-union.cpp
+++ SemaCXX/anonymous-union.cpp
@@ -80,6 +80,10 @@
   float float_val;
 };
 
+extern "C++" {
+union { }; // expected-error{{anonymous unions at namespace or global scope must be declared 'static'}}
+}
+
 static union {
   int int_val2; // expected-note{{previous definition is here}}
   float float_val2;