Index: lib/Sema/SemaExprMember.cpp
===================================================================
--- lib/Sema/SemaExprMember.cpp	(revision 157593)
+++ lib/Sema/SemaExprMember.cpp	(working copy)
@@ -115,7 +115,7 @@
     NamedDecl *D = *I;
 
     if (D->isCXXInstanceMember()) {
-      if (dyn_cast<FieldDecl>(D))
+      if (dyn_cast<FieldDecl>(D) || dyn_cast<IndirectFieldDecl>(D))
         isField = true;
 
       CXXRecordDecl *R = cast<CXXRecordDecl>(D->getDeclContext());
Index: test/SemaCXX/anonymous-union-cxx11.cpp
===================================================================
--- test/SemaCXX/anonymous-union-cxx11.cpp	(revision 0)
+++ test/SemaCXX/anonymous-union-cxx11.cpp	(working copy)
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify -pedantic %s
+
+namespace PR12866 {
+  struct bar {
+    union {
+      int member;
+    };
+  };
+
+  void foo( void ) {
+    (void)sizeof(bar::member);
+  }
+}