Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -4724,6 +4724,9 @@
 def err_standalone_class_nested_name_specifier : Error<
   "forward declaration of %select{class|struct|interface|union|enum}0 cannot "
   "have a nested name specifier">;
+def ext_standalone_class_nested_name_specifier : ExtWarn<
+  "forward declaration of %select{class|struct|interface|union|enum}0 with a "
+  "nested name specifier is a Microsoft extension">, InGroup<Microsoft>;
 def err_typecheck_sclass_func : Error<"illegal storage class on function">;
 def err_static_block_func : Error<
   "function declared in block scope cannot have 'static' storage class">;
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -3286,7 +3286,10 @@
     // nested-name-specifier unless it is an explicit instantiation
     // or an explicit specialization.
     // Per C++ [dcl.enum]p1, an opaque-enum-declaration can't either.
-    Diag(SS.getBeginLoc(), diag::err_standalone_class_nested_name_specifier)
+    Diag(SS.getBeginLoc(),
+         getLangOpts().MicrosoftExt ?
+           diag::ext_standalone_class_nested_name_specifier :
+           diag::err_standalone_class_nested_name_specifier)
       << (DS.getTypeSpecType() == DeclSpec::TST_class ? 0 :
           DS.getTypeSpecType() == DeclSpec::TST_struct ? 1 :
           DS.getTypeSpecType() == DeclSpec::TST_interface ? 2 :
Index: test/SemaCXX/MicrosoftExtensions.cpp
===================================================================
--- test/SemaCXX/MicrosoftExtensions.cpp
+++ test/SemaCXX/MicrosoftExtensions.cpp
@@ -418,3 +418,19 @@
   _Static_assert(__alignof(s1) == 8, "");
   _Static_assert(__alignof(s2) == 4, "");
 }
+
+namespace forward_decl_nested_name_NS {
+  struct X;
+  template<typename T> struct Y;
+}
+
+struct forward_decl_nested_name_NS::X; // expected-warning {{forward declaration of struct with a nested name specifier is a Microsoft extension}}
+template<typename T> struct forward_decl_nested_name_NS::Y; // expected-warning {{forward declaration of struct with a nested name specifier is a Microsoft extension}}
+
+struct forward_decl_nested_name_struct {
+  struct A;
+  union B;
+};
+
+struct forward_decl_nested_name_struct::A; // expected-warning {{forward declaration of struct with a nested name specifier is a Microsoft extension}}
+union forward_decl_nested_name_struct::B; // expected-warning {{forward declaration of union with a nested name specifier is a Microsoft extension}}