Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -997,7 +997,11 @@
   "dependent nested name specifier '%0' for friend template declaration is "
   "not supported; ignoring this friend declaration">,
   InGroup<UnsupportedFriend>;
-  
+def ext_friend_tag_redecl_outside_namespace : ExtWarn<
+  "unqualified friend declarations referring to types outside of the nearest "
+  "enclosing namespace is a Microsoft extension; add a nested name specifier to fix">,
+  InGroup<Microsoft>;
+
 def err_invalid_member_in_interface : Error<
   "%select{data member |non-public member function |static member function |"
           "user-declared constructor|user-declared destructor|operator |"
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -10910,6 +10910,10 @@
     //   the entity has been previously declared shall not consider
     //   any scopes outside the innermost enclosing namespace.
     //
+    // MSVC doesn't implement the above rule for types, so a friend tag
+    // declaration may be a redeclaration of a type declared in an enclosing
+    // scope.  They do implement this rule for friend functions.
+    //
     // Does it matter that this should be by scope instead of by
     // semantic context?
     if (!Previous.empty() && TUK == TUK_Friend) {
@@ -10920,13 +10924,22 @@
         DeclContext *DC = ND->getDeclContext()->getRedeclContext();
         if (DC->isFileContext() &&
             !EnclosingNS->Encloses(ND->getDeclContext())) {
-          F.erase();
+          if (!getLangOpts().MSVCCompat)
+            F.erase();
           FriendSawTagOutsideEnclosingNamespace = true;
         }
       }
       F.done();
+
+      if (FriendSawTagOutsideEnclosingNamespace && getLangOpts().MSVCCompat) {
+        Diag(KWLoc, diag::ext_friend_tag_redecl_outside_namespace);
+        // Set this back to false so that we don't inject a forward declaration
+        // into the enclosing namespace.  We already found a forward declaration
+        // in another namespace.
+        FriendSawTagOutsideEnclosingNamespace = false;
+      }
     }
-    
+
     // Note:  there used to be some attempt at recovery here.
     if (Previous.isAmbiguous())
       return nullptr;
Index: test/SemaCXX/MicrosoftCompatibility.cpp
===================================================================
--- test/SemaCXX/MicrosoftCompatibility.cpp
+++ test/SemaCXX/MicrosoftCompatibility.cpp
@@ -225,3 +225,58 @@
   template<int N> int *get_n() { return N; }   // expected-warning {{expression which evaluates to zero treated as a null pointer constant}}
   int *g_nullptr = get_n<0>();  // expected-note {{in instantiation of function template specialization}}
 }
+
+namespace enclosing_friend_decl {
+struct B;
+namespace ns {
+struct A {
+  friend struct B; // expected-warning-re {{unqualified friend declarations {{.*}} is a Microsoft extension}}
+protected:
+  A();
+};
+}
+struct B {
+  static void f() { ns::A x; }
+};
+}
+
+namespace enclosing_friend_qualified {
+struct B;
+namespace ns {
+struct A {
+  friend struct enclosing_friend_qualified::B; // Adding name specifiers fixes it.
+protected:
+  A();
+};
+}
+struct B {
+  static void f() { ns::A x; }
+};
+}
+
+namespace enclosing_friend_no_tag {
+struct B;
+namespace ns {
+struct A {
+  friend B; // Removing the tag decl fixes it.
+protected:
+  A();
+};
+}
+struct B {
+  static void f() { ns::A x; }
+};
+}
+
+namespace enclosing_friend_func {
+void f();
+namespace ns {
+struct A{
+  // Amusingly, in MSVC, this declares ns::f(), and doesn't find the outer f().
+  friend void f();
+protected:
+  A(); // expected-note {{declared protected here}}
+};
+}
+void f() { ns::A x; } // expected-error {{calling a protected constructor of class 'enclosing_friend_func::ns::A'}}
+}