Workaround compiler issue by hoisting access token guard outside of the function

Author: sylveon

strings/base_security.h

@@ -69,31 +69,32 @@ WINRT_EXPORT namespace winrt
             check_bool(WINRT_IMPL_SetThreadToken(nullptr, get()));
         }
 
-        /*TODO: figure out why compiler ICEs auto operator()() const
-        {
-            struct guard
-            {
-                guard(access_token&& previous) noexcept : m_previous(std::move(previous))
-                {
-                }
-
-                ~guard()
-                {
-                    m_previous.revert();
-                }
-
-                guard(guard const&)
-                {
-                    // A Visual C++ compiler bug (550631) requires the copy constructor even though it is never called.
-                    WINRT_ASSERT(false);
-                }
+        auto operator()() const;
+    };
+}
 
-            private:
+namespace winrt::impl
+{
+    struct access_token_guard
+    {
+        access_token_guard(access_token&& previous) noexcept : m_previous(std::move(previous))
+        {
+        }
 
-                access_token const m_previous;
-            };
+        ~access_token_guard()
+        {
+            m_previous.revert();
+        }
 
-            return guard(impersonate());
-        }*/
+    private:
+        access_token const m_previous;
     };
 }
+
+WINRT_EXPORT namespace winrt
+{
+    auto access_token::operator()() const
+    {
+        return impl::access_token_guard(impersonate());
+    }
+}