Workaround MSVC bug when using TrailingObjects from a template.

MSVC appears to be getting confused as to whether OverloadToken is
supposed to be public or not.

This was discovered by code in Swift, and has been reported to
microsoft by hughbe:
https://connect.microsoft.com/VisualStudio/feedback/details/3116517

Differential Revision: https://reviews.llvm.org/D29880

llvm-svn: 296497

include/llvm/Support/TrailingObjects.h

@@ -294,7 +294,14 @@ class TrailingObjects : private trailing_objects_internal::TrailingObjectsImpl<
 
 public:
   // Make this (privately inherited) member public.
+#ifndef _MSC_VER
   using ParentType::OverloadToken;
+#else
+  // MSVC bug prevents the above from working, at least up through CL
+  // 19.10.24629.
+  template <typename T>
+  using OverloadToken = typename ParentType::template OverloadToken<T>;
+#endif
 
   /// Returns a pointer to the trailing object array of the given type
   /// (which must be one of those specified in the class template). The

unittests/Support/TrailingObjectsTest.cpp

@@ -236,3 +236,24 @@ TEST(TrailingObjects, Realignment) {
                 reinterpret_cast<char *>(C + 1) + 1, alignof(long))));
 }
 }
+
+// Test the use of TrailingObjects with a template class. This
+// previously failed to compile due to a bug in MSVC's member access
+// control/lookup handling for OverloadToken.
+template <typename Derived>
+class Class5Tmpl : private llvm::TrailingObjects<Derived, float, int> {
+  using TrailingObjects = typename llvm::TrailingObjects<Derived, float>;
+  friend TrailingObjects;
+
+  size_t numTrailingObjects(
+      typename TrailingObjects::template OverloadToken<float>) const {
+    return 1;
+  }
+
+  size_t numTrailingObjects(
+      typename TrailingObjects::template OverloadToken<int>) const {
+    return 2;
+  }
+};
+
+class Class5 : public Class5Tmpl<Class5> {};