Improve error message when patching cpp.ref type

Summary:
Currently cpp.ref type is already unsupported; this diff makes the error message more obvious.

While in the future we can support patching cpp.ref type, we should ask them try to migrate to thrift.box first, since cpp.ref is deprecated.

Reviewed By: vitaut

Differential Revision: D47195678

fbshipit-source-id: fc66096aa60560c189dec3b53e395906b39fc060

thrift/lib/cpp2/op/detail/StructPatch.h

@@ -367,6 +367,24 @@ class StructPatch : public BaseEnsurePatch<Patch, StructPatch<Patch>> {
     }
   }
 
+  ~StructPatch() {
+    if (false) {
+      // Implement this check in destructor to make sure it's instantiated.
+      op::for_each_ordinal<T>([](auto id) {
+        static_assert(
+            !apache::thrift::detail::is_shared_or_unique_ptr_v<
+                op::get_field_ref<T, decltype(id)>>,
+            "Patching cpp.ref field is unsupported since we cannot distinguish "
+            "unqualified and optional fields. Why? The type of `foo.field()` "
+            "is `std::unique_ptr` regardless Whether field is optional or "
+            "unqualified. In addition, Thrift Patch has different behavior "
+            "between optional and unqualified fields, e.g. `PatchOp::Clear` "
+            "will clear an optional field, but set an unqualified field to the "
+            "intrinsic default.");
+      });
+    }
+  }
+
  private:
   using Base::data_;
 };