[GitHub] [arrow] pitrou commented on a change in pull request #8343: ARROW-9147: [C++][Dataset] Support projection from null->any type

[GitBox <gi...@apache.org>]

pitrou commented on a change in pull request #8343:
URL: https://github.com/apache/arrow/pull/8343#discussion_r499739059



##########
File path: cpp/src/arrow/dataset/projector.cc
##########
@@ -46,6 +46,15 @@ Status CheckProjectable(const Schema& from, const Schema& to) {
                                from);
     }
 
+    if (from_field->type()->id() == Type::NA) {
+      // promotion from null to any type is supported
+      if (to_field->nullable()) continue;
+
+      return Status::TypeError("field ", to_field->ToString(),
+                               " is not nullable and but has type ", NullType(),

Review comment:
       "and but"?




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