[GitHub] [arrow] kszucs commented on pull request #10093: ARROW-12420: [C++/Dataset] Reading null columns as dictionary not longer possible

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kszucs commented on pull request #10093:
URL: https://github.com/apache/arrow/pull/10093#issuecomment-822068562


   I reproduced the problem using a simple cast test from null array to a dictionary array:
   
   ```cpp
   TEST(Cast, FromNullToDictionary) {
     auto from = std::make_shared<NullArray>(10);
     auto to_type = dictionary(int8(), boolean());
   
     ASSERT_OK_AND_ASSIGN(auto expected, MakeArrayOfNull(to_type, 10));
     CheckCast(from, expected);
   }
   ```
   
   Somewhere `ArrayData::dictionary` gets set to nullptr or not set at all. I don't have a solution yet, it's a but hard to debug due to the Datum variant. 


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