pitrou commented on code in PR #36018:
URL: https://github.com/apache/arrow/pull/36018#discussion_r1254384855


##########
cpp/src/arrow/array/data.cc:
##########
@@ -384,26 +382,31 @@ void ArraySpan::FillFromScalar(const Scalar& value) {
       this->child_data[i].FillFromScalar(*scalar.value[i]);
     }
   } else if (is_union(type_id)) {
+    // Dense union needs scratch space to store both offsets and a type code
+    struct UnionScratchSpace {
+      alignas(int64_t) uint8_t type_code;
+      alignas(int64_t) uint8_t offsets[sizeof(int32_t) * 2];
+    };
+    static_assert(sizeof(UnionScratchSpace) <= 
sizeof(UnionScalar::scratch_space_));
+    auto* union_scratch_space =
+        new (checked_cast<const UnionScalar&>(value).scratch_space_) 
UnionScratchSpace{};

Review Comment:
   Why do you want to explicit create `UnionScratchSpace`? It should be a 
trivial type AFAICT.
   
   I was only mentioning padding bits in the event that your code was 
attempting to initialize them :-)



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