Github user maropu commented on a diff in the pull request:
https://github.com/apache/spark/pull/21747#discussion_r201678363
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
---
@@ -668,6 +668,56 @@ abstract class TernaryExpression extends Expression {
}
}
+/**
+ * A trait resolving nullable, containsNull, valueContainsNull flags of
the output date type.
+ * This logic is usually utilized by expressions combining data from
multiple child expressions
+ * of non-primitive types (e.g. [[CaseWhen]]).
+ */
+trait ComplexTypeMergingExpression extends Expression {
+
+ /**
+ * A collection of data types used for resolution the output type of the
expression. By default,
+ * data types of all child expressions. The collection must not be empty.
+ */
+ @transient
+ lazy val inputTypesForMerging: Seq[DataType] = children.map(_.dataType)
--- End diff --
nit: `@transient lazy val`
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