Github user viirya commented on a diff in the pull request: https://github.com/apache/spark/pull/22544#discussion_r220140660 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercion.scala --- @@ -971,9 +971,36 @@ object TypeCoercion { case (ArrayType(fromType, true), ArrayType(toType: DataType, false)) => null case (ArrayType(fromType, false), ArrayType(toType: DataType, false)) - if !Cast.forceNullable(fromType, toType) => + if !Cast.forceNullable(fromType, toType) => implicitCast(fromType, toType).map(ArrayType(_, false)).orNull + // Implicit cast between Map types. + // Follows the same semantics of implicit casting between two array types. + // Refer to documentation above. + case (MapType(fromKeyType, fromValueType, fn), MapType(toKeyType, toValueType, true)) => + val newFromType = implicitCast(fromKeyType, toKeyType).orNull + val newValueType = implicitCast(fromValueType, toValueType).orNull + if (newFromType != null && newValueType != null + && (!newFromType.sameType(fromKeyType) || !newValueType.sameType(fromValueType))) { + MapType(newFromType, newValueType, true) + } else { + null + } + + case (MapType(fromKeyType, fromValueType, true), MapType(toKeyType, toValueType, false)) => + null + + case (MapType(fromKeyType, fromValueType, false), MapType(toKeyType, toValueType, false)) + if !Cast.forceNullable(fromValueType, toValueType) => + val newFromType = implicitCast(fromKeyType, toKeyType).orNull --- End diff -- ditto, `newKeyType`.
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