Github user viirya commented on a diff in the pull request:
https://github.com/apache/spark/pull/22544#discussion_r220765052
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercion.scala
---
@@ -974,6 +974,25 @@ object TypeCoercion {
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. Make sure that both key and values
+ // can not by null after the implicit cast operation by calling
forceNullable
--- End diff --
nit: I guess this is `can not be null`?
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