## What changes were proposed in this pull request?
When turning a Dataset to another Dataset, Spark will up cast the fields in the original Dataset to the type of corresponding fields in the target DataSet.
However, the current upcast behavior is a little weird, we don't allow up casting from string to numeric, but allow non-numeric types as the target, like boolean, date, etc.
As a result, `Seq("str").toDS.as[Int]` fails, but `Seq("str").toDS.as[Boolean]` works and throw NPE during execution.
The motivation of the up cast is to prevent things like runtime NPE, it's more reasonable to make up cast stricter.
This PR does 2 things:
1. rename `Cast.canSafeCast` to `Cast.canUpcast`, and support complex typres
2. remove `Cast.mayTruncate` and replace it with `!Cast.canUpcast`
Note that, the up cast change also affects persistent view resolution. But since we don't support changing column types of an existing table, there is no behavior change here.
## How was this patch tested?
new tests
Closes#21586 from cloud-fan/cast.
Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
03c9e8adee