Smallfu666 opened a new issue, #6200:
URL: https://github.com/apache/datafusion-comet/issues/6200
### Describe the bug
`CometCast.isSupported` combines the support levels of nested casts by
returning the first one
that is not `Compatible`. That happens in the struct to struct arm, the
struct to string arm, and
the map arm (key before value). When an earlier child is `Incompatible` and
a later one is
`Unsupported`, the whole cast reports `Incompatible`.
With `spark.comet.expr.allowIncompatible=true` that cast then runs natively,
even though one of its
children is not supported natively. Without the flag it goes to the codegen
dispatcher, like
`Unsupported` would.
Today the only `Incompatible` cast is a negative-scale decimal to string with
`spark.sql.legacy.allowNegativeScaleOfDecimal=false`, and such a decimal
cannot normally be
constructed while that flag is off. So this is hard to reach. It came up in
#6179, where a TRY
cast of a map key that can fail is `Unsupported` and could be masked this
way.
### Steps to reproduce
Found by reading the code, not reproduced end to end. For example:
```scala
CometCast.isSupported(
StructType(Seq(StructField("a", DecimalType(10, -2)), StructField("b",
MapType(LongType, IntegerType)))),
StructType(Seq(StructField("a", StringType), StructField("b",
MapType(IntegerType, IntegerType)))),
None,
CometEvalMode.TRY)
```
reports `Incompatible` (from field `a`) rather than `Unsupported` (from
field `b`).
### Expected behavior
`Unsupported` in any child wins over `Incompatible`, and `Incompatible` wins
over `Compatible`.
### Additional context
The struct to struct arm has returned the first non-Compatible field since
712658e29 (2024).
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