small fix for one failing test in Kotlin#713, where GroupBy.aggregate { this into "" } would break typing

Author: Jolanrensen

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/aggregation/GroupByReceiverImpl.kt

@@ -14,6 +14,7 @@ import org.jetbrains.kotlinx.dataframe.columns.ValueColumn
 import org.jetbrains.kotlinx.dataframe.columns.shortPath
 import org.jetbrains.kotlinx.dataframe.impl.aggregation.receivers.AggregateInternalDsl
 import org.jetbrains.kotlinx.dataframe.impl.api.AggregatedPivot
+import org.jetbrains.kotlinx.dataframe.impl.createStarProjectedType
 import org.jetbrains.kotlinx.dataframe.impl.createTypeWithArgument
 import org.jetbrains.kotlinx.dataframe.impl.getListType
 import kotlin.reflect.KType
@@ -118,7 +119,19 @@ internal class GroupByReceiverImpl<T>(override val df: DataFrame<T>, override va
  pivot.aggregator.values.clear()
  }
 
- is AggregateInternalDsl<*> -> yield(value.copy(value = value.value.df))
+ is AggregateInternalDsl<*> -> {
+ // Attempt to create DataFrame<Type> from AggregateInternalDsl<Type>
+ val dfType = value.type?.arguments?.firstOrNull()?.type
+ ?.let { DataFrame::class.createTypeWithArgument(it) }
+ ?: DataFrame::class.createStarProjectedType(nullable = false)
+
+ yield(
+ value.copy(
+ value = value.value.df,
+ type = dfType,
+ ),
+ )
+ }
 
  else -> values.add(value)
  }