added tests and a fix for when an empty column was inferred as Any instead of Nothing

Author: Jolanrensen

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

@@ -197,15 +197,19 @@ internal fun commonParent(vararg classes: KClass<*>): KClass<*>? = commonParent(
 internal fun Iterable<KClass<*>>.withMostSuperclasses(): KClass<*>? = maxByOrNull { it.allSuperclasses.size }
 
 internal fun Iterable<KClass<*>>.createType(nullable: Boolean, upperBound: KType? = null): KType =
-    if (upperBound == null) {
-        (withMostSuperclasses() ?: Any::class).createStarProjectedType(nullable)
-    } else {
-        val upperClass = upperBound.classifier as KClass<*>
-        val baseClass = filter { it.isSubclassOf(upperClass) }.withMostSuperclasses() ?: withMostSuperclasses()
-        if (baseClass == null) {
-            upperBound.withNullability(nullable)
-        } else {
-            upperBound.projectTo(baseClass).withNullability(nullable)
+    when {
+        !iterator().hasNext() -> upperBound?.withNullability(nullable) ?: nothingType(nullable)
+
+        upperBound == null -> (withMostSuperclasses() ?: Any::class).createStarProjectedType(nullable)
+
+        else -> {
+            val upperClass = upperBound.classifier as KClass<*>
+            val baseClass = filter { it.isSubclassOf(upperClass) }.withMostSuperclasses() ?: withMostSuperclasses()
+            if (baseClass == null) {
+                upperBound.withNullability(nullable)
+            } else {
+                upperBound.projectTo(baseClass).withNullability(nullable)
+            }
         }
     }
 

core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/constructors.kt

@@ -1,6 +1,8 @@
 package org.jetbrains.kotlinx.dataframe.api
 
 import io.kotest.matchers.shouldBe
+import org.jetbrains.kotlinx.dataframe.impl.nothingType
+import org.jetbrains.kotlinx.dataframe.type
 import org.junit.Test
 
 class ConstructorsTests {
@@ -24,4 +26,10 @@ class ConstructorsTests {
         df.columnsCount() shouldBe 2
         df.columnNames() shouldBe listOf(column.name(), "${column.name()}1")
     }
+
+    @Test
+    fun `dataFrameOf with nothing columns`() {
+        dataFrameOf("a" to emptyList())["a"].type shouldBe nothingType(false)
+        dataFrameOf("a" to listOf(null))["a"].type shouldBe nothingType(true)
+    }
 }

core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/types/UtilTests.kt

@@ -97,8 +97,8 @@ class UtilTests {
 
     @Test
     fun `createType test`() {
-        emptyList<KClass<*>>().createType(nullable = false) shouldBe typeOf<Any>()
-        emptyList<KClass<*>>().createType(nullable = true) shouldBe typeOf<Any?>()
+        emptyList<KClass<*>>().createType(nullable = false) shouldBe nothingType(nullable = false)
+        emptyList<KClass<*>>().createType(nullable = true) shouldBe nothingType(nullable = true)
 
         listOf(Nothing::class).createType(nullable = false) shouldBe nothingType(nullable = false)
         listOf(Nothing::class).createType(nullable = true) shouldBe nothingType(nullable = true)
@@ -111,6 +111,9 @@ class UtilTests {
 
         listOf(Nothing::class).commonType(false) shouldBe nothingType(nullable = false)
         listOf(Nothing::class).commonType(true) shouldBe nothingType(nullable = true)
+
+        emptyList<KClass<*>>().commonType(false, null) shouldBe nothingType(nullable = false)
+        emptyList<KClass<*>>().commonType(true, null) shouldBe nothingType(nullable = true)
     }
 
     val a = listOf(1, 2.0, "a")
@@ -133,6 +136,7 @@ class UtilTests {
         guessValueType(sequenceOf(1, 2.0, "a", null, listOf(1, 2))) shouldBe typeOf<Any?>()
 
         guessValueType(sequenceOf(null, null)) shouldBe nothingType(nullable = true)
+        guessValueType(emptySequence()) shouldBe nothingType(nullable = false)
 
         guessValueType(sequenceOf(listOf<Int?>(null))) shouldBe typeOf<List<Nothing?>>()
         guessValueType(sequenceOf(emptyList<Int>())) shouldBe typeOf<List<Nothing>>()