Properly handle multiple nulls

Author: qwwdfsad

kotlinx-coroutines-core/common/src/flow/terminal/Reduce.kt

@@ -63,7 +63,6 @@ public suspend fun <T> Flow<T>.single(): T {
     }
 
     if (result === NULL) throw NoSuchElementException("Expected at least one element")
-    @Suppress("UNCHECKED_CAST")
     return result as T
 }
 
@@ -72,12 +71,12 @@ public suspend fun <T> Flow<T>.single(): T {
  * Throws [IllegalStateException] for flow that contains more than one element.
  */
 public suspend fun <T> Flow<T>.singleOrNull(): T? {
-    var result: T? = null
+    var result: Any? = NULL
     collect { value ->
-        if (result != null) error("Expected only one element")
+        if (result != NULL) error("Expected only one element")
         result = value
     }
-    return result
+    return if (result === NULL) null else result as T?
 }
 
 /**

kotlinx-coroutines-core/common/test/flow/terminal/SingleTest.kt

@@ -25,8 +25,14 @@ class SingleTest : TestBase() {
             emit(239L)
             emit(240L)
         }
-        assertFailsWith<RuntimeException> { flow.single() }
-        assertFailsWith<RuntimeException> { flow.singleOrNull() }
+        assertFailsWith<IllegalStateException> { flow.single() }
+        assertFailsWith<IllegalStateException> { flow.singleOrNull() }
+    }
+
+    @Test
+    fun testMultipleNulls() = runTest {
+        val flow = flowOf<Int?>(null, null)
+        assertFailsWith<IllegalStateException> { flow.singleOrNull() }
     }
 
     @Test