Throw NoSuchElementException instead of UnsupportedOperationException…

… in Flow.reduce

    * We can do it safely because reduce is still experimental
    * We will be consistent with stdlib (as soon as KT-33874 is implemented)

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

@@ -15,7 +15,7 @@ import kotlin.jvm.*
 
 /**
  * Accumulates value starting with the first element and applying [operation] to current accumulator value and each element.
- * Throws [UnsupportedOperationException] if flow was empty.
+ * Throws [NoSuchElementException] if flow was empty.
  */
 @ExperimentalCoroutinesApi
 public suspend fun <S, T : S> Flow<T>.reduce(operation: suspend (accumulator: S, value: T) -> S): S {
@@ -30,7 +30,7 @@ public suspend fun <S, T : S> Flow<T>.reduce(operation: suspend (accumulator: S,
  }
  }
 
- if (accumulator === NULL) throw UnsupportedOperationException("Empty flow can't be reduced")
+ if (accumulator === NULL) throw NoSuchElementException("Empty flow can't be reduced")
  @Suppress("UNCHECKED_CAST")
  return accumulator as S
 }

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

@@ -23,7 +23,7 @@ class ReduceTest : TestBase() {
  @Test
  fun testEmptyReduce() = runTest {
  val flow = emptyFlow<Int>()
- assertFailsWith<UnsupportedOperationException> { flow.reduce { acc, value -> value + acc } }
+ assertFailsWith<NoSuchElementException> { flow.reduce { acc, value -> value + acc } }
  }
 
  @Test
@@ -42,7 +42,7 @@ class ReduceTest : TestBase() {
  fun testReduceNulls() = runTest {
  assertNull(flowOf(null).reduce { _, value -> value })
  assertNull(flowOf(null, null).reduce { _, value -> value })
- assertFailsWith<UnsupportedOperationException> { flowOf<Nothing?>().reduce { _, value -> value } }
+ assertFailsWith<NoSuchElementException> { flowOf<Nothing?>().reduce { _, value -> value } }
  }
 
  @Test