Move a bit of bytecode out of next()

libraries/stdlib/src/kotlin/collections/SequenceBuilder.kt

@@ -76,7 +76,7 @@ private class YieldingIterator<T> : SequenceBuilder<T>(), Iterator<T>, Continuat
                     if (nextIterator!!.hasNext()) return true else nextIterator = null
                 State_Done -> return false
                 State_Ready -> return true
-                else -> throw unexpectedState()
+                else -> throw exceptionalState()
             }
 
             state = State_Failed
@@ -86,25 +86,29 @@ private class YieldingIterator<T> : SequenceBuilder<T>(), Iterator<T>, Continuat
         }
     }
 
-    override tailrec fun next(): T {
+    override fun next(): T {
         when (state) {
-            State_NotReady -> if (!hasNext()) throw NoSuchElementException() else return next()
+            State_NotReady -> return nextNotReady()
-            State_ManyReady -> {
+            State_ManyReady -> return nextIterator!!.next()
-                val iterator = nextIterator!!
-                return iterator.next()
-            }
             State_Ready -> {
                 state = State_NotReady
                 val result = nextValue as T
                 nextValue = null
                 return result
             }
-            State_Done -> throw NoSuchElementException()
+            else -> throw exceptionalState()
-            else -> throw unexpectedState()
         }
     }
 
-    private fun unexpectedState(): Throwable = IllegalStateException("Unexpected state of the iterator: $state")
+    private fun nextNotReady(): T {
+        if (!hasNext()) throw NoSuchElementException() else return next()
+    }
+
+    private fun exceptionalState(): Throwable = when (state) {
+        State_Done -> NoSuchElementException()
+        State_Failed -> IllegalStateException("Iterator has failed.")
+        else -> IllegalStateException("Unexpected state of the iterator: $state")
+    }
 
 
     suspend override fun yield(value: T) {

libraries/stdlib/test/collections/SequenceBuilderTest.kt

@@ -35,7 +35,7 @@ class SequenceBuilderTest {
         assertFalse(iterator.hasNext())
         assertFalse(iterator.hasNext())
 
-        assertTrue(assertFails { iterator.next() } is NoSuchElementException)
+        assertFailsWith<NoSuchElementException> { iterator.next() }
     }
 
     @Test
@@ -61,7 +61,7 @@ class SequenceBuilderTest {
         assertFalse(iterator.hasNext())
         assertFalse(iterator.hasNext())
 
-        assertTrue(assertFails { iterator.next() } is NoSuchElementException)
+        assertFailsWith<NoSuchElementException> { iterator.next() }
 
         assertEquals(1, result.iterator().next())
     }
@@ -74,7 +74,7 @@ class SequenceBuilderTest {
         assertFalse(iterator.hasNext())
         assertFalse(iterator.hasNext())
 
-        assertTrue(assertFails { iterator.next() } is NoSuchElementException)
+        assertFailsWith<NoSuchElementException> { iterator.next() }
     }
 
     @Test
@@ -106,7 +106,7 @@ class SequenceBuilderTest {
 
         sharedVar = -1
         assertFalse(iterator.hasNext())
-        assertTrue(assertFails { iterator.next() } is NoSuchElementException)
+        assertFailsWith<NoSuchElementException> { iterator.next() }
     }
 
     @Test
@@ -116,7 +116,7 @@ class SequenceBuilderTest {
             while (true) {
                 when (sharedVar) {
                     -1 -> return@buildSequence
-                    -2 -> error("Invalid state: -2")
+                    -2 -> throw UnsupportedOperationException("-2 is unsupported")
                     else -> yield(sharedVar)
                 }
             }
@@ -128,7 +128,9 @@ class SequenceBuilderTest {
         assertEquals(1, iterator.next())
 
         sharedVar = -2
-        assertTrue(assertFails { iterator.hasNext() } is IllegalStateException)
+        assertFailsWith<UnsupportedOperationException> { iterator.hasNext() }
+        assertFailsWith<IllegalStateException> { iterator.hasNext() }
+        assertFailsWith<IllegalStateException> { iterator.next() }
     }
 
     @Test