KT-13884 Exception "Invalid root block PSI element" on replacing trivial when-expression to if

#KT-13884 Fixed

compiler/frontend/src/org/jetbrains/kotlin/psi/createByPattern.kt

@@ -47,28 +47,30 @@ private abstract class ArgumentType<T : Any>(val klass: Class<T>)
 private class PlainTextArgumentType<T : Any>(klass: Class<T>, val toPlainText: (T) -> String) : ArgumentType<T>(klass)
 
 private abstract class PsiElementPlaceholderArgumentType<T : Any, TPlaceholder : PsiElement>(klass: Class<T>, val placeholderClass: Class<TPlaceholder>) : ArgumentType<T>(klass) {
-    abstract fun replacePlaceholderElement(placeholder: TPlaceholder, argument: T)
+    abstract fun replacePlaceholderElement(placeholder: TPlaceholder, argument: T): PsiChildRange
 }
 
 private class PsiElementArgumentType<T : PsiElement>(klass: Class<T>) : PsiElementPlaceholderArgumentType<T, T>(klass, klass) {
-    override fun replacePlaceholderElement(placeholder: T, argument: T) {
+    override fun replacePlaceholderElement(placeholder: T, argument: T): PsiChildRange {
         // if argument element has generated flag then it has not been formatted yet and we should do this manually
         // (because we cleared this flag for the whole tree above and PostprocessReformattingAspect won't format anything)
         val reformat = CodeEditUtil.isNodeGenerated(argument.node)
-        val result = placeholder.replace(argument)
+        var result = placeholder.replace(argument)
         if (reformat) {
-            CodeStyleManager.getInstance(result.project).reformat(result, true)
+            result = CodeStyleManager.getInstance(result.project).reformat(result, true)
         }
+        return PsiChildRange.singleElement(result)
     }
 }
 
 private object PsiChildRangeArgumentType : PsiElementPlaceholderArgumentType<PsiChildRange, KtElement>(PsiChildRange::class.java, KtElement::class.java) {
-    override fun replacePlaceholderElement(placeholder: KtElement, argument: PsiChildRange) {
+    override fun replacePlaceholderElement(placeholder: KtElement, argument: PsiChildRange): PsiChildRange {
         val project = placeholder.project
         val codeStyleManager = CodeStyleManager.getInstance(project)
 
         if (argument.isEmpty) {
             placeholder.delete()
+            return PsiChildRange.EMPTY
         }
         else {
             val first = placeholder.parent.addRangeBefore(argument.first!!, argument.last!!, placeholder)
@@ -79,6 +81,7 @@ private object PsiChildRangeArgumentType : PsiElementPlaceholderArgumentType<Psi
             if (last != first) {
                 codeStyleManager.reformatNewlyAddedElement(last.node.treeParent, last.node)
             }
+            return PsiChildRange(first, last)
         }
     }
 }
@@ -172,7 +175,14 @@ fun <TElement : KtElement> createByPattern(pattern: String, vararg args: Any, fa
         if (element is KtFunctionLiteral) {
             element = element.getParent() as KtLambdaExpression
         }
-        (argumentTypes[n] as PsiElementPlaceholderArgumentType<in Any, in PsiElement>).replacePlaceholderElement(element, args[n])
+        @Suppress("UNCHECKED_CAST")
+        val argumentType = argumentTypes[n] as PsiElementPlaceholderArgumentType<in Any, in PsiElement>
+        val range = argumentType.replacePlaceholderElement(element, args[n])
+
+        if (element == resultElement) {
+            assert(range.first == range.last)
+            resultElement = range.first as TElement
+        }
     }
 
     codeStyleManager.adjustLineIndent(resultElement.containingFile, resultElement.textRange)

idea/src/org/jetbrains/kotlin/idea/intentions/branchedTransformations/intentions/WhenToIfIntention.kt

@@ -33,6 +33,7 @@ class WhenToIfIntention : SelfTargetingRangeIntention<KtWhenExpression>(KtWhenEx
         if (entries.isEmpty()) return null
         val lastEntry = entries.last()
         if (entries.any { it != lastEntry && it.isElse }) return null
+        if (entries.all { it.isElse }) return null // 'when' with only 'else' branch is not supported
         return element.whenKeyword.textRange
     }
 

idea/testData/intentions/branched/ifWhen/whenToIf/kt13884.kt

@@ -0,0 +1,6 @@
+//IS_APPLICABLE: false
+fun foo() {
+    <caret>when {
+        else -> {}
+    }
+}

idea/tests/org/jetbrains/kotlin/idea/intentions/IntentionTestGenerated.java

@@ -1771,6 +1771,12 @@ public class IntentionTestGenerated extends AbstractIntentionTest {
                     doTest(fileName);
                 }
 
+                @TestMetadata("kt13884.kt")
+                public void testKt13884() throws Exception {
+                    String fileName = KotlinTestUtils.navigationMetadata("idea/testData/intentions/branched/ifWhen/whenToIf/kt13884.kt");
+                    doTest(fileName);
+                }
+
                 @TestMetadata("whenWithDotQualifiedExpression.kt")
                 public void testWhenWithDotQualifiedExpression() throws Exception {
                     String fileName = KotlinTestUtils.navigationMetadata("idea/testData/intentions/branched/ifWhen/whenToIf/whenWithDotQualifiedExpression.kt");