Replace 'when' with 'if': do not suggest if 'when' is used as expression and it has no 'else' branch

#KT-35329 Fixed

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

@@ -9,10 +9,13 @@ import com.intellij.codeInsight.intention.LowPriorityAction
 import com.intellij.openapi.editor.Editor
 import com.intellij.openapi.util.TextRange
 import org.jetbrains.kotlin.idea.KotlinBundle
+import org.jetbrains.kotlin.idea.caches.resolve.analyze
 import org.jetbrains.kotlin.idea.intentions.SelfTargetingRangeIntention
 import org.jetbrains.kotlin.idea.intentions.branchedTransformations.combineWhenConditions
 import org.jetbrains.kotlin.idea.util.CommentSaver
 import org.jetbrains.kotlin.psi.*
+import org.jetbrains.kotlin.resolve.bindingContextUtil.isUsedAsExpression
+import org.jetbrains.kotlin.resolve.lazy.BodyResolveMode
 
 class WhenToIfIntention : SelfTargetingRangeIntention<KtWhenExpression>(
     KtWhenExpression::class.java,
@@ -25,6 +28,7 @@ class WhenToIfIntention : SelfTargetingRangeIntention<KtWhenExpression>(
         if (entries.any { it != lastEntry && it.isElse }) return null
         if (entries.all { it.isElse }) return null // 'when' with only 'else' branch is not supported
         if (element.subjectExpression is KtProperty) return null
+        if (!lastEntry.isElse && element.isUsedAsExpression(element.analyze(BodyResolveMode.PARTIAL_WITH_CFA))) return null
         return element.whenKeyword.textRange
     }
 

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

@@ -0,0 +1,10 @@
+//IS_APPLICABLE: false
+enum class E { X, Y, Z}
+
+fun test(e: E) {
+    val i = <caret>when (e) {
+        E.X -> 1
+        E.Y -> 2
+        E.Z -> 3
+    }
+}

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

@@ -2653,6 +2653,11 @@ public class IntentionTestGenerated extends AbstractIntentionTest {
                     runTest("idea/testData/intentions/branched/ifWhen/whenToIf/whenWithoutElse.kt");
                 }
 
+                @TestMetadata("whenWithoutElseUsedAsExpression.kt")
+                public void testWhenWithoutElseUsedAsExpression() throws Exception {
+                    runTest("idea/testData/intentions/branched/ifWhen/whenToIf/whenWithoutElseUsedAsExpression.kt");
+                }
+
                 @TestMetadata("whenWithoutSubject.kt")
                 public void testWhenWithoutSubject() throws Exception {
                     runTest("idea/testData/intentions/branched/ifWhen/whenToIf/whenWithoutSubject.kt");