Added test and asserts

idea/src/org/jetbrains/kotlin/idea/intentions/loopToCallChain/sequence/FilterTransformation.kt

@@ -85,6 +85,7 @@ class FilterTransformation(
             // we merge filter transformations here instead of FilterTransformation.mergeWithPrevious() because of filterIndexed that won't merge otherwise
 
             var (transformation, currentState) = matchOneTransformation(state) ?: return null
+            assert(currentState.indexVariable == state.indexVariable) // indexVariable should not change
 
             if (transformation is FilterTransformation) {
                 while (true) {
@@ -92,6 +93,7 @@ class FilterTransformation(
 
                     val (nextTransformation, nextState) = matchOneTransformation(currentState) ?: break
                     if (nextTransformation !is FilterTransformation) break
+                    assert(nextState.indexVariable == currentState.indexVariable) // indexVariable should not change
 
                     val indexVariable = transformation.indexVariable ?: nextTransformation.indexVariable
                     val mergedCondition = KtPsiFactory(state.outerLoop).createExpressionByPattern(

idea/testData/intentions/loopToCallChain/filterIndexed_merge4.kt

@@ -0,0 +1,11 @@
+// WITH_RUNTIME
+// INTENTION_TEXT: "Replace with 'filterIndexed{}.firstOrNull()'"
+fun foo(list: List<String>): String? {
+    var index2 = 0
+    <caret>for ((index1, s) in list.withIndex()) {
+        if (s.length > index1) continue
+        if (s.length < index2++) continue
+        return s
+    }
+    return null
+}

idea/testData/intentions/loopToCallChain/filterIndexed_merge4.kt.after

@@ -0,0 +1,8 @@
+// WITH_RUNTIME
+// INTENTION_TEXT: "Replace with 'filterIndexed{}.firstOrNull()'"
+fun foo(list: List<String>): String? {
+    var index2 = 0
+    return list
+            .filterIndexed { index1, s -> s.length <= index1 && s.length >= index2++ }
+            .firstOrNull()
+}