diff --git a/compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/boxing/BoxingInterpreter.kt b/compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/boxing/BoxingInterpreter.kt
index eda35e00acc..b5366f64f2d 100644
--- a/compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/boxing/BoxingInterpreter.kt
+++ b/compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/boxing/BoxingInterpreter.kt
@@ -174,17 +174,20 @@ open class BoxingInterpreter(
         when {
             v == StrictBasicValue.UNINITIALIZED_VALUE || w == StrictBasicValue.UNINITIALIZED_VALUE ->
                 StrictBasicValue.UNINITIALIZED_VALUE
-            v is BoxedBasicValue && w is BoxedBasicValue -> {
-                onMergeSuccess(v, w)
-                when {
-                    v is TaintedBoxedValue -> v
-                    w is TaintedBoxedValue -> w
-                    v.type != w.type -> mergeBoxedHazardous(v, w, isLocalVariable)
-                    else -> v // two clean boxed values with the same type are equal
+            v is BoxedBasicValue -> {
+                if (w is BoxedBasicValue) {
+                    onMergeSuccess(v, w)
+                    when {
+                        v is TaintedBoxedValue -> v
+                        w is TaintedBoxedValue -> w
+                        v.type != w.type -> mergeBoxedHazardous(v, w, isLocalVariable)
+                        else -> v // two clean boxed values with the same type are equal
+                    }
+                } else {
+                    mergeBoxedHazardous(v, w, isLocalVariable)
                 }
             }
-            v is BoxedBasicValue ->
-                mergeBoxedHazardous(v, w, isLocalVariable)
+
             w is BoxedBasicValue ->
                 mergeBoxedHazardous(w, v, isLocalVariable)
             else ->
diff --git a/compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/common/ReferenceTrackingInterpreter.kt b/compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/common/ReferenceTrackingInterpreter.kt
index 2b299265cb0..6e5305d2f61 100644
--- a/compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/common/ReferenceTrackingInterpreter.kt
+++ b/compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/common/ReferenceTrackingInterpreter.kt
@@ -30,10 +30,14 @@ abstract class ReferenceTrackingInterpreter : OptimizationBasicInterpreter() {
                 else
                     createTaintedValue(v, w)
 
-            v is TrackedReferenceValue && w is TrackedReferenceValue ->
-                createPossiblyMergedValue(v, w)
+            v is TrackedReferenceValue -> {
+                if (w is TrackedReferenceValue)
+                    createPossiblyMergedValue(v, w)
+                else
+                    createTaintedValue(v, w)
+            }
 
-            v is TrackedReferenceValue || w is TrackedReferenceValue ->
+            w is TrackedReferenceValue ->
                 createTaintedValue(v, w)
 
             else ->
diff --git a/compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/nullCheck/NullabilityInterpreter.kt b/compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/nullCheck/NullabilityInterpreter.kt
index 3abf2a4ca97..7034e1fb2a0 100644
--- a/compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/nullCheck/NullabilityInterpreter.kt
+++ b/compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/nullCheck/NullabilityInterpreter.kt
@@ -96,20 +96,21 @@ class NullabilityInterpreter(private val generationState: GenerationState) : Opt
 
     override fun merge(v: BasicValue, w: BasicValue): BasicValue =
         when {
-            v is NullBasicValue && w is NullBasicValue ->
-                NullBasicValue
-            v is NullBasicValue || w is NullBasicValue ->
-                StrictBasicValue.REFERENCE_VALUE
-            v is ProgressionIteratorBasicValue && w is ProgressionIteratorBasicValue ->
-                mergeNotNullValuesOfSameKind(v, w)
-            v is ProgressionIteratorBasicValue && w is NotNullBasicValue ->
-                NotNullBasicValue.NOT_NULL_REFERENCE_VALUE
-            w is ProgressionIteratorBasicValue && v is NotNullBasicValue ->
-                NotNullBasicValue.NOT_NULL_REFERENCE_VALUE
-            v is NotNullBasicValue && w is NotNullBasicValue ->
-                mergeNotNullValuesOfSameKind(v, w)
-            else ->
-                super.merge(v, w)
+            v === NullBasicValue -> if (w === NullBasicValue) NullBasicValue else StrictBasicValue.REFERENCE_VALUE
+
+            w === NullBasicValue -> StrictBasicValue.REFERENCE_VALUE
+
+            v is ProgressionIteratorBasicValue -> when (w) {
+                is ProgressionIteratorBasicValue -> mergeNotNullValuesOfSameKind(v, w)
+                is NotNullBasicValue -> NotNullBasicValue.NOT_NULL_REFERENCE_VALUE
+                else -> super.merge(v, w)
+            }
+            v is NotNullBasicValue -> when (w) {
+                is ProgressionIteratorBasicValue -> NotNullBasicValue.NOT_NULL_REFERENCE_VALUE
+                is NotNullBasicValue -> mergeNotNullValuesOfSameKind(v, w)
+                else -> super.merge(v, w)
+            }
+            else -> super.merge(v, w)
         }
 
     private fun mergeNotNullValuesOfSameKind(v: StrictBasicValue, w: StrictBasicValue) =