diff --git a/compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/smartcasts/DelegatingDataFlowInfo.kt b/compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/smartcasts/DelegatingDataFlowInfo.kt
index 6309f8c9f93..4f754c6c2c5 100644
--- a/compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/smartcasts/DelegatingDataFlowInfo.kt
+++ b/compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/smartcasts/DelegatingDataFlowInfo.kt
@@ -17,6 +17,7 @@
 package org.jetbrains.kotlin.resolve.calls.smartcasts
 
 import com.google.common.collect.*
+import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.types.*
 import org.jetbrains.kotlin.types.KotlinType
 import org.jetbrains.kotlin.types.TypeUtils
@@ -251,6 +252,13 @@ internal class DelegatingDataFlowInfo @JvmOverloads constructor(
         return DelegatingDataFlowInfo(this, ImmutableMap.copyOf(nullabilityMapBuilder), otherTypeInfo)
     }
 
+    private fun Set<KotlinType>.containsNothing() = any { KotlinBuiltIns.isNothing(it) }
+
+    private fun Set<KotlinType>.intersect(other: Set<KotlinType>) =
+            if (other.containsNothing()) this
+            else if (this.containsNothing()) other
+            else Sets.intersection(this, other)
+
     override fun or(otherInfo: DataFlowInfo): DataFlowInfo {
         if (otherInfo === DataFlowInfo.EMPTY) return DataFlowInfo.EMPTY
         if (this === DataFlowInfo.EMPTY) return DataFlowInfo.EMPTY
@@ -270,9 +278,7 @@ internal class DelegatingDataFlowInfo @JvmOverloads constructor(
         val newTypeInfo = newTypeInfo()
 
         for (key in Sets.intersection(myTypeInfo.keySet(), otherTypeInfo.keySet())) {
-            val thisTypes = myTypeInfo.get(key)
-            val otherTypes = otherTypeInfo.get(key)
-            newTypeInfo.putAll(key, Sets.intersection(thisTypes, otherTypes))
+            newTypeInfo.putAll(key, myTypeInfo[key].intersect(otherTypeInfo[key]))
         }
 
         if (nullabilityMapBuilder.isEmpty() && newTypeInfo.isEmpty) {
diff --git a/compiler/testData/codegen/box/when/typeDisjunction.kt b/compiler/testData/codegen/box/when/typeDisjunction.kt
new file mode 100644
index 00000000000..1d2910393b0
--- /dev/null
+++ b/compiler/testData/codegen/box/when/typeDisjunction.kt
@@ -0,0 +1,15 @@
+fun foo(s: Any): String {
+    val x = when (s) {
+        is String -> s
+        is Int -> "$s"
+        else -> return ""
+    }
+
+    val y: String = x
+    return y
+}
+
+fun box() = if (foo("OK") == "OK" && foo(42) == "42" && foo(true) == "") "OK" else "Fail"
+
+
+
diff --git a/compiler/testData/diagnostics/tests/when/WhenTypeDisjunctions.kt b/compiler/testData/diagnostics/tests/when/WhenTypeDisjunctions.kt
new file mode 100644
index 00000000000..437ca0d909d
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/WhenTypeDisjunctions.kt
@@ -0,0 +1,21 @@
+fun foo(s: Any): String {
+    val x = when (s) {
+        is String -> s
+        is Int -> "$s"
+        else -> return ""
+    }
+
+    val y: String = <!DEBUG_INFO_SMARTCAST!>x<!> // should be Ok
+    return y
+}
+
+fun bar(s: Any): String {
+    val x = when (s) {
+        is String -> s <!USELESS_CAST!>as String<!> // meaningless
+        is Int -> "$s"
+        else -> return ""
+    }
+
+    val y: String = x // no error
+    return y
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/when/WhenTypeDisjunctions.txt b/compiler/testData/diagnostics/tests/when/WhenTypeDisjunctions.txt
new file mode 100644
index 00000000000..a6265b3a65a
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/WhenTypeDisjunctions.txt
@@ -0,0 +1,4 @@
+package
+
+public fun bar(/*0*/ s: kotlin.Any): kotlin.String
+public fun foo(/*0*/ s: kotlin.Any): kotlin.String
diff --git a/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java b/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java
index 20425acbf6a..602ac52763f 100644
--- a/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java
+++ b/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java
@@ -18617,6 +18617,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/When.kt");
                 doTest(fileName);
             }
+
+            @TestMetadata("WhenTypeDisjunctions.kt")
+            public void testWhenTypeDisjunctions() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/WhenTypeDisjunctions.kt");
+                doTest(fileName);
+            }
         }
     }
 
diff --git a/compiler/tests/org/jetbrains/kotlin/codegen/generated/BlackBoxCodegenTestGenerated.java b/compiler/tests/org/jetbrains/kotlin/codegen/generated/BlackBoxCodegenTestGenerated.java
index 7e7a56d0e83..b870dfc16b7 100644
--- a/compiler/tests/org/jetbrains/kotlin/codegen/generated/BlackBoxCodegenTestGenerated.java
+++ b/compiler/tests/org/jetbrains/kotlin/codegen/generated/BlackBoxCodegenTestGenerated.java
@@ -8389,6 +8389,12 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             doTest(fileName);
         }
 
+        @TestMetadata("typeDisjunction.kt")
+        public void testTypeDisjunction() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/box/when/typeDisjunction.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("whenArgumentIsEvaluatedOnlyOnce.kt")
         public void testWhenArgumentIsEvaluatedOnlyOnce() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/box/when/whenArgumentIsEvaluatedOnlyOnce.kt");