diff --git a/compiler/frontend/src/org/jetbrains/kotlin/cfg/WhenChecker.java b/compiler/frontend/src/org/jetbrains/kotlin/cfg/WhenChecker.java
index 94793287460..711fc310381 100644
--- a/compiler/frontend/src/org/jetbrains/kotlin/cfg/WhenChecker.java
+++ b/compiler/frontend/src/org/jetbrains/kotlin/cfg/WhenChecker.java
@@ -16,12 +16,12 @@
 
 package org.jetbrains.kotlin.cfg;
 
+import com.intellij.codeInsight.AnnotationUtil;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns;
-import org.jetbrains.kotlin.descriptors.ClassDescriptor;
-import org.jetbrains.kotlin.descriptors.ClassKind;
-import org.jetbrains.kotlin.descriptors.DeclarationDescriptor;
+import org.jetbrains.kotlin.descriptors.*;
+import org.jetbrains.kotlin.name.FqName;
 import org.jetbrains.kotlin.psi.*;
 import org.jetbrains.kotlin.resolve.BindingContext;
 import org.jetbrains.kotlin.resolve.BindingTrace;
@@ -46,6 +46,23 @@ public final class WhenChecker {
         return !isUnit && !isStatement && !isWhenExhaustive(expression, trace);
     }
 
+    private static final FqName notNullAnnotationName = new FqName(AnnotationUtil.NOT_NULL);
+
+    public static boolean isExhaustiveWhenOnPlatformNullableEnum(@NotNull JetWhenExpression expression, @NotNull BindingTrace trace) {
+        JetType type = whenSubjectType(expression, trace.getBindingContext());
+        if (type == null) return false;
+        ClassDescriptor classDescriptor = TypeUtils.getClassDescriptor(type);
+        return (isPlatformEnum(type, classDescriptor)
+                && isWhenOnEnumExhaustive(expression, trace, classDescriptor)
+                // nullable from Kotlin side
+                && TypeUtils.isNullableType(type)
+                // and from Java side too
+                && type.getAnnotations().findAnnotation(notNullAnnotationName) == null
+                && type.getAnnotations().findExternalAnnotation(notNullAnnotationName) == null
+                // but no null case
+                && !containsNullCase(expression, trace));
+    }
+
     public static boolean isWhenByEnum(@NotNull JetWhenExpression expression, @NotNull BindingContext context) {
         return getClassDescriptorOfTypeIfEnum(whenSubjectType(expression, context)) != null;
     }
@@ -53,9 +70,8 @@ public final class WhenChecker {
     @Nullable
     private static ClassDescriptor getClassDescriptorOfTypeIfEnum(@Nullable JetType type) {
         if (type == null) return null;
-        DeclarationDescriptor declarationDescriptor = type.getConstructor().getDeclarationDescriptor();
-        if (!(declarationDescriptor instanceof ClassDescriptor)) return null;
-        ClassDescriptor classDescriptor = (ClassDescriptor) declarationDescriptor;
+        ClassDescriptor classDescriptor = TypeUtils.getClassDescriptor(type);
+        if (classDescriptor == null) return null;
         if (classDescriptor.getKind() != ClassKind.ENUM_CLASS || classDescriptor.getModality().isOverridable()) return null;
 
         return classDescriptor;
@@ -98,6 +114,12 @@ public final class WhenChecker {
         return notEmpty;
     }
 
+    private static boolean isPlatformEnum(@NotNull JetType type, @Nullable ClassDescriptor classDescriptor) {
+        // instanceof JetClass are Kotlin types, as well as nullable types
+        return classDescriptor != null && classDescriptor.getKind() == ClassKind.ENUM_CLASS
+               && !(type instanceof JetClass) && !type.isMarkedNullable();
+    }
+    
     public static boolean isWhenExhaustive(@NotNull JetWhenExpression expression, @NotNull BindingTrace trace) {
         JetType type = whenSubjectType(expression, trace.getBindingContext());
         if (type == null) return false;
@@ -116,7 +138,7 @@ public final class WhenChecker {
         else {
             exhaustive = isWhenOnEnumExhaustive(expression, trace, classDescriptor);
         }
-        if (exhaustive && (!TypeUtils.isNullableType(type) || containsNullCase(expression, trace))) {
+        if (exhaustive && (!TypeUtils.isNullableType(type) || containsNullCase(expression, trace) || isPlatformEnum(type, classDescriptor))) {
             trace.record(BindingContext.EXHAUSTIVE_WHEN, expression);
             return true;
         }
diff --git a/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java b/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java
index d17fb7e6d2d..bc1aac565dc 100644
--- a/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java
+++ b/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java
@@ -618,6 +618,7 @@ public interface Errors {
     DiagnosticFactory0<JetWhenCondition> EXPECTED_CONDITION = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory0<JetWhenEntry> ELSE_MISPLACED_IN_WHEN = DiagnosticFactory0.create(ERROR, ELSE_ENTRY);
     DiagnosticFactory0<JetWhenExpression> NO_ELSE_IN_WHEN = DiagnosticFactory0.create(ERROR, WHEN_EXPRESSION);
+    DiagnosticFactory0<JetExpression> WHEN_ENUM_CAN_BE_NULL_IN_JAVA = DiagnosticFactory0.create(WARNING);
 
     // Type mismatch
 
diff --git a/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java b/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java
index f8ea8a6d2a5..170195ccce1 100644
--- a/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java
+++ b/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java
@@ -385,6 +385,8 @@ public class DefaultErrorMessages {
         MAP.put(ELSE_MISPLACED_IN_WHEN, "'else' entry must be the last one in a when-expression");
 
         MAP.put(NO_ELSE_IN_WHEN, "'when' expression must contain 'else' branch");
+        MAP.put(WHEN_ENUM_CAN_BE_NULL_IN_JAVA, "Enum argument ''{0}'' can be null in Java, but exhaustive when contains no null branch");
+
         MAP.put(TYPE_MISMATCH_IN_RANGE, "Type mismatch: incompatible types of range and element checked in it");
         MAP.put(CYCLIC_INHERITANCE_HIERARCHY, "There's a cycle in the inheritance hierarchy for this type");
 
diff --git a/compiler/frontend/src/org/jetbrains/kotlin/types/expressions/PatternMatchingTypingVisitor.java b/compiler/frontend/src/org/jetbrains/kotlin/types/expressions/PatternMatchingTypingVisitor.java
index 1418ec69ccf..c6f362812db 100644
--- a/compiler/frontend/src/org/jetbrains/kotlin/types/expressions/PatternMatchingTypingVisitor.java
+++ b/compiler/frontend/src/org/jetbrains/kotlin/types/expressions/PatternMatchingTypingVisitor.java
@@ -21,6 +21,7 @@ import com.intellij.openapi.util.Ref;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns;
+import org.jetbrains.kotlin.cfg.WhenChecker;
 import org.jetbrains.kotlin.diagnostics.Errors;
 import org.jetbrains.kotlin.psi.*;
 import org.jetbrains.kotlin.resolve.BindingContext;
@@ -129,6 +130,11 @@ public class PatternMatchingTypingVisitor extends ExpressionTypingVisitor {
         if (commonDataFlowInfo == null) {
             commonDataFlowInfo = context.dataFlowInfo;
         }
+        // Check for conditionally-exhaustive platform enums, see KT-6399
+        if (expression.getElseExpression() == null
+            && WhenChecker.isExhaustiveWhenOnPlatformNullableEnum(expression, context.trace)) {
+            context.trace.report(WHEN_ENUM_CAN_BE_NULL_IN_JAVA.on(expression.getSubjectExpression()));
+        }
 
         return TypeInfoFactoryPackage.createTypeInfo(expressionTypes.isEmpty() ? null : DataFlowUtils.checkImplicitCast(
                                                              CommonSupertypes.commonSupertype(expressionTypes), expression,
diff --git a/compiler/testData/diagnostics/tests/when/ElseOnNullableEnum.kt b/compiler/testData/diagnostics/tests/when/ElseOnNullableEnum.kt
index 787d09c9927..ed89017a4c7 100644
--- a/compiler/testData/diagnostics/tests/when/ElseOnNullableEnum.kt
+++ b/compiler/testData/diagnostics/tests/when/ElseOnNullableEnum.kt
@@ -22,7 +22,7 @@ fun withNullableNothing(e: E?) = when (e) {
     nullableNothing() -> null
 }
 
-fun platformType() = <!NO_ELSE_IN_WHEN!>when<!> (J.foo()) {
+fun platformType() = when (<!WHEN_ENUM_CAN_BE_NULL_IN_JAVA!>J.foo()<!>) {
     E.A -> 7
     E.B -> 8
 }
diff --git a/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnum.kt b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnum.kt
new file mode 100644
index 00000000000..dff3fe24e60
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnum.kt
@@ -0,0 +1,21 @@
+// See KT-6399: exhaustive whens on platform enums
+
+// FILE: J.java
+
+public enum J {
+    A, B;
+
+    public static J create() {
+        return J.A;
+    }
+}
+
+// FILE: K.kt
+
+fun foo(): Int {
+    // When is exhaustive (count a platform enum as a special case)
+    return when (<!WHEN_ENUM_CAN_BE_NULL_IN_JAVA!>J.create()<!>) {
+        J.A -> 1
+        J.B -> 2
+    }
+}
diff --git a/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnum.txt b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnum.txt
new file mode 100644
index 00000000000..86c67a05e99
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnum.txt
@@ -0,0 +1,38 @@
+package
+
+internal fun foo(): kotlin.Int
+
+public final enum class J : kotlin.Enum<J!> {
+    public enum entry A : J {
+        private constructor A()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public enum entry B : J {
+        private constructor B()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public constructor J()
+    public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+    public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    // Static members
+    public open fun create(): J!
+    public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): J
+    public final /*synthesized*/ fun values(): kotlin.Array<J>
+}
diff --git a/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumAnnotated.kt b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumAnnotated.kt
new file mode 100644
index 00000000000..6820a883e59
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumAnnotated.kt
@@ -0,0 +1,23 @@
+// See KT-6399: exhaustive whens on platform enums
+
+// FILE: J.java
+
+import org.jetbrains.annotations.*;
+
+public enum J {
+    A, B;
+
+    @NotNull public static J create() {
+        return J.A;
+    }
+}
+
+// FILE: K.kt
+
+fun foo(): Int {
+    // When is exhaustive (count a platform enum as a special case)
+    return when (J.create()) {
+        J.A -> 1
+        J.B -> 2
+    }
+}
diff --git a/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumAnnotated.txt b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumAnnotated.txt
new file mode 100644
index 00000000000..0f5c9acb5f8
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumAnnotated.txt
@@ -0,0 +1,38 @@
+package
+
+internal fun foo(): kotlin.Int
+
+public final enum class J : kotlin.Enum<J!> {
+    public enum entry A : J {
+        private constructor A()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public enum entry B : J {
+        private constructor B()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public constructor J()
+    public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+    public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    // Static members
+    org.jetbrains.annotations.NotNull() public open fun create(): J!
+    public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): J
+    public final /*synthesized*/ fun values(): kotlin.Array<J>
+}
diff --git a/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumElse.kt b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumElse.kt
new file mode 100644
index 00000000000..bd311096cf2
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumElse.kt
@@ -0,0 +1,21 @@
+// See KT-6399: exhaustive whens on platform enums
+
+// FILE: J.java
+
+public enum J {
+    A, B;
+
+    public static J create() {
+        return J.A;
+    }
+}
+
+// FILE: K.kt
+
+fun foo(): Int {
+    return when (J.create()) {
+        J.A -> 1
+        J.B -> 2
+        else -> 0
+    }
+}
diff --git a/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumElse.txt b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumElse.txt
new file mode 100644
index 00000000000..86c67a05e99
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumElse.txt
@@ -0,0 +1,38 @@
+package
+
+internal fun foo(): kotlin.Int
+
+public final enum class J : kotlin.Enum<J!> {
+    public enum entry A : J {
+        private constructor A()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public enum entry B : J {
+        private constructor B()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public constructor J()
+    public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+    public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    // Static members
+    public open fun create(): J!
+    public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): J
+    public final /*synthesized*/ fun values(): kotlin.Array<J>
+}
diff --git a/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumNull.kt b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumNull.kt
new file mode 100644
index 00000000000..c809bee2c2b
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumNull.kt
@@ -0,0 +1,22 @@
+// See KT-6399: exhaustive whens on platform enums
+
+// FILE: J.java
+
+public enum J {
+    A, B;
+
+    public static J create() {
+        return J.A;
+    }
+}
+
+// FILE: K.kt
+
+fun foo(): Int {
+    // When is exhaustive including null
+    return when (J.create()) {
+        J.A -> 1
+        J.B -> 2
+        null -> 0
+    }
+}
diff --git a/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumNull.txt b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumNull.txt
new file mode 100644
index 00000000000..86c67a05e99
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumNull.txt
@@ -0,0 +1,38 @@
+package
+
+internal fun foo(): kotlin.Int
+
+public final enum class J : kotlin.Enum<J!> {
+    public enum entry A : J {
+        private constructor A()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public enum entry B : J {
+        private constructor B()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public constructor J()
+    public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+    public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    // Static members
+    public open fun create(): J!
+    public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): J
+    public final /*synthesized*/ fun values(): kotlin.Array<J>
+}
diff --git a/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumStatement.kt b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumStatement.kt
new file mode 100644
index 00000000000..744c6fbe580
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumStatement.kt
@@ -0,0 +1,20 @@
+// See KT-6399: exhaustive whens on platform enums
+
+// FILE: J.java
+
+public enum J {
+    A, B;
+
+    public static J create() {
+        return J.A;
+    }
+}
+
+// FILE: K.kt
+
+fun foo(): Int {
+    when (<!WHEN_ENUM_CAN_BE_NULL_IN_JAVA!>J.create()<!>) {
+        J.A -> return 1
+        J.B -> return 2
+    }
+}
diff --git a/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumStatement.txt b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumStatement.txt
new file mode 100644
index 00000000000..86c67a05e99
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumStatement.txt
@@ -0,0 +1,38 @@
+package
+
+internal fun foo(): kotlin.Int
+
+public final enum class J : kotlin.Enum<J!> {
+    public enum entry A : J {
+        private constructor A()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public enum entry B : J {
+        private constructor B()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public constructor J()
+    public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+    public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    // Static members
+    public open fun create(): J!
+    public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): J
+    public final /*synthesized*/ fun values(): kotlin.Array<J>
+}
diff --git a/compiler/testData/diagnostics/tests/when/NonExhaustivePlatformEnum.kt b/compiler/testData/diagnostics/tests/when/NonExhaustivePlatformEnum.kt
new file mode 100644
index 00000000000..da8465bb7dd
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/NonExhaustivePlatformEnum.kt
@@ -0,0 +1,20 @@
+// See KT-6399: exhaustive whens on platform enums
+
+// FILE: J.java
+
+public enum J {
+    A, B;
+
+    public static J create() {
+        return J.A;
+    }
+}
+
+// FILE: K.kt
+
+fun foo(): Int {
+    // When is not-exhaustive
+    return <!NO_ELSE_IN_WHEN!>when<!> (J.create()) {
+        J.A -> 1
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/when/NonExhaustivePlatformEnum.txt b/compiler/testData/diagnostics/tests/when/NonExhaustivePlatformEnum.txt
new file mode 100644
index 00000000000..86c67a05e99
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/when/NonExhaustivePlatformEnum.txt
@@ -0,0 +1,38 @@
+package
+
+internal fun foo(): kotlin.Int
+
+public final enum class J : kotlin.Enum<J!> {
+    public enum entry A : J {
+        private constructor A()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public enum entry B : J {
+        private constructor B()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public constructor J()
+    public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: J!): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+    public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    // Static members
+    public open fun create(): J!
+    public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): J
+    public final /*synthesized*/ fun values(): kotlin.Array<J>
+}
diff --git a/compiler/tests/org/jetbrains/kotlin/checkers/JetDiagnosticsTestGenerated.java b/compiler/tests/org/jetbrains/kotlin/checkers/JetDiagnosticsTestGenerated.java
index 696624602b7..4439504a88e 100644
--- a/compiler/tests/org/jetbrains/kotlin/checkers/JetDiagnosticsTestGenerated.java
+++ b/compiler/tests/org/jetbrains/kotlin/checkers/JetDiagnosticsTestGenerated.java
@@ -13026,6 +13026,36 @@ public class JetDiagnosticsTestGenerated extends AbstractJetDiagnosticsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("ExhaustivePlatformEnum.kt")
+            public void testExhaustivePlatformEnum() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnum.kt");
+                doTest(fileName);
+            }
+
+            @TestMetadata("ExhaustivePlatformEnumAnnotated.kt")
+            public void testExhaustivePlatformEnumAnnotated() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumAnnotated.kt");
+                doTest(fileName);
+            }
+
+            @TestMetadata("ExhaustivePlatformEnumElse.kt")
+            public void testExhaustivePlatformEnumElse() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumElse.kt");
+                doTest(fileName);
+            }
+
+            @TestMetadata("ExhaustivePlatformEnumNull.kt")
+            public void testExhaustivePlatformEnumNull() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumNull.kt");
+                doTest(fileName);
+            }
+
+            @TestMetadata("ExhaustivePlatformEnumStatement.kt")
+            public void testExhaustivePlatformEnumStatement() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/ExhaustivePlatformEnumStatement.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("ExhaustiveReturn.kt")
             public void testExhaustiveReturn() throws Exception {
                 String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/ExhaustiveReturn.kt");
@@ -13086,6 +13116,12 @@ public class JetDiagnosticsTestGenerated extends AbstractJetDiagnosticsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("NonExhaustivePlatformEnum.kt")
+            public void testNonExhaustivePlatformEnum() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/NonExhaustivePlatformEnum.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("PropertyNotInitialized.kt")
             public void testPropertyNotInitialized() throws Exception {
                 String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/PropertyNotInitialized.kt");