Solution for KT-7838: now data flow information at the end of when expression is determined correctly.

#KT-7838  Fixed.

compiler/frontend/src/org/jetbrains/kotlin/types/expressions/PatternMatchingTypingVisitor.java

@@ -130,6 +130,11 @@ public class PatternMatchingTypingVisitor extends ExpressionTypingVisitor {
         if (commonDataFlowInfo == null) {
             commonDataFlowInfo = context.dataFlowInfo;
         }
+        else if (expression.getElseExpression() == null && !WhenChecker.isWhenExhaustive(expression, context.trace)) {
+            // Without else expression in non-exhaustive when, we *must* take initial data flow info into account,
+            // because data flow can bypass all when branches in this case
+            commonDataFlowInfo = commonDataFlowInfo.or(context.dataFlowInfo);
+        }
 
         return TypeInfoFactoryPackage.createTypeInfo(expressionTypes.isEmpty() ? null : DataFlowUtils.checkType(
                                                              DataFlowUtils.checkImplicitCast(

compiler/testData/diagnostics/tests/when/BranchBypassVal.kt

@@ -0,0 +1,11 @@
+class A
+
+fun test(a: Any): String {
+    val q: String? = null
+
+    when (a) {
+        is A -> q!!
+    }
+    // When is not exhaustive
+    return <!TYPE_MISMATCH!>q<!>
+}

compiler/testData/diagnostics/tests/when/BranchBypassVal.txt

@@ -0,0 +1,10 @@
+package
+
+internal fun test(/*0*/ a: kotlin.Any): kotlin.String
+
+internal final class A {
+    public constructor A()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/when/BranchBypassVar.kt

@@ -0,0 +1,11 @@
+class A
+
+fun test(a: Any) {
+    var q: String? = null
+
+    when (a) {
+        is A -> q = "1"
+    }
+    // When is not exhaustive
+    return <!TYPE_MISMATCH!>q<!>
+}

compiler/testData/diagnostics/tests/when/BranchBypassVar.txt

@@ -0,0 +1,10 @@
+package
+
+internal fun test(/*0*/ a: kotlin.Any): kotlin.Unit
+
+internal final class A {
+    public constructor A()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/when/BranchFalseBypass.kt

@@ -0,0 +1,12 @@
+enum class My { A, B }
+
+fun test(a: My): String {
+    val q: String?
+
+    when (a) {
+        My.A -> q = "1"
+        My.B -> q = "2"
+    }
+    // When is exhaustive
+    return <!DEBUG_INFO_SMARTCAST!>q<!>
+}

compiler/testData/diagnostics/tests/when/BranchFalseBypass.txt

@@ -0,0 +1,37 @@
+package
+
+internal fun test(/*0*/ a: My): kotlin.String
+
+internal final enum class My : kotlin.Enum<My> {
+    public enum entry A : My {
+        private constructor A()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: My): 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 : My {
+        private constructor B()
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: My): 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
+    }
+
+    private constructor My()
+    public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: My): 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 final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): My
+    public final /*synthesized*/ fun values(): kotlin.Array<My>
+}

compiler/testData/diagnostics/tests/when/BranchFalseBypassElse.kt

@@ -0,0 +1,12 @@
+class A
+
+fun test(a: Any): String {
+    var q: String?
+
+    when (a) {
+        is A -> q = "1"
+        else -> q = "2"
+    }
+    // When is not exhaustive
+    return <!DEBUG_INFO_SMARTCAST!>q<!>
+}

compiler/testData/diagnostics/tests/when/BranchFalseBypassElse.txt

@@ -0,0 +1,10 @@
+package
+
+internal fun test(/*0*/ a: kotlin.Any): kotlin.String
+
+internal final class A {
+    public constructor A()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/tests/org/jetbrains/kotlin/checkers/JetDiagnosticsTestGenerated.java

@@ -13548,6 +13548,30 @@ public class JetDiagnosticsTestGenerated extends AbstractJetDiagnosticsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("BranchBypassVal.kt")
+            public void testBranchBypassVal() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/BranchBypassVal.kt");
+                doTest(fileName);
+            }
+
+            @TestMetadata("BranchBypassVar.kt")
+            public void testBranchBypassVar() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/BranchBypassVar.kt");
+                doTest(fileName);
+            }
+
+            @TestMetadata("BranchFalseBypass.kt")
+            public void testBranchFalseBypass() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/BranchFalseBypass.kt");
+                doTest(fileName);
+            }
+
+            @TestMetadata("BranchFalseBypassElse.kt")
+            public void testBranchFalseBypassElse() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/BranchFalseBypassElse.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("ElseOnNullableEnum.kt")
             public void testElseOnNullableEnum() throws Exception {
                 String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/when/ElseOnNullableEnum.kt");