[JS Legacy] Fix Unit materialization in overridden methods

^KT-51878 Fixed

core/descriptors/src/org/jetbrains/kotlin/builtins/KotlinBuiltIns.java

@@ -895,6 +895,24 @@ public abstract class KotlinBuiltIns {
         return isNotNullConstructedFromGivenClass(type, FqNames.unit);
     }
 
+    /**
+     * Returns <code>true</code> if the <code>descriptor</code>'s return type is not <code>Unit</code>,
+     * or it overrides a function with a non-<code>Unit</code> return type.
+     */
+    public static boolean mayReturnNonUnitValue(@NotNull FunctionDescriptor descriptor) {
+        KotlinType functionReturnType = descriptor.getReturnType();
+        assert functionReturnType != null : "Function return typed type must be resolved.";
+        boolean mayReturnNonUnitValue = !isUnit(functionReturnType);
+        for (FunctionDescriptor overriddenDescriptor : descriptor.getOriginal().getOverriddenDescriptors()) {
+            if (mayReturnNonUnitValue)
+                break;
+            KotlinType overriddenFunctionReturnType = overriddenDescriptor.getReturnType();
+            assert overriddenFunctionReturnType != null : "Function return typed type must be resolved.";
+            mayReturnNonUnitValue = !isUnit(overriddenFunctionReturnType);
+        }
+        return mayReturnNonUnitValue;
+    }
+
     public static boolean isUnitOrNullableUnit(@NotNull KotlinType type) {
         return isConstructedFromGivenClass(type, FqNames.unit);
     }

js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/BoxJsTestGenerated.java

@@ -750,6 +750,12 @@ public class BoxJsTestGenerated extends AbstractBoxJsTest {
             runTest("js/js.translator/testData/box/coercion/unitIsAs.kt");
         }
 
+        @Test
+        @TestMetadata("unitMaterializationInOverriddenMethod.kt")
+        public void testUnitMaterializationInOverriddenMethod() throws Exception {
+            runTest("js/js.translator/testData/box/coercion/unitMaterializationInOverriddenMethod.kt");
+        }
+
         @Test
         @TestMetadata("unitNullCheck.kt")
         public void testUnitNullCheck() throws Exception {

js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/ir/IrBoxJsTestGenerated.java

@@ -750,6 +750,12 @@ public class IrBoxJsTestGenerated extends AbstractIrBoxJsTest {
             runTest("js/js.translator/testData/box/coercion/unitIsAs.kt");
         }
 
+        @Test
+        @TestMetadata("unitMaterializationInOverriddenMethod.kt")
+        public void testUnitMaterializationInOverriddenMethod() throws Exception {
+            runTest("js/js.translator/testData/box/coercion/unitMaterializationInOverriddenMethod.kt");
+        }
+
         @Test
         @TestMetadata("unitNullCheck.kt")
         public void testUnitNullCheck() throws Exception {

js/js.translator/src/org/jetbrains/kotlin/js/translate/expression/ExpressionVisitor.java

@@ -160,7 +160,12 @@ public final class ExpressionVisitor extends TranslatorVisitor<JsNode> {
 
         JsReturn jsReturn;
         if (returned == null) {
-            jsReturn = new JsReturn(null);
+            JsExpression returnExpression = null;
+            if (returnTarget != null && KotlinBuiltIns.mayReturnNonUnitValue(returnTarget)) {
+                ClassDescriptor unit = context.getCurrentModule().getBuiltIns().getUnit();
+                returnExpression = ReferenceTranslator.translateAsValueReference(unit, context);
+            }
+            jsReturn = new JsReturn(returnExpression);
         }
         else {
             JsExpression jsReturnExpression = translateAsExpression(returned, context);

js/js.translator/src/org/jetbrains/kotlin/js/translate/utils/FunctionBodyTranslator.java

@@ -161,9 +161,7 @@ public final class FunctionBodyTranslator extends AbstractTranslator {
     }
 
     private boolean mustAddReturnToGeneratedFunctionBody() {
-        KotlinType functionReturnType = descriptor.getReturnType();
-        assert functionReturnType != null : "Function return typed type must be resolved.";
-        return (!declaration.hasBlockBody()) && !(KotlinBuiltIns.isUnit(functionReturnType) && !descriptor.isSuspend());
+        return !declaration.hasBlockBody() && (KotlinBuiltIns.mayReturnNonUnitValue(descriptor) || descriptor.isSuspend());
     }
 
     @NotNull

js/js.translator/testData/box/coercion/unitMaterializationInOverriddenMethod.kt

@@ -0,0 +1,24 @@
+// EXPECTED_REACHABLE_NODES: 1279
+// KT-51878
+
+open class A {
+    open fun foo(): Any = "string"
+}
+
+var condition = true
+
+class B: A() {
+    override fun foo(): Unit {
+        if (condition) {
+            return
+        }
+        condition = !condition
+    }
+}
+
+fun box(): String {
+    val b: A = B()
+    if (b.foo() !is Unit) return "fail1"
+    if (b.foo() !is Unit) return "fail2"
+    return "OK"
+}