[IR BE] Don't override private declarations

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/CheckIrElementVisitor.kt

@@ -17,9 +17,9 @@
 package org.jetbrains.kotlin.backend.common
 
 import org.jetbrains.kotlin.descriptors.CallableMemberDescriptor
+import org.jetbrains.kotlin.descriptors.Visibilities
 import org.jetbrains.kotlin.ir.IrElement
-import org.jetbrains.kotlin.ir.declarations.IrClass
+import org.jetbrains.kotlin.ir.declarations.*
-import org.jetbrains.kotlin.ir.declarations.IrFunction
 import org.jetbrains.kotlin.ir.descriptors.IrBuiltIns
 import org.jetbrains.kotlin.ir.expressions.*
 import org.jetbrains.kotlin.ir.symbols.IrClassSymbol
@@ -283,6 +283,19 @@ class CheckIrElementVisitor(
         expression.symbol.ensureBound(expression)
     }
 
+    override fun visitDeclaration(declaration: IrDeclaration) {
+        super.visitDeclaration(declaration)
+
+        if (declaration is IrOverridableDeclaration<*>) {
+            for (overriddenSymbol in declaration.overriddenSymbols) {
+                val overriddenDeclaration = overriddenSymbol.owner as? IrDeclarationWithVisibility ?: continue
+                if (overriddenDeclaration.visibility == Visibilities.PRIVATE) {
+                    reportError(declaration, "Overrides private declaration $overriddenDeclaration")
+                }
+            }
+        }
+    }
+
     override fun visitFunctionAccess(expression: IrFunctionAccessExpression) {
         super.visitFunctionAccess(expression)
 

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/ir/IrUtils.kt

@@ -464,6 +464,7 @@ fun IrClass.addFakeOverrides() {
             irClass.declarations
                 .flatMap { it.toList() }
                 .filter { it !in overriddenFunctions }
+                .filter { it.visibility != Visibilities.PRIVATE }
         }
         .toMutableSet()
 

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/JsLoweringPhases.kt

@@ -24,7 +24,7 @@ private fun ClassLoweringPass.runOnFilesPostfix(moduleFragment: IrModuleFragment
 
 private fun validationCallback(context: JsIrBackendContext, module: IrModuleFragment) {
     val validatorConfig = IrValidatorConfig(
-        abortOnError = false,
+        abortOnError = true,
         ensureAllNodesAreDifferent = true,
         checkTypes = false,
         checkDescriptors = false