Support access to companion private members from nested classes

#KT-5363 Fixed
  #KT-6804 Fixed

compiler/backend/src/org/jetbrains/kotlin/codegen/context/CodegenContext.java

@@ -206,7 +206,26 @@ public abstract class CodegenContext<T extends DeclarationDescriptor> {
 
     @NotNull
     public ClassContext intoClass(ClassDescriptor descriptor, OwnerKind kind, GenerationState state) {
-        return new ClassContext(state.getTypeMapper(), descriptor, kind, this, null);
+        if (descriptor.isCompanionObject()) {
+            CodegenContext companionContext = this.findChildContext(descriptor);
+            if (companionContext != null) {
+                assert companionContext.getContextKind() == kind : "Kinds should be same, but: " +
+                                                                   companionContext.getContextKind() + "!= " + kind;
+                return (ClassContext) companionContext;
+            }
+        }
+        ClassContext classContext = new ClassContext(state.getTypeMapper(), descriptor, kind, this, null);
+
+        //We can't call descriptor.getCompanionObjectDescriptor() on light class generation
+        // because it triggers companion light class generation via putting it to BindingContext.CLASS
+        // (so MemberCodegen doesn't skip it in genClassOrObject).
+        if (state.getTypeMapper().getClassBuilderMode() != ClassBuilderMode.LIGHT_CLASSES &&
+            descriptor.getCompanionObjectDescriptor() != null) {
+            //We need to create companion object context ahead of time
+            // because otherwise we can't generate synthetic accessor for private members in companion object
+            classContext.intoClass(descriptor.getCompanionObjectDescriptor(), OwnerKind.IMPLEMENTATION, state);
+        }
+        return classContext;
     }
 
     @NotNull

compiler/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/checkers/NeedSyntheticChecker.java

@@ -43,7 +43,7 @@ public class NeedSyntheticChecker implements CallChecker {
     //Necessary synthetic accessors in outer classes generated via old logic: CodegenContext.getAccessor
     //Generation of accessors in nested classes (to invoke from outer,
     //      e.g.: from class to companion object) controlled via NEED_SYNTHETIC_ACCESSOR slice
-    private boolean needSyntheticAccessor(LexicalScope invokationScope, CallableDescriptor targetDescriptor) {
+    private static boolean needSyntheticAccessor(LexicalScope invokationScope, CallableDescriptor targetDescriptor) {
         return targetDescriptor instanceof CallableMemberDescriptor &&
                Visibilities.isPrivate(targetDescriptor.getVisibility()) &&
                targetDescriptor.getContainingDeclaration() != invokationScope.getOwnerDescriptor().getContainingDeclaration();

compiler/testData/codegen/box/innerNested/kt5363.kt

@@ -0,0 +1,13 @@
+class Outer {
+    class Nested{
+        fun foo(s: String) = s.extension()
+    }
+
+    companion object {
+        private fun String.extension(): String = this
+    }
+}
+
+fun box(): String {
+    return Outer.Nested().foo("OK")
+}

compiler/testData/codegen/box/innerNested/kt6804.kt

@@ -0,0 +1,13 @@
+class Outer {
+    class Nested {
+        fun fn() = s
+    }
+
+    companion object {
+        private val s = "OK"
+    }
+}
+
+fun box(): String {
+    return Outer.Nested().fn()
+}

compiler/tests/org/jetbrains/kotlin/codegen/generated/BlackBoxCodegenTestGenerated.java

@@ -4498,6 +4498,18 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             doTest(fileName);
         }
 
+        @TestMetadata("kt5363.kt")
+        public void testKt5363() throws Exception {
+            String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/box/innerNested/kt5363.kt");
+            doTest(fileName);
+        }
+
+        @TestMetadata("kt6804.kt")
+        public void testKt6804() throws Exception {
+            String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/box/innerNested/kt6804.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("nestedClassInObject.kt")
         public void testNestedClassInObject() throws Exception {
             String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/box/innerNested/nestedClassInObject.kt");