Do not use LanguageVersionSettings.languageVersion directly

compiler/backend/src/org/jetbrains/kotlin/codegen/ExpressionCodegen.java

@@ -51,7 +51,6 @@ import org.jetbrains.kotlin.codegen.state.KotlinTypeMapper;
 import org.jetbrains.kotlin.codegen.when.SwitchCodegen;
 import org.jetbrains.kotlin.codegen.when.SwitchCodegenUtil;
 import org.jetbrains.kotlin.config.ApiVersion;
-import org.jetbrains.kotlin.config.LanguageVersion;
 import org.jetbrains.kotlin.descriptors.*;
 import org.jetbrains.kotlin.descriptors.impl.LocalVariableDescriptor;
 import org.jetbrains.kotlin.descriptors.impl.SyntheticFieldDescriptor;
@@ -145,8 +144,6 @@ public class ExpressionCodegen extends KtVisitor<StackValue, StackValue> impleme
     private boolean shouldMarkLineNumbers = true;
     private int finallyDepth = 0;
 
-    private static final ApiVersion apiVersion1_1 = ApiVersion.createByLanguageVersion(LanguageVersion.KOTLIN_1_1);
-
     public ExpressionCodegen(
             @NotNull MethodVisitor mv,
             @NotNull FrameMap frameMap,
@@ -3656,8 +3653,7 @@ public class ExpressionCodegen extends KtVisitor<StackValue, StackValue> impleme
                 // of version 1.0 is used and the user forgot to explicitly specify language version / API version.
                 // This should be enabled when/if we decide to drop support for "-language-version 1.0" / "-api-version 1.0"
                 //noinspection ConstantConditions, PointlessBooleanExpression
-                if (false && state.getLanguageVersionSettings().getLanguageVersion() != LanguageVersion.KOTLIN_1_0 &&
-                    state.getLanguageVersionSettings().getApiVersion().compareTo(apiVersion1_1) >= 0) {
+                if (false && state.getLanguageVersionSettings().getApiVersion().compareTo(ApiVersion.KOTLIN_1_1) >= 0) {
                     return StackValue.operation(Type.BOOLEAN_TYPE, new Function1<InstructionAdapter, Unit>() {
                         @Override
                         public Unit invoke(InstructionAdapter v) {

compiler/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/JvmDelegationFilter.kt

@@ -17,7 +17,7 @@
 package org.jetbrains.kotlin.resolve.jvm
 
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns
-import org.jetbrains.kotlin.config.LanguageVersion
+import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.descriptors.CallableMemberDescriptor
 import org.jetbrains.kotlin.descriptors.Modality
@@ -29,7 +29,8 @@ import org.jetbrains.kotlin.serialization.deserialization.PLATFORM_DEPENDENT_ANN
 object JvmDelegationFilter : DelegationFilter {
 
     override fun filter(interfaceMember: CallableMemberDescriptor, languageVersionSettings: LanguageVersionSettings): Boolean {
-        if (languageVersionSettings.languageVersion == LanguageVersion.KOTLIN_1_0) return true
+        if (!languageVersionSettings.supportsFeature(LanguageFeature.NoDelegationToJavaDefaultInterfaceMembers)) return true
+
         //We always have only one implementation otherwise it's an error in kotlin and java
         val realMember = DescriptorUtils.unwrapFakeOverride(interfaceMember)
         return !isJavaDefaultMethod(realMember) && !isBuiltInMemberMappedToJavaDefault(realMember)

compiler/util/src/org/jetbrains/kotlin/config/ApiVersion.kt

@@ -34,6 +34,9 @@ class ApiVersion private constructor(
     override fun toString() = versionString
 
     companion object {
+        @JvmField
+        val KOTLIN_1_1 = createByLanguageVersion(LanguageVersion.KOTLIN_1_1)
+
         @JvmField
         val LATEST: ApiVersion = createByLanguageVersion(LanguageVersion.Companion.LATEST)
 

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -41,6 +41,7 @@ enum class LanguageFeature(val sinceVersion: LanguageVersion?, val hintUrl: Stri
     RefinedSamAdaptersPriority(KOTLIN_1_1),
     SafeCallBoundSmartCasts(KOTLIN_1_1),
     TypeInferenceOnGenericsForCallableReferences(KOTLIN_1_1),
+    NoDelegationToJavaDefaultInterfaceMembers(KOTLIN_1_1),
 
     // Experimental features
     MultiPlatformProjects(null),
@@ -89,8 +90,10 @@ enum class LanguageVersion(val major: Int, val minor: Int) : DescriptionAware {
 interface LanguageVersionSettings {
     fun supportsFeature(feature: LanguageFeature): Boolean
 
-    val languageVersion: LanguageVersion
     val apiVersion: ApiVersion
+
+    // Please do not use this to enable/disable specific features/checks. Instead add a new LanguageFeature entry and call supportsFeature
+    val languageVersion: LanguageVersion
 }
 
 class LanguageVersionSettingsImpl @JvmOverloads constructor(