Generate 'DefaultImpls' for jvm 8 target only within compiler option

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JVMCompilerArguments.java

@@ -98,6 +98,9 @@ public class K2JVMCompilerArguments extends CommonCompilerArguments {
     @Argument(value = "Xsingle-module", description = "Combine modules for source files and binary dependencies into a single module")
     public boolean singleModule;
 
+    @Argument(value = "Xinterface-compatibility", description = "Generate DefaultImpls classes for interfaces in JVM target bytecode version 1.8 for binary compatibility with 1.6")
+    public boolean interfaceCompatibility;
+
     // Paths to output directories for friend modules.
     public String[] friendPaths;
 

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/K2JVMCompiler.kt

@@ -139,6 +139,17 @@ class K2JVMCompiler : CLICompiler<K2JVMCompilerArguments>() {
             }
         }
 
+        if (arguments.interfaceCompatibility) {
+            val target = configuration.get(JVMConfigurationKeys.JVM_TARGET)
+            if (target != JvmTarget.JVM_1_8) {
+                val errorMessage = "The -Xinterface-compatibility option has effect only for JVM target bytecode version 1.8."
+                messageCollector.report(CompilerMessageSeverity.WARNING, errorMessage, CompilerMessageLocation.NO_LOCATION)
+            }
+            else {
+                configuration.put(JVMConfigurationKeys.INTERFACE_COMPATIBILITY, true)
+            }
+        }
+
         putAdvancedOptions(configuration, arguments)
 
         messageCollector.report(CompilerMessageSeverity.LOGGING, "Configuring the compilation environment", CompilerMessageLocation.NO_LOCATION)