Enable JVM IR by default if language version is >= 1.5

#KT-44021 Fixed

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

@@ -165,7 +165,12 @@ fun CompilerConfiguration.configureAdvancedJvmOptions(arguments: K2JVMCompilerAr
 
     put(JVMConfigurationKeys.PARAMETERS_METADATA, arguments.javaParameters)
 
-    val useIR = (arguments.useIR && !arguments.useOldBackend) || arguments.useFir
+    val useIR = arguments.useFir ||
+            if (languageVersionSettings.supportsFeature(LanguageFeature.JvmIrEnabledByDefault)) {
+                !arguments.useOldBackend
+            } else {
+                arguments.useIR && !arguments.useOldBackend
+            }
     put(JVMConfigurationKeys.IR, useIR)
 
     val abiStability = JvmAbiStability.fromStringOrNull(arguments.abiStability)

compiler/testData/cli/jvm/jvmIrByDefault.kt

@@ -0,0 +1,13 @@
+// This test checks that with -language-version 1.4 we're using the old JVM backend,
+// and with -language-version 1.5 -- the new JVM IR backend.
+// JVM IR doesn't produce classes for local functions, so the test checks which backend
+// is used by asserting that the file for the anonymous class does or doesn't exist.
+
+// Feel free to remove both _1_4 and _1_5 tests as soon as either the old JVM backend
+// or LV 1.4 is removed, whichever happens earlier.
+
+class C {
+    fun test() {
+        fun local() {}
+    }
+}

compiler/testData/cli/jvm/jvmIrByDefault1_4.args

@@ -0,0 +1,5 @@
+$TESTDATA_DIR$/jvmIrByDefault.kt
+-d
+$TEMP_DIR$
+-language-version
+1.4

compiler/testData/cli/jvm/jvmIrByDefault1_4.out

@@ -0,0 +1 @@
+OK

compiler/testData/cli/jvm/jvmIrByDefault1_4.test

@@ -0,0 +1 @@
+// EXISTS: C$test$1.class

compiler/testData/cli/jvm/jvmIrByDefault1_5.args

@@ -0,0 +1,5 @@
+$TESTDATA_DIR$/jvmIrByDefault.kt
+-d
+$TEMP_DIR$
+-language-version
+1.5

compiler/testData/cli/jvm/jvmIrByDefault1_5.out

@@ -0,0 +1,2 @@
+warning: language version 1.5 is experimental, there are no backwards compatibility guarantees for new language and library features
+OK

compiler/testData/cli/jvm/jvmIrByDefault1_5.test

@@ -0,0 +1 @@
+// ABSENT: C$test$1.class

compiler/tests-gen/org/jetbrains/kotlin/cli/CliTestGenerated.java

@@ -541,6 +541,16 @@ public class CliTestGenerated extends AbstractCliTest {
             runTest("compiler/testData/cli/jvm/jvmDefaultAll.args");
         }
 
+        @TestMetadata("jvmIrByDefault1_4.args")
+        public void testJvmIrByDefault1_4() throws Exception {
+            runTest("compiler/testData/cli/jvm/jvmIrByDefault1_4.args");
+        }
+
+        @TestMetadata("jvmIrByDefault1_5.args")
+        public void testJvmIrByDefault1_5() throws Exception {
+            runTest("compiler/testData/cli/jvm/jvmIrByDefault1_5.args");
+        }
+
         @TestMetadata("jvmRecordOk.args")
         public void testJvmRecordOk() throws Exception {
             runTest("compiler/testData/cli/jvm/jvmRecordOk.args");

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

@@ -144,6 +144,7 @@ enum class LanguageFeature(
 
     AllowSealedInheritorsInDifferentFilesOfSamePackage(KOTLIN_1_5),
     SealedInterfaces(KOTLIN_1_5),
+    JvmIrEnabledByDefault(KOTLIN_1_5),
 
     /*
      * Improvements include the following: