Prohibit using old JVM backend with LV >= 1.6

#KT-48928 Fixed

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/CommonCompilerArguments.kt

@@ -531,9 +531,9 @@ abstract class CommonCompilerArguments : CommonToolArguments() {
     }
 
     fun toLanguageVersionSettings(collector: MessageCollector): LanguageVersionSettings {
-
         // If only "-api-version" is specified, language version is assumed to be the latest stable
-        val languageVersion = parseVersion(collector, languageVersion, "language") ?: LanguageVersion.LATEST_STABLE
+        val languageVersion = parseVersion(collector, languageVersion, "language")
+            ?: defaultLanguageVersion(collector)
 
         // If only "-language-version" is specified, API version is assumed to be equal to the language version
         // (API version cannot be greater than the language version)
@@ -554,6 +554,8 @@ abstract class CommonCompilerArguments : CommonToolArguments() {
             checkProgressiveMode(languageVersion, collector)
         }
 
+        checkPlatformSpecificSettings(languageVersionSettings, collector)
+
         return languageVersionSettings
     }
 
@@ -622,6 +624,12 @@ abstract class CommonCompilerArguments : CommonToolArguments() {
         }
     }
 
+    protected open fun defaultLanguageVersion(collector: MessageCollector): LanguageVersion =
+        LanguageVersion.LATEST_STABLE
+
+    protected open fun checkPlatformSpecificSettings(languageVersionSettings: LanguageVersionSettings, collector: MessageCollector) {
+    }
+
     private enum class VersionKind(val text: String) {
         LANGUAGE("Language"), API("API")
     }

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

@@ -534,4 +534,26 @@ default: `indy-with-constants` for JVM target 9 or greater, `inline` otherwise""
         }
         return result
     }
+
+    override fun defaultLanguageVersion(collector: MessageCollector): LanguageVersion =
+        if (useOldBackend) {
+            if (!suppressVersionWarnings) {
+                collector.report(
+                    CompilerMessageSeverity.STRONG_WARNING,
+                    "Language version is automatically inferred to ${LanguageVersion.KOTLIN_1_5.versionString} when using " +
+                            "the old JVM backend. Consider specifying -language-version explicitly, or remove -Xuse-old-backend"
+                )
+            }
+            LanguageVersion.KOTLIN_1_5
+        } else super.defaultLanguageVersion(collector)
+
+    override fun checkPlatformSpecificSettings(languageVersionSettings: LanguageVersionSettings, collector: MessageCollector) {
+        if (useOldBackend && languageVersionSettings.languageVersion >= LanguageVersion.KOTLIN_1_6) {
+            collector.report(
+                CompilerMessageSeverity.ERROR,
+                "Old JVM backend does not support language version 1.6 or above. " +
+                        "Please use language version 1.5 or below, or remove -Xuse-old-backend"
+            )
+        }
+    }
 }