Make EXPERIMENTAL_ANNOTATION_ON_OVERRIDE warning till 1.6

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java

@@ -292,6 +292,7 @@ public interface Errors {
 
     DiagnosticFactory1<KtAnnotationEntry, String> EXPERIMENTAL_ANNOTATION_ON_WRONG_TARGET = DiagnosticFactory1.create(ERROR);
     DiagnosticFactory0<KtAnnotationEntry> EXPERIMENTAL_ANNOTATION_ON_OVERRIDE = DiagnosticFactory0.create(ERROR);
+    DiagnosticFactory0<KtAnnotationEntry> EXPERIMENTAL_ANNOTATION_ON_OVERRIDE_WARNING = DiagnosticFactory0.create(WARNING);
 
     DiagnosticFactory1<PsiElement, String> EXPERIMENTAL_UNSIGNED_LITERALS = DiagnosticFactory1.create(WARNING);
     DiagnosticFactory1<PsiElement, String> EXPERIMENTAL_UNSIGNED_LITERALS_ERROR = DiagnosticFactory1.create(ERROR);

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -174,6 +174,7 @@ public class DefaultErrorMessages {
 
         MAP.put(EXPERIMENTAL_ANNOTATION_ON_WRONG_TARGET, "Opt-in requirement marker annotation cannot be used on {0}", STRING);
         MAP.put(EXPERIMENTAL_ANNOTATION_ON_OVERRIDE, "Opt-in requirement marker annotation on override requires the same marker on base declaration");
+        MAP.put(EXPERIMENTAL_ANNOTATION_ON_OVERRIDE_WARNING, "Opt-in requirement marker annotation on override requires the same marker on base declaration (will be an error in 1.6)");
 
         MAP.put(EXPERIMENTAL_UNSIGNED_LITERALS, "{0}", STRING);
         MAP.put(EXPERIMENTAL_UNSIGNED_LITERALS_ERROR, "{0}", STRING);

compiler/frontend/src/org/jetbrains/kotlin/resolve/AnnotationChecker.kt

@@ -230,7 +230,7 @@ class AnnotationChecker(
         }
 
         for (checker in additionalCheckers) {
-            checker.checkEntries(entries, actualTargets.defaultTargets, trace)
+            checker.checkEntries(entries, actualTargets.defaultTargets, trace, languageVersionSettings)
         }
     }
 
@@ -381,5 +381,10 @@ private typealias TargetLists = AnnotationTargetLists
 private typealias TargetList = AnnotationTargetList
 
 interface AdditionalAnnotationChecker {
-    fun checkEntries(entries: List<KtAnnotationEntry>, actualTargets: List<KotlinTarget>, trace: BindingTrace)
+    fun checkEntries(
+        entries: List<KtAnnotationEntry>,
+        actualTargets: List<KotlinTarget>,
+        trace: BindingTrace,
+        languageVersionSettings: LanguageVersionSettings
+    )
 }

compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/ExperimentalMarkerDeclarationAnnotationChecker.kt

@@ -6,6 +6,8 @@
 package org.jetbrains.kotlin.resolve.checkers
 
 import org.jetbrains.kotlin.builtins.StandardNames
+import org.jetbrains.kotlin.config.LanguageFeature
+import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.descriptors.CallableMemberDescriptor
 import org.jetbrains.kotlin.descriptors.ClassDescriptor
 import org.jetbrains.kotlin.descriptors.ModuleDescriptor
@@ -37,7 +39,8 @@ class ExperimentalMarkerDeclarationAnnotationChecker(private val module: ModuleD
     override fun checkEntries(
         entries: List<KtAnnotationEntry>,
         actualTargets: List<KotlinTarget>,
-        trace: BindingTrace
+        trace: BindingTrace,
+        languageVersionSettings: LanguageVersionSettings
     ) {
         var isAnnotatedWithExperimental = false
 
@@ -88,7 +91,11 @@ class ExperimentalMarkerDeclarationAnnotationChecker(private val module: ModuleD
                     if (descriptor is CallableMemberDescriptor &&
                         !descriptor.hasExperimentalOverriddenDescriptors(annotationClass.fqNameSafe)
                     ) {
-                        trace.report(Errors.EXPERIMENTAL_ANNOTATION_ON_OVERRIDE.on(entry))
+                        if (languageVersionSettings.supportsFeature(LanguageFeature.OptInOnOverrideForbidden)) {
+                            trace.report(Errors.EXPERIMENTAL_ANNOTATION_ON_OVERRIDE.on(entry))
+                        } else {
+                            trace.report(Errors.EXPERIMENTAL_ANNOTATION_ON_OVERRIDE_WARNING.on(entry))
+                        }
                     }
                 }
             }