IDEA-91665 (highlight extension methods outside interfaces)

java/java-impl/src/com/intellij/codeInsight/daemon/impl/analysis/HighlightMethodUtil.java

@@ -850,16 +850,21 @@ public class HighlightMethodUtil {
   static HighlightInfo checkMethodCanHaveBody(PsiMethod method) {
     if (method.getBody() == null) return null;
     PsiClass aClass = method.getContainingClass();
+    boolean isInterface = aClass != null && aClass.isInterface();
+    boolean isExtension = PsiUtil.isExtensionMethod(method);
 
     String message = null;
-    if (aClass != null && aClass.isInterface()) {
-      if (!PsiUtil.isExtensionMethod(method)) {
+    if (isInterface) {
+      if (!isExtension) {
         message = JavaErrorMessages.message("interface.methods.cannot.have.body");
       }
       else {
         return HighlightUtil.checkExtensionMethodsFeature(method);
       }
     }
+    else if (isExtension) {
+      message = JavaErrorMessages.message("extension.method.in.class");
+    }
     else if (method.hasModifierProperty(PsiModifier.ABSTRACT)) {
       message = JavaErrorMessages.message("abstract.methods.cannot.have.a.body");
     }
@@ -871,7 +876,7 @@ public class HighlightMethodUtil {
     TextRange textRange = HighlightNamesUtil.getMethodDeclarationTextRange(method);
     HighlightInfo info = HighlightInfo.createHighlightInfo(HighlightInfoType.ERROR, textRange, message);
     QuickFixAction.registerQuickFixAction(info, new DeleteMethodBodyFix(method));
-    if (method.hasModifierProperty(PsiModifier.ABSTRACT) && aClass != null && !aClass.isInterface()) {
+    if (method.hasModifierProperty(PsiModifier.ABSTRACT) && isInterface) {
       IntentionAction fix = QUICK_FIX_FACTORY.createModifierListFix(method, PsiModifier.ABSTRACT, false, false);
       QuickFixAction.registerQuickFixAction(info, fix);
     }

java/java-psi-impl/src/messages/JavaErrorMessages.properties

@@ -173,6 +173,7 @@ incompatible.types.html.tooltip=\
 interface.methods.cannot.have.body=Interface methods cannot have body
 abstract.methods.cannot.have.a.body=Abstract methods cannot have a body
 native.methods.cannot.have.a.body=Native methods cannot have a body
+extension.method.in.class=Extension methods can only be used within an interface
 
 instance.method.cannot.override.static.method=Instance method ''{0}'' in ''{1}'' cannot override static method ''{2}'' in ''{3}''
 static.method.cannot.override.instance.method=Static method ''{0}'' in ''{1}'' cannot override instance method ''{2}'' in ''{3}''

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/advHighlighting7/ExtensionMethods.java

@@ -43,4 +43,8 @@ class C {
       }
     }.m();
   }
+}
+
+class D {
+  <error descr="Extension methods can only be used within an interface">void m()</error> default { }
 }