Forbid ExtensionFunctionType on functional types without parameters

#KT-43527 Fixed

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

@@ -278,6 +278,7 @@ public interface Errors {
     DiagnosticFactory0<KtAnnotationEntry> REPEATED_ANNOTATION = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory0<KtAnnotationEntry> REPEATED_ANNOTATION_WARNING = DiagnosticFactory0.create(WARNING);
     DiagnosticFactory0<KtAnnotationEntry> NON_SOURCE_ANNOTATION_ON_INLINED_LAMBDA_EXPRESSION = DiagnosticFactory0.create(ERROR);
+    DiagnosticFactory0<KtAnnotationEntry> WRONG_EXTENSION_FUNCTION_TYPE = DiagnosticFactory0.create(ERROR);
 
     // Annotations
 

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

@@ -146,6 +146,7 @@ public class DefaultErrorMessages {
         MAP.put(REPEATED_ANNOTATION, "This annotation is not repeatable");
         MAP.put(REPEATED_ANNOTATION_WARNING, "This annotation is not repeatable");
         MAP.put(NON_SOURCE_ANNOTATION_ON_INLINED_LAMBDA_EXPRESSION, "The lambda expression here is an inlined argument so this annotation cannot be stored anywhere");
+        MAP.put(WRONG_EXTENSION_FUNCTION_TYPE, "ExtensionFunctionType is forbidden on a function type without parameters");
 
         MAP.put(INAPPLICABLE_TARGET_ON_PROPERTY, "''@{0}:'' annotations could be applied only to property declarations", TO_STRING);
         MAP.put(INAPPLICABLE_TARGET_PROPERTY_IMMUTABLE, "''@{0}:'' annotations could be applied only to mutable properties", TO_STRING);

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

@@ -6,6 +6,7 @@
 package org.jetbrains.kotlin.resolve
 
 import org.jetbrains.kotlin.builtins.StandardNames
+import org.jetbrains.kotlin.builtins.isFunctionOrKFunctionTypeWithAnySuspendability
 import org.jetbrains.kotlin.config.LanguageFeature.*
 import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.descriptors.ClassDescriptor
@@ -19,11 +20,13 @@ import org.jetbrains.kotlin.diagnostics.Errors
 import org.jetbrains.kotlin.diagnostics.reportDiagnosticOnce
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
+import org.jetbrains.kotlin.name.StandardClassIds
 import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.psi.psiUtil.getAnnotationEntries
 import org.jetbrains.kotlin.resolve.constants.ArrayValue
 import org.jetbrains.kotlin.resolve.constants.EnumValue
 import org.jetbrains.kotlin.resolve.descriptorUtil.annotationClass
+import org.jetbrains.kotlin.resolve.descriptorUtil.classId
 import org.jetbrains.kotlin.resolve.descriptorUtil.getAnnotationRetention
 import org.jetbrains.kotlin.resolve.descriptorUtil.isAnnotatedWithKotlinRepeatable
 import org.jetbrains.kotlin.resolve.inline.InlineUtil
@@ -163,9 +166,9 @@ class AnnotationChecker(
         checkWithoutLanguageFeature: Boolean = false
     ) {
         val shouldRunCheck = isSuperType || shouldCheckReferenceItself
-        if (shouldRunCheck) {
-            for (entry in reference.annotationEntries) {
-                val descriptor = trace.get(BindingContext.ANNOTATION, entry)
+        for (entry in reference.annotationEntries) {
+            val descriptor = trace.get(BindingContext.ANNOTATION, entry)
+            if (shouldRunCheck) {
                 if (descriptor is LazyAnnotationDescriptor) {
                     /*
                      * There are no users of type annotations until backend, so if there are errors
@@ -181,6 +184,12 @@ class AnnotationChecker(
                     checkAnnotationEntry(entry, actualTargets, trace)
                 }
             }
+            if (descriptor?.annotationClass?.classId == StandardClassIds.Annotations.ExtensionFunctionType) {
+                val type = trace[BindingContext.TYPE, reference]
+                if (type != null && type.isFunctionOrKFunctionTypeWithAnySuspendability && type.arguments.size <= 1) {
+                    trace.report(Errors.WRONG_EXTENSION_FUNCTION_TYPE.on(entry))
+                }
+            }
         }
         val typeArguments = reference.typeElement?.typeArgumentsAsTypes ?: return
         for (typeArgument in typeArguments) {