diff --git a/compiler/fir/resolve/tests/org/jetbrains/kotlin/fir/FirOldFrontendDiagnosticsTestGenerated.java b/compiler/fir/resolve/tests/org/jetbrains/kotlin/fir/FirOldFrontendDiagnosticsTestGenerated.java
index 1fc8264d58d..d75c50f22b5 100644
--- a/compiler/fir/resolve/tests/org/jetbrains/kotlin/fir/FirOldFrontendDiagnosticsTestGenerated.java
+++ b/compiler/fir/resolve/tests/org/jetbrains/kotlin/fir/FirOldFrontendDiagnosticsTestGenerated.java
@@ -7924,6 +7924,11 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirOldFronte
             runTest("compiler/testData/diagnostics/tests/funInterface/basicFunInterfaceDisabled.kt");
         }
 
+        @TestMetadata("funInterfaceDeclarationCheck.kt")
+        public void testFunInterfaceDeclarationCheck() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/funInterface/funInterfaceDeclarationCheck.kt");
+        }
+
         @TestMetadata("funInterfaceSyntheticConstructors.kt")
         public void testFunInterfaceSyntheticConstructors() throws Exception {
             runTest("compiler/testData/diagnostics/tests/funInterface/funInterfaceSyntheticConstructors.kt");
diff --git a/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java b/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java
index c196b5cfa98..f9c6ae8ec17 100644
--- a/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java
+++ b/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java
@@ -360,6 +360,13 @@ public interface Errors {
     DiagnosticFactory0<PsiElement> RESULT_CLASS_IN_RETURN_TYPE = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory1<PsiElement, String> RESULT_CLASS_WITH_NULLABLE_OPERATOR = DiagnosticFactory1.create(ERROR);
 
+    // Fun interfaces
+
+    DiagnosticFactory0<PsiElement> FUN_INTERFACE_WRONG_COUNT_OF_ABSTRACT_MEMBERS = DiagnosticFactory0.create(ERROR);
+    DiagnosticFactory0<PsiElement> FUN_INTERFACE_CANNOT_HAVE_ABSTRACT_PROPERTIES = DiagnosticFactory0.create(ERROR);
+    DiagnosticFactory0<PsiElement> FUN_INTERFACE_ABSTRACT_METHOD_WITH_TYPE_PARAMETERS = DiagnosticFactory0.create(ERROR);
+    DiagnosticFactory0<PsiElement> FUN_INTERFACE_ABSTRACT_METHOD_WITH_DEFAULT_VALUE = DiagnosticFactory0.create(ERROR);
+
     // Secondary constructors
 
     DiagnosticFactory0<KtConstructorDelegationReferenceExpression> CYCLIC_CONSTRUCTOR_DELEGATION_CALL = DiagnosticFactory0.create(ERROR);
diff --git a/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java b/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java
index 31b9edd30c3..047486ff380 100644
--- a/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java
+++ b/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java
@@ -700,6 +700,11 @@ public class DefaultErrorMessages {
         MAP.put(RESULT_CLASS_IN_RETURN_TYPE, "'kotlin.Result' cannot be used as a return type");
         MAP.put(RESULT_CLASS_WITH_NULLABLE_OPERATOR, "Expression of type 'kotlin.Result' cannot be used as a left operand of ''{0}''", STRING);
 
+        MAP.put(FUN_INTERFACE_WRONG_COUNT_OF_ABSTRACT_MEMBERS, "Fun interfaces must have exactly one abstract method");
+        MAP.put(FUN_INTERFACE_CANNOT_HAVE_ABSTRACT_PROPERTIES, "Fun interfaces cannot have abstract properties");
+        MAP.put(FUN_INTERFACE_ABSTRACT_METHOD_WITH_TYPE_PARAMETERS, "Single abstract member cannot declare type parameters");
+        MAP.put(FUN_INTERFACE_ABSTRACT_METHOD_WITH_DEFAULT_VALUE, "Single abstract member cannot declare default values");
+
         MAP.put(VARIANCE_ON_TYPE_PARAMETER_NOT_ALLOWED, "Variance annotations are only allowed for type parameters of classes and interfaces");
         MAP.put(BOUND_ON_TYPE_ALIAS_PARAMETER_NOT_ALLOWED, "Bounds are not allowed on type alias parameters");
 
diff --git a/compiler/frontend/src/org/jetbrains/kotlin/resolve/PlatformConfiguratorBase.kt b/compiler/frontend/src/org/jetbrains/kotlin/resolve/PlatformConfiguratorBase.kt
index b3543fa49bf..6293c9cbbc1 100644
--- a/compiler/frontend/src/org/jetbrains/kotlin/resolve/PlatformConfiguratorBase.kt
+++ b/compiler/frontend/src/org/jetbrains/kotlin/resolve/PlatformConfiguratorBase.kt
@@ -35,7 +35,8 @@ private val DEFAULT_DECLARATION_CHECKERS = listOf(
     ExplicitApiDeclarationChecker(),
     TailrecFunctionChecker,
     TrailingCommaDeclarationChecker,
-    MissingDependencySupertypeChecker.ForDeclarations
+    MissingDependencySupertypeChecker.ForDeclarations,
+    FunInterfaceDeclarationChecker()
 )
 
 private val DEFAULT_CALL_CHECKERS = listOf(
diff --git a/compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/FunInterfaceDeclarationChecker.kt b/compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/FunInterfaceDeclarationChecker.kt
new file mode 100644
index 00000000000..d584dfea9ed
--- /dev/null
+++ b/compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/FunInterfaceDeclarationChecker.kt
@@ -0,0 +1,71 @@
+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.resolve.checkers
+
+import com.intellij.psi.PsiElement
+import org.jetbrains.kotlin.descriptors.*
+import org.jetbrains.kotlin.diagnostics.Errors
+import org.jetbrains.kotlin.psi.*
+import org.jetbrains.kotlin.resolve.calls.components.hasDefaultValue
+import org.jetbrains.kotlin.resolve.sam.getAbstractMembers
+import org.jetbrains.kotlin.resolve.source.getPsi
+
+class FunInterfaceDeclarationChecker : DeclarationChecker {
+    override fun check(declaration: KtDeclaration, descriptor: DeclarationDescriptor, context: DeclarationCheckerContext) {
+        if (declaration !is KtClass) return
+        if (descriptor !is ClassDescriptor || !descriptor.isFun) return
+
+        val funKeyword = declaration.getFunKeyword() ?: return
+
+        val abstractMembers = getAbstractMembers(descriptor)
+        for (abstractMember in abstractMembers) {
+            if (abstractMember !is PropertyDescriptor) continue
+
+            val reportOnProperty = abstractMember.containingDeclaration == descriptor
+            val reportOn = if (reportOnProperty) {
+                (abstractMember.source.getPsi() as? KtProperty)?.valOrVarKeyword ?: funKeyword
+            } else {
+                funKeyword
+            }
+
+            context.trace.report(Errors.FUN_INTERFACE_CANNOT_HAVE_ABSTRACT_PROPERTIES.on(reportOn))
+
+            if (!reportOnProperty) return // It's enough to report only once if abstract properties are in the base class
+        }
+
+        val abstractMember = abstractMembers.filterIsInstance<FunctionDescriptor>().singleOrNull()
+
+        if (abstractMember == null) {
+            context.trace.report(Errors.FUN_INTERFACE_WRONG_COUNT_OF_ABSTRACT_MEMBERS.on(funKeyword))
+            return
+        }
+
+        checkSingleAbstractMember(abstractMember, funKeyword, context)
+    }
+
+    private fun checkSingleAbstractMember(
+        abstractMember: FunctionDescriptor,
+        funInterfaceKeyword: PsiElement,
+        context: DeclarationCheckerContext,
+    ) {
+        val ktFunction = abstractMember.source.getPsi() as? KtNamedFunction
+        if (abstractMember.typeParameters.isNotEmpty()) {
+            val reportOn = ktFunction?.typeParameterList ?: ktFunction?.funKeyword ?: funInterfaceKeyword
+            context.trace.report(Errors.FUN_INTERFACE_ABSTRACT_METHOD_WITH_TYPE_PARAMETERS.on(reportOn))
+            return
+        }
+
+        for (parameter in abstractMember.valueParameters) {
+            if (parameter.hasDefaultValue()) {
+                val reportOn = parameter.source.getPsi() ?: ktFunction?.funKeyword ?: funInterfaceKeyword
+                context.trace.report(Errors.FUN_INTERFACE_ABSTRACT_METHOD_WITH_DEFAULT_VALUE.on(reportOn))
+                return
+            }
+        }
+    }
+}
+
+
diff --git a/compiler/psi/src/org/jetbrains/kotlin/psi/KtClass.kt b/compiler/psi/src/org/jetbrains/kotlin/psi/KtClass.kt
index d4dfcd7092a..7ca970386d0 100644
--- a/compiler/psi/src/org/jetbrains/kotlin/psi/KtClass.kt
+++ b/compiler/psi/src/org/jetbrains/kotlin/psi/KtClass.kt
@@ -40,6 +40,8 @@ open class KtClass : KtClassOrObject {
     fun getClassOrInterfaceKeyword(): PsiElement? = findChildByType(TokenSet.create(KtTokens.CLASS_KEYWORD, KtTokens.INTERFACE_KEYWORD))
 
     fun getClassKeyword(): PsiElement? = findChildByType(KtTokens.CLASS_KEYWORD)
+
+    fun getFunKeyword(): PsiElement? = modifierList?.getModifier(KtTokens.FUN_KEYWORD)
 }
 
 fun KtClass.createPrimaryConstructorIfAbsent(): KtPrimaryConstructor {
diff --git a/compiler/testData/diagnostics/tests/funInterface/funInterfaceDeclarationCheck.fir.kt b/compiler/testData/diagnostics/tests/funInterface/funInterfaceDeclarationCheck.fir.kt
new file mode 100644
index 00000000000..b93f7b75793
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/funInterface/funInterfaceDeclarationCheck.fir.kt
@@ -0,0 +1,104 @@
+// !LANGUAGE: +FunctionalInterfaceConversion
+
+fun interface Good {
+    fun invoke()
+}
+
+fun interface Foo1
+
+fun interface Foo2 {
+
+}
+
+fun interface Foo3 {
+    fun foo()
+    fun bar()
+}
+
+interface BaseWithSAM {
+    fun base()
+}
+
+fun interface Foo4 : BaseWithSAM {
+    fun oneMore()
+}
+
+fun interface Foo4WithDefault : BaseWithSAM {
+    fun oneMore() {}
+}
+
+interface BaseWithDefault {
+    fun def() {}
+}
+
+fun interface Foo4WithBaseDefault : BaseWithDefault {
+    fun oneMore()
+}
+
+fun interface GoodWithBase : BaseWithSAM
+
+fun interface Foo5 {
+    val prop: Int
+}
+
+fun interface Foo6 {
+    fun foo()
+    val prop: Int
+}
+
+fun interface Foo7 : BaseWithSAM {
+    val prop: Int
+}
+
+fun interface GoodWithPropAndBase : BaseWithSAM {
+    val prop: Int get() = 42
+}
+
+fun interface Foo8 {
+    fun <T> invoke(x: T)
+}
+
+fun interface GoodGeneric<T> {
+    fun invoke(x: T)
+}
+
+interface BaseWithGeneric {
+    fun <T> invoke(x: T)
+}
+
+fun interface Foo9 : BaseWithGeneric
+
+fun interface GoodExtensionGeneric : GoodGeneric<String>
+
+fun interface GoodSuspend {
+    suspend fun invoke()
+}
+
+class WithNestedFun<K> {
+    fun interface NestedSimple
+
+    fun interface GoodFun {
+        fun invoke()
+    }
+
+    fun interface NestedFun {
+        fun inovke(element: K)
+    }
+}
+
+fun <T> local() {
+    fun interface LocalFun {
+        fun invoke(element: T)
+    }
+}
+
+fun interface WithDefaultValue {
+    fun invoke(s: String = "")
+}
+
+interface BaseWithDefaultValue {
+    fun invoke(s: String = "")
+}
+
+fun interface DeriveDefault : BaseWithDefaultValue
+
diff --git a/compiler/testData/diagnostics/tests/funInterface/funInterfaceDeclarationCheck.kt b/compiler/testData/diagnostics/tests/funInterface/funInterfaceDeclarationCheck.kt
new file mode 100644
index 00000000000..2fef1295262
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/funInterface/funInterfaceDeclarationCheck.kt
@@ -0,0 +1,103 @@
+// !LANGUAGE: +FunctionalInterfaceConversion
+
+fun interface Good {
+    fun invoke()
+}
+
+<!FUN_INTERFACE_WRONG_COUNT_OF_ABSTRACT_MEMBERS!>fun<!> interface Foo1
+
+<!FUN_INTERFACE_WRONG_COUNT_OF_ABSTRACT_MEMBERS!>fun<!> interface Foo2 {
+
+}
+
+<!FUN_INTERFACE_WRONG_COUNT_OF_ABSTRACT_MEMBERS!>fun<!> interface Foo3 {
+    fun foo()
+    fun bar()
+}
+
+interface BaseWithSAM {
+    fun base()
+}
+
+<!FUN_INTERFACE_WRONG_COUNT_OF_ABSTRACT_MEMBERS!>fun<!> interface Foo4 : BaseWithSAM {
+    fun oneMore()
+}
+
+fun interface Foo4WithDefault : BaseWithSAM {
+    fun oneMore() {}
+}
+
+interface BaseWithDefault {
+    fun def() {}
+}
+
+fun interface Foo4WithBaseDefault : BaseWithDefault {
+    fun oneMore()
+}
+
+fun interface GoodWithBase : BaseWithSAM
+
+<!FUN_INTERFACE_WRONG_COUNT_OF_ABSTRACT_MEMBERS!>fun<!> interface Foo5 {
+    <!FUN_INTERFACE_CANNOT_HAVE_ABSTRACT_PROPERTIES!>val<!> prop: Int
+}
+
+fun interface Foo6 {
+    fun foo()
+    <!FUN_INTERFACE_CANNOT_HAVE_ABSTRACT_PROPERTIES!>val<!> prop: Int
+}
+
+fun interface Foo7 : BaseWithSAM {
+    <!FUN_INTERFACE_CANNOT_HAVE_ABSTRACT_PROPERTIES!>val<!> prop: Int
+}
+
+fun interface GoodWithPropAndBase : BaseWithSAM {
+    val prop: Int get() = 42
+}
+
+fun interface Foo8 {
+    fun <!FUN_INTERFACE_ABSTRACT_METHOD_WITH_TYPE_PARAMETERS!><T><!> invoke(x: T)
+}
+
+fun interface GoodGeneric<T> {
+    fun invoke(x: T)
+}
+
+interface BaseWithGeneric {
+    fun <T> invoke(x: T)
+}
+
+<!FUN_INTERFACE_ABSTRACT_METHOD_WITH_TYPE_PARAMETERS!>fun<!> interface Foo9 : BaseWithGeneric
+
+fun interface GoodExtensionGeneric : GoodGeneric<String>
+
+fun interface GoodSuspend {
+    suspend fun invoke()
+}
+
+class WithNestedFun<K> {
+    <!FUN_INTERFACE_WRONG_COUNT_OF_ABSTRACT_MEMBERS!>fun<!> interface NestedSimple
+
+    fun interface GoodFun {
+        fun invoke()
+    }
+
+    fun interface NestedFun {
+        fun inovke(element: <!UNRESOLVED_REFERENCE!>K<!>)
+    }
+}
+
+fun <T> local() {
+    <!LOCAL_INTERFACE_NOT_ALLOWED!>fun interface LocalFun<!> {
+        fun invoke(element: T)
+    }
+}
+
+fun interface WithDefaultValue {
+    fun invoke(<!FUN_INTERFACE_ABSTRACT_METHOD_WITH_DEFAULT_VALUE!>s: String = ""<!>)
+}
+
+interface BaseWithDefaultValue {
+    fun invoke(s: String = "")
+}
+
+<!FUN_INTERFACE_ABSTRACT_METHOD_WITH_DEFAULT_VALUE!>fun<!> interface DeriveDefault : BaseWithDefaultValue
diff --git a/compiler/testData/diagnostics/tests/funInterface/funInterfaceDeclarationCheck.txt b/compiler/testData/diagnostics/tests/funInterface/funInterfaceDeclarationCheck.txt
new file mode 100644
index 00000000000..97988bad390
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/funInterface/funInterfaceDeclarationCheck.txt
@@ -0,0 +1,196 @@
+package
+
+public fun </*0*/ T> local(): kotlin.Unit
+
+public interface BaseWithDefault {
+    public open fun def(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface BaseWithDefaultValue {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract fun invoke(/*0*/ s: kotlin.String = ...): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface BaseWithGeneric {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract fun </*0*/ T> invoke(/*0*/ x: T): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface BaseWithSAM {
+    public abstract fun base(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface DeriveDefault : BaseWithDefaultValue {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*1*/ /*fake_override*/ fun invoke(/*0*/ s: kotlin.String = ...): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface Foo1 {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface Foo2 {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface Foo3 {
+    public abstract fun bar(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface Foo4 : BaseWithSAM {
+    public abstract override /*1*/ /*fake_override*/ fun base(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract fun oneMore(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface Foo4WithBaseDefault : BaseWithDefault {
+    public open override /*1*/ /*fake_override*/ fun def(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract fun oneMore(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface Foo4WithDefault : BaseWithSAM {
+    public abstract override /*1*/ /*fake_override*/ fun base(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open fun oneMore(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface Foo5 {
+    public abstract val prop: kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface Foo6 {
+    public abstract val prop: kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface Foo7 : BaseWithSAM {
+    public abstract val prop: kotlin.Int
+    public abstract override /*1*/ /*fake_override*/ fun base(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface Foo8 {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract fun </*0*/ T> invoke(/*0*/ x: T): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface Foo9 : BaseWithGeneric {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*1*/ /*fake_override*/ fun </*0*/ T> invoke(/*0*/ x: T): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface Good {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract fun invoke(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface GoodExtensionGeneric : GoodGeneric<kotlin.String> {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*1*/ /*fake_override*/ fun invoke(/*0*/ x: kotlin.String): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface GoodGeneric</*0*/ T> {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract fun invoke(/*0*/ x: T): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface GoodSuspend {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract suspend fun invoke(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface GoodWithBase : BaseWithSAM {
+    public abstract override /*1*/ /*fake_override*/ fun base(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface GoodWithPropAndBase : BaseWithSAM {
+    public open val prop: kotlin.Int
+    public abstract override /*1*/ /*fake_override*/ fun base(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface WithDefaultValue {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract fun invoke(/*0*/ s: kotlin.String = ...): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class WithNestedFun</*0*/ K> {
+    public constructor WithNestedFun</*0*/ K>()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    public fun interface GoodFun {
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public abstract fun invoke(): kotlin.Unit
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public fun interface NestedFun {
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public abstract fun inovke(/*0*/ element: [ERROR : K]): kotlin.Unit
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public fun interface NestedSimple {
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}
diff --git a/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java b/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java
index a7c46fc948f..13cd0dcdc1f 100644
--- a/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java
+++ b/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java
@@ -7931,6 +7931,11 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 runTest("compiler/testData/diagnostics/tests/funInterface/basicFunInterfaceDisabled.kt");
             }
 
+            @TestMetadata("funInterfaceDeclarationCheck.kt")
+            public void testFunInterfaceDeclarationCheck() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/funInterface/funInterfaceDeclarationCheck.kt");
+            }
+
             @TestMetadata("funInterfaceSyntheticConstructors.kt")
             public void testFunInterfaceSyntheticConstructors() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/funInterface/funInterfaceSyntheticConstructors.kt");
diff --git a/compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java b/compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java
index 7f25f19028a..7b3758729cd 100644
--- a/compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java
+++ b/compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java
@@ -7926,6 +7926,11 @@ public class DiagnosticsUsingJavacTestGenerated extends AbstractDiagnosticsUsing
                 runTest("compiler/testData/diagnostics/tests/funInterface/basicFunInterfaceDisabled.kt");
             }
 
+            @TestMetadata("funInterfaceDeclarationCheck.kt")
+            public void testFunInterfaceDeclarationCheck() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/funInterface/funInterfaceDeclarationCheck.kt");
+            }
+
             @TestMetadata("funInterfaceSyntheticConstructors.kt")
             public void testFunInterfaceSyntheticConstructors() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/funInterface/funInterfaceSyntheticConstructors.kt");