diff --git a/core/descriptors/src/org/jetbrains/kotlin/builtins/functionTypes.kt b/core/descriptors/src/org/jetbrains/kotlin/builtins/functionTypes.kt
index bb6656e018d..2b6a625af10 100644
--- a/core/descriptors/src/org/jetbrains/kotlin/builtins/functionTypes.kt
+++ b/core/descriptors/src/org/jetbrains/kotlin/builtins/functionTypes.kt
@@ -17,10 +17,10 @@
 package org.jetbrains.kotlin.builtins
 
 import org.jetbrains.kotlin.builtins.functions.BuiltInFictitiousFunctionClassFactory
-import org.jetbrains.kotlin.descriptors.SourceElement
 import org.jetbrains.kotlin.builtins.functions.FunctionClassDescriptor
 import org.jetbrains.kotlin.descriptors.ClassDescriptor
 import org.jetbrains.kotlin.descriptors.DeclarationDescriptor
+import org.jetbrains.kotlin.descriptors.SourceElement
 import org.jetbrains.kotlin.descriptors.annotations.AnnotationDescriptorImpl
 import org.jetbrains.kotlin.descriptors.annotations.Annotations
 import org.jetbrains.kotlin.descriptors.annotations.AnnotationsImpl
@@ -80,6 +80,8 @@ val KotlinType.isBuiltinFunctionalType: Boolean
     }
 
 fun isBuiltinFunctionClass(classId: ClassId): Boolean {
+    if (!classId.startsWith(KotlinBuiltIns.BUILT_INS_PACKAGE_NAME)) return false
+
     val kind = classId.asSingleFqName().toUnsafe().getFunctionalClassKind()
     return kind == FunctionClassDescriptor.Kind.Function ||
            kind == FunctionClassDescriptor.Kind.SuspendFunction
@@ -102,27 +104,21 @@ private val KotlinType.isTypeAnnotatedWithExtensionFunctionType: Boolean
  * e.g. kotlin.Function1 (but NOT kotlin.reflect.KFunction1)
  */
 fun isNumberedFunctionClassFqName(fqName: FqNameUnsafe): Boolean {
-    if (!fqName.startsWith(KotlinBuiltIns.BUILT_INS_PACKAGE_NAME)) return false
-
-    val segments = fqName.pathSegments()
-    if (segments.size != 2) return false
-
-    val shortName = segments.last().asString()
-    return BuiltInFictitiousFunctionClassFactory.isFunctionClassName(shortName, KotlinBuiltIns.BUILT_INS_PACKAGE_FQ_NAME)
+    return fqName.startsWith(KotlinBuiltIns.BUILT_INS_PACKAGE_NAME) &&
+           fqName.getFunctionalClassKind() == FunctionClassDescriptor.Kind.Function
 }
 
 fun DeclarationDescriptor.getFunctionalClassKind(): FunctionClassDescriptor.Kind? {
     if (this !is ClassDescriptor) return null
+    if (!KotlinBuiltIns.isUnderKotlinPackage(this)) return null
 
-    val fqNameUnsafe = this.fqNameUnsafe
     return fqNameUnsafe.getFunctionalClassKind()
 }
 
-fun FqNameUnsafe.getFunctionalClassKind(): FunctionClassDescriptor.Kind? {
+private fun FqNameUnsafe.getFunctionalClassKind(): FunctionClassDescriptor.Kind? {
     if (!isSafe || isRoot) return null
-    val fqName = toSafe()
 
-    return BuiltInFictitiousFunctionClassFactory.getFunctionalClassKind(fqName.shortName().asString(), fqName.parent())
+    return BuiltInFictitiousFunctionClassFactory.getFunctionalClassKind(shortName().asString(), toSafe().parent())
 }
 
 
diff --git a/core/descriptors/src/org/jetbrains/kotlin/builtins/functions/BuiltInFictitiousFunctionClassFactory.kt b/core/descriptors/src/org/jetbrains/kotlin/builtins/functions/BuiltInFictitiousFunctionClassFactory.kt
index a4ff5b3ea6c..e141ccff1c4 100644
--- a/core/descriptors/src/org/jetbrains/kotlin/builtins/functions/BuiltInFictitiousFunctionClassFactory.kt
+++ b/core/descriptors/src/org/jetbrains/kotlin/builtins/functions/BuiltInFictitiousFunctionClassFactory.kt
@@ -48,18 +48,10 @@ class BuiltInFictitiousFunctionClassFactory(
             return KindWithArity(kind, arity)
         }
 
-        @JvmStatic fun getFunctionalClassKind(className: String, packageFqName: FqName) =
+        @JvmStatic
+        fun getFunctionalClassKind(className: String, packageFqName: FqName) =
                 parseClassName(className, packageFqName)?.kind
 
-        @JvmStatic fun getFunctionalClassArity(className: String, packageFqName: FqName) =
-                parseClassName(className, packageFqName)?.arity
-
-        @JvmStatic fun isFunctionClassName(className: String, packageFqName: FqName) =
-                getFunctionalClassKind(className, packageFqName) == Kind.Function
-
-        @JvmStatic fun isSuspendFunctionClassName(className: String, packageFqName: FqName) =
-                getFunctionalClassKind(className, packageFqName) == Kind.SuspendFunction
-
         private fun toInt(s: String): Int? {
             if (s.isEmpty()) return null
 
diff --git a/core/descriptors/src/org/jetbrains/kotlin/name/ClassId.java b/core/descriptors/src/org/jetbrains/kotlin/name/ClassId.java
index 547cd70cf3b..fa5fbf4d8a4 100644
--- a/core/descriptors/src/org/jetbrains/kotlin/name/ClassId.java
+++ b/core/descriptors/src/org/jetbrains/kotlin/name/ClassId.java
@@ -90,6 +90,10 @@ public final class ClassId {
         return new FqName(packageFqName.asString() + "." + relativeClassName.asString());
     }
 
+    public boolean startsWith(@NotNull Name segment) {
+        return packageFqName.startsWith(segment);
+    }
+
     /**
      * @return a string where packages are delimited by '/' and classes by '.', e.g. "kotlin/Map.Entry"
      */