JS: don't report error when FAKE function overrides external function with optional parameters. Report only when it overrides at least two such functions. See KT-15961

compiler/testData/diagnostics/testsWithJsStdLib/name/jsNameAndNamedNative.kt

@@ -1,4 +1,4 @@
-// !DIAGNOSTICS:  -DEPRECATION
+// !DIAGNOSTICS:  -DEPRECATION, -DEPRECATION_ERROR
 @native("B")
 <!JS_NAME_PROHIBITED_FOR_NAMED_NATIVE!>@JsName("C")<!>
 class A

compiler/testData/diagnostics/testsWithJsStdLib/native/overrideOptionalParam.kt

@@ -6,6 +6,8 @@ class B : A() {
     <!OVERRIDING_EXTERNAL_FUN_WITH_OPTIONAL_PARAMS!>override fun f(x: Int)<!> {}
 }
 
+class BB : A()
+
 external class C : A {
     override fun f(x: Int)
 }
@@ -19,6 +21,12 @@ interface J {
     fun f(x: Int = 23)
 }
 
+interface II {
+    fun f(x: Int)
+}
+
+interface IIJ : II, J
+
 open external class D {
     open fun f(x: Int)
 }
@@ -45,6 +53,8 @@ open external class XE {
 
 class <!OVERRIDING_EXTERNAL_FUN_WITH_OPTIONAL_PARAMS_WITH_FAKE!>Y<!> : X(), I
 
+class <!OVERRIDING_EXTERNAL_FUN_WITH_OPTIONAL_PARAMS_WITH_FAKE!>YY<!> : A(), II
+
 external class YE: XE, I
 
 class Z : X(), J

compiler/testData/diagnostics/testsWithJsStdLib/native/overrideOptionalParam.txt

@@ -16,6 +16,14 @@ public final class B : A {
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }
 
+public final class BB : A {
+    public constructor BB()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun f(/*0*/ x: kotlin.Int = ...): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
 public external final class C : A {
     public constructor C()
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
@@ -63,6 +71,20 @@ public external interface I {
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }
 
+public interface II {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun f(/*0*/ x: kotlin.Int): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IIJ : II, J {
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*2*/ /*fake_override*/ fun f(/*0*/ x: kotlin.Int = ...): kotlin.Unit
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
 public interface J {
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
     public abstract fun f(/*0*/ x: kotlin.Int = ...): kotlin.Unit
@@ -102,6 +124,14 @@ public external final class YE : XE, I {
     public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
 }
 
+public final class YY : A, II {
+    public constructor YY()
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun f(/*0*/ x: kotlin.Int = ...): kotlin.Unit
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
 public final class Z : X, J {
     public constructor Z()
     public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsInheritanceChecker.kt

@@ -71,7 +71,7 @@ object JsInheritanceChecker : SimpleDeclarationChecker {
     private fun findFakeMethodOverridingExternalWithOptionalParams(cls: ClassDescriptor): FunctionDescriptor? {
         val members = cls.unsubstitutedMemberScope.getContributedDescriptors(DescriptorKindFilter.CALLABLES)
                 .mapNotNull { it as? FunctionDescriptor }
-                .filter { it.containingDeclaration == cls && !it.kind.isReal }
+                .filter { it.containingDeclaration == cls && !it.kind.isReal && it.overriddenDescriptors.size > 1 }
 
         return members.firstOrNull { isOverridingExternalWithOptionalParams(it) }
     }