Do not generate certain interface mixin forwarders if not necessary

compiler/src/dotty/tools/dotc/transform/MixinOps.scala

@@ -67,9 +67,16 @@ class MixinOps(cls: ClassSymbol, thisPhase: DenotTransformer)(using Context) {
     def generateSerializationForwarder: Boolean =
        (meth.name == nme.readResolve || meth.name == nme.writeReplace) && meth.info.paramNamess.flatten.isEmpty
 
+    // Even when mixinForwarderChoices option is true, we might not want to generate certain
+    // mixin forwarders to avoid discrepancies between java getInterfaces() and getGenericInterfaces(),
+    // as adding some unnecessary mixin forwarders forces us to extend the `interfaces` array
+    // of the generated class in its classfile
+    def isJavaInterfaceIndirectlyExtended =
+      meth.owner.isAllOf(Flags.JavaInterface) && !cls.parentSyms.contains(meth.owner)
+
     !meth.isConstructor &&
     meth.is(Method, butNot = PrivateOrAccessorOrDeferred) &&
-    (ctx.settings.mixinForwarderChoices.isTruthy || meth.owner.is(Scala2x) || needsDisambiguation || hasNonInterfaceDefinition ||
+    ((ctx.settings.mixinForwarderChoices.isTruthy && !isJavaInterfaceIndirectlyExtended)|| meth.owner.is(Scala2x) || needsDisambiguation || hasNonInterfaceDefinition ||
      generateJUnitForwarder || generateSerializationForwarder) &&
     isInImplementingClass(meth)
   }

tests/run/i21177a/BaseRootJava.java

@@ -0,0 +1,4 @@
+public interface BaseRootJava  {
+    default void someMethod() {
+    }
+}

tests/run/i21177a/GenericBaseIntermidiateJava.java

@@ -0,0 +1,2 @@
+public interface GenericBaseIntermidiateJava<T> extends BaseRootJava {
+}

tests/run/i21177a/Test.scala

@@ -0,0 +1,12 @@
+class ChildScala extends GenericBaseIntermidiateJava[Int] {
+  // override def someMethod() = ???
+}
+object Test {
+  def main(args: Array[String]): Unit = {
+    val c = classOf[ChildScala]
+    assert(
+      c.getGenericInterfaces.length == c.getInterfaces.length,
+      s"mismatch between ${c.getGenericInterfaces.mkString("Array(", ", ", ")")} and ${c.getInterfaces.mkString("Array(", ", ", ")")}"
+    )
+  }
+}

tests/run/i21177b/A.java

@@ -0,0 +1 @@
+interface A { default int m() { return 1; } }

tests/run/i21177b/Test.scala

@@ -0,0 +1,11 @@
+abstract class B { def m(): Int }
+trait T extends B with A
+class C extends T
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val cls = classOf[C]
+    assert(cls.getInterfaces().length == cls.getGenericInterfaces().length)
+    new C().m() // lack of the mixin forwarder for m() will crash this on runtime
+  }
+}