Add regression tests for #24619

Add test cases for nested class generic signatures when
method type parameters shadow enclosing class type parameters.
The tests verify that Scala 3 correctly handles type parameter name conflicts in nested generic classes by renaming method-level type parameters to avoid clashing with outer class type parameters.

Closes #24619

Author: tanishiking

compiler/test/dotc/pos-test-pickling.excludelist

@@ -34,6 +34,7 @@ i11982a.scala
 i17255
 i17735.scala
 i24134
+i24134-nested
 
 # Tree is huge and blows stack for printing Text
 i7034.scala

tests/generic-java-signatures/nestedClassTypeParams.check

@@ -0,0 +1,4 @@
+noShadow: public scala.Tuple3<T, U, V> Outer$Middle$Inner.noShadow(T,U,V)
+shadowOuter: public <T1> T Outer$Middle$Inner.shadowOuter()
+shadowMiddle: public <U1> U Outer$Middle$Inner.shadowMiddle()
+shadowInner: public <V1> V Outer$Middle$Inner.shadowInner()

tests/generic-java-signatures/nestedClassTypeParams.scala

@@ -0,0 +1,27 @@
+class Outer[T]:
+  def outerValue: T = ???
+
+  class Middle[U]:
+    def middleValue: U = ???
+
+    class Inner[V]:
+      def innerValue: V = ???
+      def noShadow(x: T, y: U, z: V): (T, U, V) = (x, y, z)
+      def shadowOuter[T] = outerValue
+      def shadowMiddle[U] = middleValue
+      def shadowInner[V] = innerValue
+
+@main def Test(): Unit =
+  val innerMethods = classOf[Outer[_]].getDeclaredClasses()
+    .find(_.getName.contains("Middle")).get.getDeclaredClasses()
+    .find(_.getName.contains("Inner")).get.getDeclaredMethods()
+
+  printMethodSig(innerMethods, "noShadow")
+  printMethodSig(innerMethods, "shadowOuter")
+  printMethodSig(innerMethods, "shadowMiddle")
+  printMethodSig(innerMethods, "shadowInner")
+
+def printMethodSig(methods: Array[java.lang.reflect.Method], name: String): Unit =
+  methods.find(_.getName.endsWith(name)).foreach { m =>
+    println(s"$name: ${m.toGenericString}")
+  }

tests/pos/i24134-nested/Container.scala

@@ -0,0 +1,2 @@
+class Container[A]:
+  abstract class JavaPartialFunction[B] extends PartialFunction[A, B]

tests/pos/i24134-nested/Main.java

@@ -0,0 +1,15 @@
+public class Main {
+    public static void main(String[] args) {
+        Container<String> container = new Container<>();
+        Container<String>.JavaPartialFunction<Integer> pf = container.new JavaPartialFunction<Integer>() {
+            @Override
+            public boolean isDefinedAt(String x) {
+                return x != null && !x.isEmpty();
+            }
+            @Override
+            public Integer apply(String x) {
+                return x.length();
+            }
+        };
+    }
+}