Fix scala#18658: Handle varargs of generic types in JSExportsGen.

When extracting the type of a varargs parameter, we have to go back
to before erasure. However, that gives us a non-erased type inside
as well. We need to re-erase that type to get something sensible
for the back-end.

Author: sjrd

compiler/src/dotty/tools/dotc/transform/sjs/JSSymUtils.scala

@@ -181,11 +181,14 @@ object JSSymUtils {
       def paramNamesAndTypes(using Context): List[(Names.TermName, Type)] =
         sym.info.paramNamess.flatten.zip(sym.info.paramInfoss.flatten)
 
+      def reerase(tpe: Type): Type =
+        TypeErasure.erasure(tpe)
+
       val paramInfosAtElimRepeated = atPhase(elimRepeatedPhase) {
         val list =
           for ((name, info) <- paramNamesAndTypes) yield {
             val v =
-              if (info.isRepeatedParam) Some(info.repeatedToSingle.widenDealias)
+              if (info.isRepeatedParam) Some(reerase(info.repeatedToSingle.widenDealias))
               else None
             name -> v
           }
@@ -206,7 +209,7 @@ object JSSymUtils {
             new JSParamInfo(info, repeated = true)
 
           case Some(None) =>
-            val info = paramInfosAtElimEVT.getOrElse(paramName, paramInfoNow)
+            val info = paramInfosAtElimEVT.get(paramName).fold(paramInfoNow)(reerase(_))
             new JSParamInfo(info)
         }
       }

tests/sjs-junit/test/org/scalajs/testsuite/jsinterop/NonNativeJSTypeTestScala3.scala

@@ -0,0 +1,21 @@
+package org.scalajs.testsuite.jsinterop
+
+import org.junit.Assert.*
+import org.junit.Test
+
+import scala.scalajs.js
+import scala.scalajs.js.annotation.*
+
+class NonNativeJSTypeTestScala3 {
+  @Test
+  def overloadWithVarargOfGenericType(): Unit = {
+    class OverloadWithVarargOfGenericType extends js.Object {
+      def overloaded(x: Int): Int = x
+      def overloaded(xs: (Int, Int)*): Int = xs.size
+    }
+
+    val obj = new OverloadWithVarargOfGenericType
+    assertEquals(5, obj.overloaded(5))
+    assertEquals(2, obj.overloaded((1, 2), (3, 4)))
+  }
+}