Fix #15419: Handle Unit parameters with default values in JS types.

Both at call site (for native and non-native JS types) and at
definition sites (for non-native JS types).

Forward port of
scala-js/scala-js@1de54d7

Author: sjrd

compiler/src/dotty/tools/backend/sjs/JSCodeGen.scala

@@ -1829,7 +1829,15 @@ class JSCodeGen()(using genCtx: Context) {
         }
 
       case Block(stats, expr) =>
-        js.Block(stats.map(genStat) :+ genStatOrExpr(expr, isStat))
+        // #15419 Collapse { <undefined-param>; BoxedUnit } to <undefined-param>
+        val genStatsAndExpr0 = stats.map(genStat(_)) :+ genStatOrExpr(expr, isStat)
+        val genStatsAndExpr = genStatsAndExpr0 match {
+          case (undefParam @ js.Transient(UndefinedParam)) :: js.Undefined() :: Nil =>
+            undefParam :: Nil
+          case _ =>
+            genStatsAndExpr0
+        }
+        js.Block(genStatsAndExpr)
 
       case Typed(expr, _) =>
         expr match {

compiler/src/dotty/tools/backend/sjs/JSExportsGen.scala

@@ -761,7 +761,7 @@ final class JSExportsGen(jsCodeGen: JSCodeGen)(using Context) {
     // Pass previous arguments to defaultGetter
     val defaultGetterArgs = previousArgsValues(defaultGetter.info.paramInfoss.head.size)
 
-    if (targetSym.isJSType) {
+    val callGetter = if (targetSym.isJSType) {
       if (defaultGetter.owner.isNonNativeJSClass) {
         if (defaultGetter.hasAnnotation(jsdefn.JSOptionalAnnot))
           js.Undefined()
@@ -784,6 +784,12 @@ final class JSExportsGen(jsCodeGen: JSCodeGen)(using Context) {
     } else {
       genApplyMethod(targetTree, defaultGetter, defaultGetterArgs)
     }
+
+    // #15419 If the getter returns void, we must "box" it by returning undefined
+    if (callGetter.tpe == jstpe.NoType)
+      js.Block(callGetter, js.Undefined())
+    else
+      callGetter
   }
 
   private def targetSymForDefaultGetter(sym: Symbol): Symbol =