Fix repeated args when type is union

Author: noti0na1

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

@@ -171,9 +171,10 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
    *        of generic Java varargs in `elimRepeated`.
    */
   private def adaptToArray(tree: Tree, elemPt: Type)(implicit ctx: Context): Tree =
-    val elemTp = tree.tpe.elemType
+    val treeTpe = tree.tpe.widenUnion
+    val elemTp = treeTpe.elemType
     val elemTpMatches = elemTp <:< elemPt
-    val treeIsArray = tree.tpe.derivesFrom(defn.ArrayClass)
+    val treeIsArray = treeTpe.derivesFrom(defn.ArrayClass)
     if elemTpMatches && treeIsArray then
       tree // No adaptation necessary
     else tree match

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -753,10 +753,10 @@ class Typer extends Namer
           if (ctx.mode.is(Mode.QuotedPattern)) pt.translateFromRepeated(toArray = false, translateWildcard = true)
           else pt.translateFromRepeated(toArray = false, translateWildcard = true) |
                pt.translateFromRepeated(toArray = true,  translateWildcard = true)
-        val tpdExpr = typedExpr(tree.expr, ptArg)
         val expr1 = typedExpr(tree.expr, ptArg)
-        val fromCls = if expr1.tpe.derivesFrom(defn.ArrayClass) then defn.ArrayClass else defn.SeqClass
-        val tpt1 = TypeTree(expr1.tpe.widen.translateToRepeated(fromCls)).withSpan(tree.tpt.span)
+        val exprTpeWiden = expr1.tpe.widenUnion
+        val fromCls = if exprTpeWiden.derivesFrom(defn.ArrayClass) then defn.ArrayClass else defn.SeqClass
+        val tpt1 = TypeTree(exprTpeWiden.translateToRepeated(fromCls)).withSpan(tree.tpt.span)
         assignType(cpy.Typed(tree)(expr1, tpt1), tpt1)
       }
       cases(

tests/neg/repeatedArgs213.scala

@@ -15,4 +15,16 @@ class repeatedArgs {
     Paths.get("Hello", ys: _*) // error: immutable.Seq expected, found Seq
     Paths.get("Hello", zs: _*)
   }
+
+  def test2(xs: immutable.Seq[String] | Null, ys: collection.Seq[String] | Null, zs: Array[String] | Null): Unit = {
+    bar("a", "b", "c")
+    bar(xs: _*)
+    bar(ys: _*) // error: immutable.Seq expected, found Seq
+    bar(zs: _*) // old-error: Remove (compiler generated) Array to Seq conversion in 2.13?
+
+    Paths.get("Hello", "World")
+    Paths.get("Hello", xs: _*)
+    Paths.get("Hello", ys: _*) // error: immutable.Seq expected, found Seq
+    Paths.get("Hello", zs: _*)
+  }
 }

tests/pos/repeatedArgs213.scala

@@ -15,4 +15,15 @@ class repeatedArgs {
     val List(_, others: _*) = xs.toList // toList should not be needed, see #4790
     val x: immutable.Seq[String] = others
   }
+
+  def test2(xs: immutable.Seq[String] | Null): Unit = {
+    bar("a", "b", "c")
+    bar(xs: _*)
+
+    Paths.get("Hello", "World")
+    Paths.get("Hello", xs: _*)
+
+    val List(_, others: _*) = xs.toList // toList should not be needed, see #4790
+    val x: immutable.Seq[String] = others
+  }
 }