Fix scala#5140: Ycheck failure in covariant java varargs

Allow passing `Array[Dog]` to a Java varargs that expects an
`Array[Animal]`. This types because Java vargs are represented as
`RepeatedParam[+T]`, which is covariant.

However, after `elimRepeated`, the `RP`s go away and the true type
(Array) is revealed, which was causing a Ycheck error because arrays are
invariant before erasure.

Fix the Ycheck error by casting `Array[Dog]` to `Array[Animal]`. This is
unsound but consistent with the typer behaviour and what scalac does.

Author: abeln

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

@@ -91,8 +91,11 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
   private def seqToArray(tree: Tree, pt: Type)(implicit ctx: Context): Tree = tree match {
     case SeqLiteral(elems, elemtpt) =>
       JavaSeqLiteral(elems, elemtpt)
-    case app@Apply(fun, args) if defn.WrapArrayMethods().contains(fun.symbol) => // rewrite a call to `wrapXArray(arr)` to `arr`
-      args.head
+    case app@Apply(fun, args) if defn.WrapArrayMethods().contains(fun.symbol) =>
+      // Rewrite a call to `wrapXArray(arr)` to `arr`.
+      // Additionally, a cast might be necessary in some instances due to arrays being invariant before
+      // erasure. See `tests/pos/i5140`.
+      args.head.ensureConforms(pt)
     case _ =>
       val elemType = tree.tpe.elemType
       var elemClass = elemType.classSymbol

tests/pos/i5140/J.java

@@ -0,0 +1,5 @@
+class Animal {}
+class Dog extends Animal {}
+class J {
+  void foo(Animal... animal) {}
+}

tests/pos/i5140/S.scala

@@ -0,0 +1,8 @@
+class S {
+  val j = new J()
+  val x: Array[Dog] = ???
+  // Check that x is cast to an `Array[Animal]` after elimRepeated.
+  // Otherwise, the call below would fail in -Ycheck:elimRepeated because arrays are invariant before erasure.
+  // This is unsound but allowed.
+  j.foo(x: _*)
+}