Fix #5140: Ycheck failure in covariant java varargs

compiler/src/dotty/tools/dotc/core/TypeApplications.scala

@@ -435,15 +435,19 @@ class TypeApplications(val self: Type) extends AnyVal {
     case _ => if (self.isMatch) MatchAlias(self) else TypeAlias(self)
   }
 
-  /** Translate a type of the form From[T] to To[T], keep other types as they are.
+  /** Translate a type of the form From[T] to either To[T] or To[_ <: T] (if `wildcardArg` is set). Keep other types as they are.
    *  `from` and `to` must be static classes, both with one type parameter, and the same variance.
    *  Do the same for by name types => From[T] and => To[T]
    */
-  def translateParameterized(from: ClassSymbol, to: ClassSymbol)(implicit ctx: Context): Type = self match {
+  def translateParameterized(from: ClassSymbol, to: ClassSymbol, wildcardArg: Boolean = false)(implicit ctx: Context): Type = self match {
     case self @ ExprType(tp) =>
       self.derivedExprType(tp.translateParameterized(from, to))
     case _ =>
-      if (self.derivesFrom(from)) to.typeRef.appliedTo(self.baseType(from).argInfos)
+      if (self.derivesFrom(from)) {
+        val arg = self.baseType(from).argInfos.head
+        val arg1 = if (wildcardArg) TypeBounds.upper(arg) else arg
+        to.typeRef.appliedTo(arg1)
+      }
       else self
   }
 
@@ -453,7 +457,9 @@ class TypeApplications(val self: Type) extends AnyVal {
   def underlyingIfRepeated(isJava: Boolean)(implicit ctx: Context): Type =
     if (self.isRepeatedParam) {
       val seqClass = if (isJava) defn.ArrayClass else defn.SeqClass
-      translateParameterized(defn.RepeatedParamClass, seqClass)
+      // If `isJava` is set, then we want to turn `RepeatedParam[T]` into `Array[_ <: T]`,
+      // since arrays aren't covariant until after erasure. See `tests/pos/i5140`.
+      translateParameterized(defn.RepeatedParamClass, seqClass, wildcardArg = isJava)
     }
     else self
 

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

@@ -125,6 +125,13 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
    *  @return  a thicket consisting of `ddef` and a varargs bridge method
    *           which overrides the Java varargs method JM from this phase on
    *           and forwards to `ddef`.
+   *
+   *  A bridge is necessary because the following hold
+   *    - the varargs in `ddef` will change from `RepeatedParam[T]` to `Seq[T]` after this phase
+   *    - _but_ the callers of `ddef` expect its varargs to be changed to `Array[_ <: T]`, since it overrides
+   *      a Java varargs
+   *  The solution is to add a "bridge" method that converts its argument from `Array[_ <: T]` to `Seq[T]` and
+   *  forwards it to `ddef`.
    */
   private def addVarArgsBridge(ddef: DefDef)(implicit ctx: Context): Tree = {
     val original = ddef.symbol.asTerm
@@ -133,7 +140,9 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
       info = toJavaVarArgs(ddef.symbol.info)).enteredAfter(thisPhase).asTerm
     val bridgeDef = polyDefDef(bridge, trefs => vrefss => {
       val (vrefs :+ varArgRef) :: vrefss1 = vrefss
-      val elemtp = varArgRef.tpe.widen.argTypes.head
+      // Can't call `.argTypes` here because the underlying array type is of the
+      // form `Array[_ <: SomeType]`, so we need `.argInfos` to get the `TypeBounds`.
+      val elemtp = varArgRef.tpe.widen.argInfos.head
       ref(original.termRef)
         .appliedToTypes(trefs)
         .appliedToArgs(vrefs :+ tpd.wrapArray(varArgRef, elemtp))

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 +1,9 @@
+class S {
+  val j = new J()
+  val x: Array[Dog] = ???
+  // Check that the java varargs for `foo` gets typed as `Array[_ <: Animal]`.
+  // Otherwise, the call below would fail in -Ycheck:elimRepeated because arrays are invariant before erasure.
+  // This is unsound but allowed.
+  j.foo(x: _*)
+  j.foo(new Dog, new Dog)
+}