Fix #5140: Ycheck failure in covariant java varargs

[abeln]

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 RPs 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.

[allanrenucci]

If this is what scalac does then I suppose we should reproduce it

[smarter]

Could we instead change elimRepeated to replace RepeatedParam[X] by Array[_ <: X] ?

[abeln]

Switched to @smarter's suggestion that we change the type to Array[_ <: T], as opposed to a cast. PTAL

[abeln]

@smarter Here's the problem I run into with the Array[_ <: Animal] approach. If a Scala method overrides a Java varargs method, then elimRepeated has to add a bridge from the Scala method, which expects a Seq[Animal], to the Java signature of the method (which is what the callers see), which is Array[_ <: Animal].

The code generated by elimRepeated after this change then looks like

  ) class Sub() extends Base() { 
    def foo(x: Seq[Object] @Repeated): Object = x
    override def foo(x: Array[_ <: Object]): Object = 
      this.foo(Predef.wrapRefArray[Object](x))
  }
}

Which doesn't work because there's no sound way to convert an Array[_ <: Object] into a Seq[Object]. Or is there?

If there's no sound way, then I suggest we revert back to the asInstanceOf approach of the first commit.

[smarter]

The problem is that passing Object as the type argument to wrapRefArray doesn't work here, but this compile:

object Test {
  val x: Array[_ <: Object] = Array()
  val y = Predef.wrapRefArray(x)
  val z: Seq[Object] = y
}

The infered type argument is Array[_ <: Object]#T which is a bit awkward and can't be written in source code but can be inferred by the compiler. We could change whatever code is currently generating the wrapRefArray to do the same.

[abeln]

PTAL. Changed elimRepeated to generate

  ) class Sub() extends Base() { 
    def foo(x: Seq[Object] @Repeated): Object = x
      override def foo(x: Array[_ <: Object]): Object =
      this.foo(Predef.wrapRefArray[ <: Object](x))
  }
}

Which can't be written with surface syntax (unbound wildcard), but apparently typechecks.

[abeln]

Addressed style comments and collapsed review commits.

[allanrenucci]

Thanks @abeln!