Erasure of `Int | Nothing`

[nicolasstucki]

Compiler version

3.1.3

Minimized example

def foo: Int | Nothing  = 1

Output

def foo(): Object = scala.Int.box(1)

Expectation

Int | Nothing should erase to int

[nicolasstucki]

Another strange case is the following

class A:
  def foo: Int = 1
class B extends A:
  override def foo: Int | Nothing  = 2
def test(b: B): Int = b.foo

which erases to

class A:
  def foo: Int = 1
class B extends A:
  override def foo(): Object = scala.Int.box(2)
  override def foo(): Int = scala.Int.unbox(this.foo())
def test(b: B): Int = scala.Int.unbox(b.foo())

This forces boxing the value for no good reason.

[nicolasstucki]

Note that def foo: Int & Any = 1 does erase to def foo: Int = 1

[nicolasstucki]

As shown in #14971 (comment) this can also cause fundamental issues with signatures containing Array[Int | Nothing].