Type inference failure for type variable in intersection type

[heaven-born]

Compiler version

2.13.5 and 3.0.0-RC2

Minimized code

This code compiles in 2.13.5 but not in 3.0.0-RC2.

    trait Wrapper[K]
    trait Has0[T]
    
    def test[R](v: Wrapper[Has0[String] with R]):R = ???

    val zz:Wrapper[Has0[String] with Has0[Int]] = ???
    test(zz)

Output

Error in 3.0.0-RC2

Found:    (zz : Wrapper[Has0[String] & Has0[Int]])
Required: Wrapper[Has0[String] & R]

where:    R is a type variable

      test(zz)

Expectation

[heaven-born]

this issue and #12051 may have some connections

[smarter]

This is kind of a weird encoding (Has0[String] with Has0[Int] is an empty type like Nothing: because its type parameter is invariant and String is not the same type as Int, there's no value of this type), so I'm not sure how much we can or should do here. Alternatively, you could use unions:

trait Wrapper[K]
trait Has0[T]

object Test {
  def test[R](v: Wrapper[Has0[String] | R]): R = ???

  val zz: Wrapper[Has0[String] | Has0[Int]] = ???

  test(zz) // Infers R=Has0[Int]
}

[heaven-born]

@smarter not sure if this is an issue or not, but it seems that ZIO uses this kind of encoding now. Except that K is contravariant. Like: ZIO[Has[Database] & Has[Logging],..,..]

[heaven-born]

btw, the inference works for zero-kinded types val zz: Wrapper[Has0[String] & Int] = ???
And this is example of the library that use this type trick, so it will not be able to survive the migration.