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existential type bound check fails in value class #11562

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kailuowang opened this issue Jun 7, 2019 · 1 comment

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@kailuowang
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commented Jun 7, 2019

Happens in 2.12 and 2.13 (didn't check in 2.11)

object Test {
  trait M[A] {
    def d[AA <: A](a: AA): Unit
  }
  
  class MWorks[A](val a: A) {
    def d(M: M[_ >: A]): Unit = M.d(a)
  }

  class MFails[A](val a: A) extends AnyVal {
    def d(M: M[_ >: A]): Unit = M.d(a)  //fails compilation in value class
  }
}

The same simple code in a value class causes compilation error:

type arguments [A] do not conform to method d's type parameter bounds [AA <: _$2]

Scastie https://scastie.scala-lang.org/Uyhm8ZzAQkqJTiNP02zXmw

To be clear, the bound AA <: A in trait M is clear redundant, but it's needed in the real use case where the parameter is of slightly more complex type.

@hrhino

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commented Jun 7, 2019

Sounds similar to #11383 (comment), but with the type parameter on the class rather than the method. I suspect the root cause is the same.

@hrhino hrhino added this to the Backlog milestone Jun 7, 2019

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