Member type does not conform to type projection on supertype

[DarkDimius]

Can be reproduced here:
dotty-staging/dotty-1@82cdf60

./src/dotty/tools/backend/jvm/DottyBackendInterface.scala:789: error: type mismatch:
 found   : DottyBackendInterface.this.Symbol(in DottyBackendInterface)(sym)
 required: scala.tools.nsc.backend.jvm.BackendInterface#Symbol(in BackendInterface)
        primitiveTypeMap.getOrElse(sym, storage.getClassBTypeAndRegisterInnerClass(sym.asInstanceOf[ct.int.Symbol]))

[DarkDimius]

I can workaround this.

[samuelgruetter]

Minimal example:

trait A {
  type X
}

class B extends A {
  def x: X = ???
}

object test {
  def f(): Unit = {
    val b = new B
    val a: A#X = b.x
  }
}

Compiles with scalac, dotty says

[error] examples/hash1.scala:13: error: type mismatch:
[error]  found   : b.X
[error]  required: A#X
[error]     val a: A#X = b.x
[error]                    ^
[error] one error found

[samuelgruetter]

But dotty seems to be right, because b.X is not a subtype of ex(y: A)y.X.

[DarkDimius]

@samuelgruetter thanks 👍

[smarter]

The rules for type projections in Dotty have changed and this actually compiles now, @DarkDimius you might be able to get rid of your workaround.

[samuelgruetter]

That's good, and it's also compatible with the rules that I've typeset last summer: By <:-∃-R on page 4, we get b.X <: ∃(b: A)b.X, and by alpha-renaming, b.X <: ∃(y: A)y.X.