Cannot abstract over erased classes

[nicolasstucki]

Compiler version

3.0.1

Minimized example

import language.experimental.erasedDefinitions

erased class Capability

def f(using Capability): Int = 1
def g[C <: Capability](using C): Int = 1

Output

6 |def g[C <: Capability](using C): Int = 1
  |                             ^
  |illegal reference to erased trait Capability in definition that is not itself erased

Expectation

We should be able to abstract over erased classes.

[arturopala]

Does it work with (using erased C)?

[nicolasstucki]

Yes. But the erased classes are there to make it possible to avoid writing erased in the first place.

If you don't use erased classes you would write

import language.experimental.erasedDefinitions

type Capability

def f(using erased Capability): Int = 1
def g[C <: Capability](using erased C): Int = 1

[arturopala]

You are right, the doc says:

If the type of a val definition or parameter is a (possibly aliased, refined, or instantiated) erased class, the definition is assumed to be erased itself.

Surprisingly, the examples are still using erased for params in https://dotty.epfl.ch/docs/reference/experimental/erased-defs.html#erased-classes

[dwijnand]

I guess subtypes (C) of erased classes should also be implicitly erased?

[odersky]

If the type of a val definition or parameter is a (possibly aliased, refined, or instantiated) erased class, the definition is assumed to be erased itself.

So that does not cover the example, since there C is neither aliased, nor refined nor instantiated. Should it also work? I am not sure about that yet. Maybe better be conservative at first?

If we wanted to generalize it, what would the right rule be? In particular considering that bounds can be unions or intersections?