`selectDynamic` cannot be an extension method

[prolativ]

Compiler version

3.3.1-RC1-bin-20230313-f28d708-NIGHTLY and before

Minimized code

trait Sel extends Selectable

extension (s: Sel)
  def selectDynamic(name: String) = ???

val sel = (new Sel {}).asInstanceOf[Sel{ def foo: String }]
val foo = sel.foo

Output

[error] ./Selectable.scala:7:11
[error] value selectDynamic is not a member of Sel{def foo: String}, but could be made available as an extension method.
[error] 
[error] The following import might fix the problem:
[error] 
[error]   import reflect.Selectable.reflectiveSelectable
[error] 
[error] val foo = sel.foo
[error]           ^^^

Expectation

According to our reference documentation:

Implementations of Selectable have to make available one or both of the methods selectDynamic and applyDynamic. The methods could be members of the Selectable implementation or they could be extension methods.

To my understanding this means that the code snippet should compile successfully, by applying the syntax desugarings:

sel.foo

sel.selectDynamic("foo").asInstanceOf[String]

selectDynamic(sel)("foo").asInstanceOf[String]

Similarly it should be possible to provide the implementation of applyDynamic as an extension method.

[prolativ]

@odersky did I get the specification right? The only case that works and that I know about in which a structural selection is desugared to selectDynamic even though the receiver does not inherit from scala.Selectable is the case of scala.reflect.Selectable. But this works in the way that a refined type which does NOT extends scala.Selectable is converted (via an implicit conversion) to scala.reflect.Selectable (or more specifically to its subtype scala.reflect.Selectable.DefaultSelectable), which actually does implement selectDynamic as a member method.

[odersky]

Yes, this should work.

[plokhotnyuk]

Should it work for extension of Dynamic like in this issue?

[odersky]

No

[plokhotnyuk]

Just to be clear.

If I will replace Selectable by Dynamic should it work for the following code too?

trait Sel extends Dynamic

extension (s: Sel)
  def selectDynamic(name: String) = ???

val sel = (new Sel {}).asInstanceOf[Sel{ def foo: String }]
val foo = sel.foo

[prolativ]

@plokhotnyuk Actually in case of Dynamic this seems to work (checked with 3.nightly and 3.0.0 so I assume other versions are OK too):

import scala.language.dynamics

trait Sel extends Dynamic

extension (s: Sel)
  def selectDynamic(name: String) = name

val sel = new Sel {}
val foo = sel.foo

The point is that you need to import scala.language.dynamics and you should NOT explicitly add the member declaration in a refinement (because Dynamic is used for members that are not statically guaranteed to exist).

[odersky]

@prolativ Yes, that's the current case. But it seems reasonable to also allow Dynamic even if the qualifier type is structural. I am trying to to this in 2808f3d.