Setters for variables in refinements not working

[prolativ]

Compiler version

3.1.0-RC3

Minimized code

trait Foo extends reflect.Selectable

@main def test: Unit =
  val f = new Foo { var i: Int = 0 }
  f.i = 1

Output

[error] (run-main-3) java.lang.NoSuchMethodException: test$package$$anon$1.i_=(int)
[error] java.lang.NoSuchMethodException: test$package$$anon$1.i_=(int)
[error]         at java.lang.Class.getMethod(Class.java:1786)
[error]         at scala.reflect.Selectable.applyDynamic(Selectable.scala:38)
[error]         at scala.reflect.Selectable.applyDynamic$(Selectable.scala:11)
[error]         at test$package$$anon$1.applyDynamic(test.scala:4)
[error]         at test$package$.test(test.scala:5)
[error]         at test.main(test.scala:3)
[error]         at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error]         at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[error]         at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error]         at java.lang.reflect.Method.invoke(Method.java:498)

Expectation

I'm not really sure what is the expected level of support for variables inside refinements.
Currently trying to explicitly define a refined type with a variable member results in a compilation error

type F = Foo { var i: Int }

refinement cannot be `mutable`

If this is so by design we should disallow creating instances of subtypes of Selectable with variable members.

[odersky]

Variables setters in anonymous classes are supported, but there was a problem in code generation. Using var directly in a refinement is not supported. I improved the error message.