scala: using method of extending class based on macros invokes trait default impl if type of instance is statically the base class

Question

I have a trait T, I implement it with macro in class C. i create an instance of C and invoke its methods. if the type of the val containing an instance of C is C - works as expected. if the type of the val containing an instance of C is T - invokes methods as if T.

best way to describe it that i can come up with is "virtual table broken" in scala-macros, but i don't know if that's a thing...

example code:

Type in expressions for evaluation. Or try :help.

scala>

scala>

scala> import language.experimental.macros
import language.experimental.macros

scala>

scala>

scala> trait T { def doSomething(): Unit = println ("trait") }
defined trait T

scala>

scala>

scala> import scala.reflect.macros.Context
import scala.reflect.macros.Context

scala>

scala>

scala> object Macro {
     |   def doSomething(c: Context)(): c.universe.Tree = {
     |     import c.universe._
     |     q"""println ("macro")"""
     |   }
     | }
warning: there was one deprecation warning (since 2.11.0); for details, enable `:setting -deprecation' or `:replay -deprecation'
defined object Macro

scala>

scala>

scala> class C extends T { override def doSomething(): Unit = macro Macro.doSomething }
defined class C

scala>

scala> val c: C = new C()
c: C = C@3bd1883a

scala> c.doSomething()
macro

scala>

scala> val t: T = new C()
t: T = C@4079fec7

scala> t.doSomething()
trait

Answer 1

Def macros are expanded at compile time so late binding / dynamic dispatch (which is a runtime feature) is impossible for them.

At compile time it's unknown that t has type C , at compile time it's only known that t has type T .

See details here: Eugene Burmako. Unification of Compile-Time and Runtime Metaprogramming in Scalahttps://infoscience.epfl.ch/record/226166/files/EPFL_TH7159.pdf p. 98, §4.6.1 "Inheritance"