It's a little hard to find the best wording for this, but it looks like trying to access external val s from within a closure passed into a macro materializer breaks on Scala 2.12.

Using the following macro:

trait Scope {}

object Macros {
  def materializeImpl[T: c.WeakTypeTag](c: Context)(generator: c.Expr[() => T]): c.Expr[Unit] = {
    import c.universe._
    c.Expr[Unit](q"""
      new Scope {
        $generator()
      }
    """)
  }
}

def generate[T](generator: () => T): Unit = macro Macros.materializeImpl[T]

The following will compile on Scala 2.11 but not 2.12:

val someValue = 1
generate { () => someValue }

The compiler seem to have problems dealing with someValue

If the macro doesn't wrap the generator with

new Scope {...}

it compiles just fine. If an external method is used from within the closure, it also compiles just fine.

I've created a Github repo that can be cloned and run, to quickly reproduce this issue:
https://github.com/bfil/scala-compiler-bug

The original issue originated when trying to upgrade the following project to Scala 2.12:
https://github.com/bfil/scala-automapper