Partially applied macros

Test.scala

import scala.language.experimental.macros
import scala.reflect.macros.blackbox

object Test {
  def foo1[A, B]: Unit = macro impl[A, B]
  def foo2[A]: Unit = macro impl[A, Option[Int]]

  def impl[A: c.WeakTypeTag, B: c.WeakTypeTag](c: blackbox.Context): c.Expr[Unit] = {
    import c.universe._
    c.echo(c.enclosingPosition, s"A=${weakTypeOf[A]}, B=${weakTypeOf[B]}")
    reify(())
  }
}

/*
scala> Test.foo1[Int, Option[Int]]
<console>:12: A=Int, B=Option[Int]
       Test.foo1[Int, Option[Int]]
                ^

scala> Test.foo2[Int]
<console>:12: A=Int, B=Option[A] // <--- Expected: A=Int, B=Option[Int]
       Test.foo2[Int]
*/

Solution:

import scala.language.experimental.macros
import scala.reflect.macros.blackbox
import scala.reflect.runtime.universe.TypeTag

object Test {
  def foo1[A, B](implicit bTag: TypeTag[B]): Unit = macro impl[A, B]
  def foo2[A](implicit bTag: TypeTag[Option[Int]]): Unit = macro impl[A, Option[Int]]

  def impl[A: c.WeakTypeTag, B](c: blackbox.Context)(bTag: c.Expr[TypeTag[B]]): c.Expr[Unit] = {
    import c.universe._
    c.echo(c.enclosingPosition, s"A=${weakTypeOf[A]}, B=${bTag.actualType.typeArgs.head}")
    reify(())
  }
}

/*
scala> Test.foo1[Int, Option[Int]]
<console>:12: A=Int, B=Option[Int]
       Test.foo1[Int, Option[Int]]
                ^

scala> Test.foo2[Int]
<console>:12: A=Int, B=Option[Int]
       Test.foo2[Int]
*/