Macro.scala

import scala.quoted.*

trait Bar {
  type X
}

object Foo {
  def makeImpl[A : Type](a: Expr[A])(using Quotes): Expr[Any] = {
    import quotes.reflect.*
    val tree  = TypeSelect(a.asTerm, "A").tpe.asType match {
      case '[t] => '{
        new Bar {
          type X = t
        }
      }
    }
    println(tree.show)
    tree
  }

  inline transparent def make[A](a: A): Any = ${ makeImpl[A]('a) }
}

Use.scala

trait Baz {
  type A
}

def smth(baz: Baz): Bar = {
  Foo.make(baz)
}