Byname implicits: could not find proxy for lazy var

[OlivierBlanvillain]

This is test case taken scala/scala#6050:

trait Foo[T]

object Foo {
  implicit def pair[T, U]
    (implicit
      fooT: => Foo[(T, U)],
      fooU: => Foo[(U, T)]
    ): Foo[(T, U)] = ???

  implicit def int: Foo[Int] = ???
  implicit def string: Foo[String] = ???
}

object Test {
  implicitly[Foo[(Int, String)]]
}

Exception in thread "main" java.lang.IllegalArgumentException: Could not find proxy for lazy var $_lazy_implicit_$3$lzy1: dotty.run
time.LazyRef in List(val $_lazy_implicit_$3$lzy1, val <local Test$>, module class Test$, module class <empty>, module class <root>), encl = package <empty>, owners = package <empty>, package <root>; enclosures = package <empty>, package <root>
        at dotty.tools.dotc.transform.LambdaLift$Lifter.searchIn$1(LambdaLift.scala:374)
        at dotty.tools.dotc.transform.LambdaLift$Lifter.proxy(LambdaLift.scala:387)
        at dotty.tools.dotc.transform.LambdaLift$Lifter.proxyRef(LambdaLift.scala:405)
        at dotty.tools.dotc.transform.LambdaLift$Lifter.$anonfun$addFreeArgs$1(LambdaLift.scala:411)
        at scala.collection.immutable.List.map(List.scala:283)
        at dotty.tools.dotc.transform.LambdaLift$Lifter.addFreeArgs(LambdaLift.scala:411)
        at dotty.tools.dotc.transform.LambdaLift.transformApply(LambdaLift.scala:532)
        at dotty.tools.dotc.transform.LambdaLift.transformApply(LambdaLift.scala:486)

[OlivierBlanvillain]

Here is the code generated by typer:

object Test {
  implicitly[Foo[Tuple2[Int, String]]](
    Foo.pair[Int, String](
      {
        lazy val $_lazy_implicit_$1: Foo[(Int, String)] =
          Foo.pair[Int, String]($_lazy_implicit_$1,
            {
              lazy val $_lazy_implicit_$3: Foo[(String, Int)] =
                Foo.pair[String, Int]($_lazy_implicit_$3, $_lazy_implicit_$1)
              $_lazy_implicit_$3
            }
          )
        $_lazy_implicit_$1
      }
    ,
      {
        lazy val $_lazy_implicit_$6: Foo[(String, Int)] =
          Foo.pair[String, Int]($_lazy_implicit_$6,
            {
              lazy val $_lazy_implicit_$8: Foo[(Int, String)] =
                Foo.pair[Int, String]($_lazy_implicit_$8, $_lazy_implicit_$6)
              $_lazy_implicit_$8
            }
          )
        $_lazy_implicit_$6
      }
    )
  )
}

Compiling with -Ycheck leads to a different error:

Exception in thread "main" java.lang.AssertionError: assertion failed: bad owner; lazy value $_lazy_implicit_$3 has owner value <lo
cal Test$>, expected was lazy value $_lazy_implicit_$1