Slack LUBs fail to capture constraints between type parameters and blow up in RefChecks if used in a TypeTree

[scabug]

trait A
trait B

trait L[A2, B2 <: A2]

object Lub {
  def lub = if (true) (null: L[A, A]) else (null: L[B, B])
  def foo(a: Any, b: Any) = (a, b)
  foo(b = lub, a = lub) // use named args transforms to include TypeTree(<lub.tpe>
}

<console>:13: error: type arguments [_1,_2] do not conform to trait L's type parameter bounds [A2,B2 <: A2]
         foo(b = lub, a = lub) // use named args transforms to include TypeTree(<lub.tpe>
                 ^

I suspect calculating the correct LUB will be to difficult, and we should instead disable the check for synthetic TypeTrees.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-7694?orig=1
Reporter: @retronym
Affected Versions: 2.10.0
Other Milestones: 2.11.0-M6

[scabug]

@retronym said:
scala/scala#2771