`-Yexplicit-nulls` breaks givens resolution when depending on libraries without this flag

[WojciechMazur]

Based on OpenCB failure in broij/zarrow - build logs

The compilation succeeds if both compilation units use -Yexplicit-nulls or if both do not use it. In case of mixed mode it fails to build

Compiler version

Last good release: 3.8.0-RC1-bin-20250821-0a7f843-NIGHTLY
First bad release: 3.8.0-RC1-bin-20250822-658c8bd-NIGHTLY

Minimized code

Requires separate compilation units

// 1_defs.scala

object ZIO:
  def attempt[A](code: => A): ZIO[Any, Throwable, A] = ???

trait ZIO[-R, +E, +A]:
  final def flatten[R1 <: R, E1 >: E, B](using A IsSubtypeOfOutput ZIO[R1, E1, B]): ZIO[R1, E1, B] = ???

infix sealed abstract class IsSubtypeOfOutput[-A, +B]
object IsSubtypeOfOutput:
  given [A, B](using A <:< B): IsSubtypeOfOutput[A, B] = ???

// 2_usage.scala
//> using options -Yexplicit-nulls

opaque type ZArrow[-I, -R, +E, +O] = I => ZIO[R, E, O]
object ZArrow:
  def fromZIOAttempt[I, R, E, O](f: I => ZIO[R, E, O]): ZArrow[I, R, Throwable | E, O] =
    (in: I) => ZIO.attempt(f(in)).flatten
    

Output

-- [E172] Type Error: /Users/wmazur/projects/scala/community-build3/test2.test.scala:7:41 ---------------------------------------------------------------------------------------------------
7 |    (in: I) => ZIO.attempt(f(in)).flatten
  |                                         ^
  |               No given instance of type ((ZIO[R, E, O])? IsSubtypeOfOutput (ZIO[(R1)?, ((Throwable)?)?, (B)?])?)? was found for parameter x$1 of method flatten in trait ZIO.
  |               I found:
  |
  |                   IsSubtypeOfOutput.given_IsSubtypeOfOutput_A_B[(ZIO[R, E, O])?, (((ZIO[R, E, O])?)?)?](<:<.refl[((ZIO[R, E, O])?)?])
  |
  |               But given instance given_IsSubtypeOfOutput_A_B in object IsSubtypeOfOutput does not match type ((ZIO[R, E, O])? IsSubtypeOfOutput (ZIO[(R1)?, ((Throwable)?)?, (B)?])?)?
  |
  |               where:    B  is a type variable with constraint <: O
  |                         R1 is a type variable with constraint >: R and <: (Any)?
  |               .
1 error found

Expectation