Pickling difference error with explicit nulls and unsafe nulls

[noti0na1]

I was trying to enable explicit nulls for all compiling. But I get several errors in BootstrappedOnlyCompilationTests.picklingWithCompiler test. See #14946

Minimized code

I am able to minimize some of the errors:

class C {
  def g: String | Null = ???

  def f =
    import scala.language.unsafeNulls
    if true then "" else g
}

Output

scalac -Xprint-types -Ytest-pickler -Yexplicit-nulls -color never Stest.scala

exception occurred while compiling Stest.scala
class dotty.tools.dotc.reporting.Diagnostic$Error at ?: pickling difference for class C in Stest.scala, for details:

  diff before-pickling.txt after-pickling.txt while compiling Stest.scalaException in thread "main" 
class dotty.tools.dotc.reporting.Diagnostic$Error at ?: pickling difference for class C in Stest.scala, for details:

  diff before-pickling.txt after-pickling.txt
        at dotty.tools.dotc.report$.error(report.scala:63)
        at dotty.tools.dotc.transform.Pickler.testSame(Pickler.scala:151)
        at dotty.tools.dotc.transform.Pickler.testUnpickler$$anonfun$2(Pickler.scala:138)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:563)
        at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:561)
        at scala.collection.AbstractIterable.foreach(Iterable.scala:926)
        at scala.collection.IterableOps$WithFilter.foreach(Iterable.scala:896)
        at dotty.tools.dotc.transform.Pickler.testUnpickler(Pickler.scala:139)
        at dotty.tools.dotc.transform.Pickler.runOn(Pickler.scala:122)
        at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:225)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1328)
        at dotty.tools.dotc.Run.runPhases$1(Run.scala:236)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:244)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:253)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:253)
        at dotty.tools.dotc.Run.compileSources(Run.scala:186)
        at dotty.tools.dotc.Run.compile(Run.scala:170)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:35)
        at dotty.tools.dotc.Driver.process(Driver.scala:195)
        at dotty.tools.dotc.Driver.process(Driver.scala:163)
        at dotty.tools.dotc.Driver.process(Driver.scala:175)
        at dotty.tools.dotc.Driver.main(Driver.scala:205)
        at dotty.tools.dotc.Main.main(Main.scala)

before-pickling.txt

<package <empty>.type {
  @SourceFile(<"Stest.scala":("Stest.scala" : String)>) class C() extends <
    <<new Object:Object>:((): Object)>
  ():Object> {
    def g: scala.Predef.String | scala.Null = 
      <scala.Predef.???:=> scala.Nothing>
    def f: scala.Predef.String = 
      <
        {
          <import <<scala:scala>.language:scala.language>.unsafeNulls:
            <import>.type
          >
          <if <true:(true : scala.Boolean)> then <"":("" : String)> else 
            <<this:(C.this : C)>.g:=> scala.Predef.String | scala.Null>
          :String>
        }
      :String>
  }
}:<empty>.type>

diff before-pickling.txt after-pickling.txt

15c15
<           :String>
---
>           :String | scala.Null>
17c17
<       :String>
---
>       :String | scala.Null>

It seems the if expression gets different types before and after pickling?