Fix problem when making package objects explicit during unpickling

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -1073,12 +1073,10 @@ class TreeUnpickler(reader: TastyReader,
       def makeSelect(qual: Tree, name: Name, denot: Denotation): Select =
         var qualType = qual.tpe.widenIfUnstable
         val owner = denot.symbol.maybeOwner
-        if (owner.isPackageObject && qualType.termSymbol.is(Package))
-          qualType = qualType.select(owner.sourceModule)
-        val tpe = name match {
+        val tpe0 = name match
           case name: TypeName => TypeRef(qualType, name, denot)
           case name: TermName => TermRef(qualType, name, denot)
-        }
+        val tpe = TypeOps.makePackageObjPrefixExplicit(tpe0)
         ConstFold.Select(untpd.Select(qual, name).withType(tpe))
 
       def completeSelect(name: Name, sig: Signature, target: Name): Select =

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3591,6 +3591,7 @@ class Typer extends Namer
             gadts.println(i"insert GADT cast from $tree to $target")
             tree.cast(target)
           case _ =>
+            //typr.println(i"OK ${tree.tpe}\n${TypeComparer.explained(_.isSubType(tree.tpe, pt))}") // uncomment for unexpected successes
             tree
     }
 

tests/neg/i13377/LeakFoo_1.scala

@@ -0,0 +1,3 @@
+import scala.util.NotGiven
+type LeakFoo[T] = core.LeakingFoo[T]
+val ok = summon[NotGiven[LeakFoo[1] =:= LeakFoo[2]]]

tests/neg/i13377/Test_2.scala

@@ -0,0 +1,3 @@
+import scala.util.NotGiven
+val x: LeakFoo[1] = ??? : LeakFoo[2]  // error
+val notok = summon[NotGiven[LeakFoo[1] =:= LeakFoo[2]]] // ok

tests/neg/i13377/coreFoo_1.scala

@@ -0,0 +1,4 @@
+package core
+
+opaque type Foo[T] <: Int = Int
+type LeakingFoo[T] = Foo[T]