SI-8502 create PackageClass instead of Class symbol stubs for pkgs

https://github.com/scala/scala/pull/4111 creates a stub type symbol
for missing packages, deferring (or avoiding) a crash if a package
is missing.

The symbol created was a ClassSymbol, which could lead to an assertion
failure in flattten:

    case TypeRef(pre, sym, args) if isFlattenablePrefix(pre) =>
      assert(args.isEmpty && sym.enclosingTopLevelClass != NoSymbol, sym.ownerChain)

`pre` is the stub ClassSymbol, so `isFlattenablePrefix` is true (but
it should be false). The assertion then fails because the enclosing
class of a top-level class defined in a missing package is NoSymbol.

This failed only with GenBCode, which traverses more of the symbol
graph while building ClassBTypes: it looks collects the nested classes
of `Outer` into a `NestedInfo`.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/reflect/scala/reflect/internal/pickling/UnPickler.scala b/src/reflect/scala/reflect/internal/pickling/UnPickler.scala
index 1fc7aebab0..1f643b2b23 100644
--- a/src/reflect/scala/reflect/internal/pickling/UnPickler.scala
+++ b/src/reflect/scala/reflect/internal/pickling/UnPickler.scala
@@ -398,7 +398,7 @@ abstract class UnPickler {
         val sym = readSymbolRef() match {
           case stub: StubSymbol if !stub.isClass =>
             // SI-8502 This allows us to create a stub for a unpickled reference to `missingPackage.Foo`.
-            stub.owner.newStubSymbol(stub.name.toTypeName, stub.missingMessage)
+            stub.owner.newStubSymbol(stub.name.toTypeName, stub.missingMessage, isPackage = true)
           case sym => sym
         }
         ThisType(sym)