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, 46 insertions, 0 deletions

diff --git a/test/files/run/t8502b.scala b/test/files/run/t8502b.scala
new file mode 100644
index 0000000000..4f70d13bb0
--- /dev/null
+++ b/test/files/run/t8502b.scala
@@ -0,0 +1,46 @@
+import scala.tools.partest._
+import java.io.File
+
+// used to crash with an assertion failure in flatten because the type symbol created for the missing
+// package was a ClassSymbol, not a PackageClassSymbol
+//   - isFlattenablePrefix(vanishingPackage) was true (wrongly)
+//   - therefore flatten tried to flatten the class defined in the package, but the class is
+//     top-level, vanishingClass.enclosingTopLevelClass is NoSymbol
+object Test extends StoreReporterDirectTest {
+  def code = ???
+
+  def compileCode(code: String) = {
+    val classpath = List(sys.props("partest.lib"), testOutput.path) mkString sys.props("path.separator")
+    compileString(newCompiler("-cp", classpath, "-d", testOutput.path))(code)
+  }
+
+  def show(): Unit = {
+    compileCode("""
+      class Outer {
+        class Nested extends vanishing.Vanishing
+      }
+
+      package vanishing {
+        class Vanishing
+      }
+      """)
+    assert(filteredInfos.isEmpty, filteredInfos)
+    deletePackage("vanishing")
+    compileCode("""
+      class Test {
+        def f(o: Outer): Outer = o
+      }
+    """)
+    assert(storeReporter.infos.isEmpty, storeReporter.infos.mkString("\n")) // Included a MissingRequirementError before.
+  }
+
+  def deletePackage(name: String) {
+    val directory = new File(testOutput.path, name)
+    for (f <- directory.listFiles()) {
+      assert(f.getName.endsWith(".class"))
+      assert(f.delete())
+    }
+    assert(directory.listFiles().isEmpty)
+    assert(directory.delete())
+  }
+}