Avoid stub symbol related crash in backend

In this test case, the backend forces the specialization
info transform of `Sub` during computation of its
inner class metadata. This in turn runs the info transforms
of the `Base`. This leads to the uncurry info tranform
transforming a signature that has a type alias as a
method parameter type. Subsequent substution of the new
method symbol into the result type, which includes a
stub symbol for an absent class, tripped an assertion:

```
requirement failed: package b
java.lang.IllegalArgumentException: requirement failed: package b
	at scala.Predef$.require(Predef.scala:277)
	at scala.reflect.internal.Types$ModuleTypeRef.<init>(Types.scala:1879)
	at scala.reflect.internal.Types$PackageTypeRef.<init>(Types.scala:1897)
	at scala.reflect.internal.Types$TypeRef$.apply(Types.scala:2401)
	at scala.reflect.internal.Types.typeRef(Types.scala:3553)
	at scala.reflect.internal.Types.typeRef$(Types.scala:3536)
	at scala.reflect.internal.SymbolTable.typeRef(SymbolTable.scala:16)
	at scala.reflect.internal.Symbols$TypeSymbol.newTypeRef(Symbols.scala:3026)
	at scala.reflect.internal.Symbols$TypeSymbol.updateTypeCache(Symbols.scala:3079)
	at scala.reflect.internal.Symbols$TypeSymbol.maybeUpdateTypeCache(Symbols.scala:3065)
	at scala.reflect.internal.Symbols$TypeSymbol.tpe_$times(Symbols.scala:3043)
	at scala.reflect.internal.Symbols$Symbol.typeOfThis(Symbols.scala:2020)
	at scala.reflect.internal.Types$ThisType.underlying(Types.scala:1184)
	at scala.reflect.internal.Types$SimpleTypeProxy.boundSyms(Types.scala:150)
	at scala.reflect.internal.Types$SimpleTypeProxy.boundSyms$(Types.scala:150)
	at scala.reflect.internal.Types$SingletonType.boundSyms(Types.scala:1088)
	at scala.reflect.internal.tpe.TypeMaps$SubstMap.apply(TypeMaps.scala:726)
	at scala.reflect.internal.tpe.TypeMaps$SubstSymMap.apply(TypeMaps.scala:789)
	at scala.reflect.internal.tpe.TypeMaps$TypeMap.mapOver(TypeMaps.scala:102)
	at scala.reflect.internal.tpe.TypeMaps$SubstSymMap.apply(TypeMaps.scala:783)
	at scala.reflect.internal.tpe.TypeMaps$TypeMap.mapOver(TypeMaps.scala:102)
	at scala.reflect.internal.tpe.TypeMaps$SubstSymMap.apply(TypeMaps.scala:783)
	at scala.reflect.internal.Types$Type.substSym(Types.scala:727)
	at scala.reflect.internal.tpe.TypeMaps$TypeMap.mapOver(TypeMaps.scala:123)
	at scala.reflect.internal.transform.UnCurry$$anon$1.apply(UnCurry.scala:53)
	at scala.reflect.internal.transform.UnCurry.transformInfo(UnCurry.scala:154)
```

This commit address the direct failure above by setting coherent flags on
the stub package class symbol (it also needs the MODULE flag).

1 files changed, 59 insertions, 0 deletions

diff --git a/test/files/run/t10171/Test.scala b/test/files/run/t10171/Test.scala
new file mode 100644
index 0000000000..37a2cfc67f
--- /dev/null
+++ b/test/files/run/t10171/Test.scala
@@ -0,0 +1,59 @@
+import scala.tools.partest._
+import java.io.File
+
+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 library = """
+package a {
+  package b {
+    class C { class D }
+  }
+}
+package z {
+  class Base {
+    type S = String
+    def foo(s: S): a.b.C#D = null
+  }
+  class Sub extends Base {
+    def sub = "sub"
+  }
+}
+  """
+
+  def client = """
+    class Client { new z.Sub().sub }
+  """
+
+  def deleteClass(s: String) = {
+    val f = new File(testOutput.path, s + ".class")
+    assert(f.exists)
+    f.delete()
+  }
+
+  def deletePackage(s: String) = {
+    val f = new File(testOutput.path, s)
+    assert(f.exists)
+    f.delete()
+  }
+
+  def assertNoErrors(): Unit = {
+    assert(storeReporter.infos.isEmpty, storeReporter.infos.mkString("\n"))
+    storeReporter.reset()
+  }
+  def show(): Unit = {
+    compileCode(library)
+    assertNoErrors()
+    deleteClass("a/b/C$D")
+    deleteClass("a/b/C")
+    deletePackage("a/b")
+    compileCode(client)
+    assertNoErrors()
+  }
+}
+