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).

3 files changed, 3 insertions, 2 deletions

diff --git a/src/reflect/scala/reflect/internal/Symbols.scala b/src/reflect/scala/reflect/internal/Symbols.scala
index 890a5796e9..9d71136fc5 100644
--- a/src/reflect/scala/reflect/internal/Symbols.scala
+++ b/src/reflect/scala/reflect/internal/Symbols.scala
@@ -809,7 +809,7 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
 
     final def isDerivedValueClass =
       isClass && !hasFlag(PACKAGE | TRAIT) &&
-      info.firstParent.typeSymbol == AnyValClass && !isPrimitiveValueClass
+      !phase.erasedTypes && info.firstParent.typeSymbol == AnyValClass && !isPrimitiveValueClass
 
     final def isMethodWithExtension =
       isMethod && owner.isDerivedValueClass && !isParamAccessor && !isConstructor && !hasFlag(SUPERACCESSOR) && !isMacro && !isSpecialized
diff --git a/src/reflect/scala/reflect/internal/Types.scala b/src/reflect/scala/reflect/internal/Types.scala
index b46f071717..0318acee2d 100644
--- a/src/reflect/scala/reflect/internal/Types.scala
+++ b/src/reflect/scala/reflect/internal/Types.scala
@@ -1197,6 +1197,7 @@ trait Types
       else super.safeToString
     override def narrow: Type = this
     override def kind = "ThisType"
+    override def boundSyms = if (sym.isInstanceOf[StubSymbol]) emptySymbolSet else super.boundSyms
   }
 
   final class UniqueThisType(sym: Symbol) extends ThisType(sym) { }
diff --git a/src/reflect/scala/reflect/internal/pickling/UnPickler.scala b/src/reflect/scala/reflect/internal/pickling/UnPickler.scala
index 08ccac8069..b4152c9b8c 100644
--- a/src/reflect/scala/reflect/internal/pickling/UnPickler.scala
+++ b/src/reflect/scala/reflect/internal/pickling/UnPickler.scala
@@ -377,7 +377,7 @@ abstract class UnPickler {
 
       def readThisType(): Type = {
         val sym = readSymbolRef() match {
-          case stub: StubSymbol => stub.setFlag(PACKAGE)
+          case stub: StubSymbol => stub.setFlag(PACKAGE | MODULE)
           case sym => sym
         }
         ThisType(sym)