SI-5919 TypeTags and Exprs should be serializable

- Make TypeCreator and TreeCreator extend Serializable.
- When replacing a SerializedTypeTag with a TypeTag or WeakTypeTag,
  do not use scala.reflect.runtime.universe.rootMirror, since
  it is unlikely to find user classes; instead, create a runtime
  mirror using the context ClassLoader of the current thread.
  Use the same logic for SerializedExpr.
- Remove writeObject/readObject methods from SerializedTypeTag
  and SerializedExpr since they are unused.
- Add @throws annotation on writeReplace and readResolve methods.
- Handle SecurityException if the current thread cannot access the
  context ClassLoader.
- To make type tags of primitive value classes serializable, make
  PredefTypeCreator a top-level class. Otherwise, it would
  retain a reference to the enclosing Universe,
  rendering the TypeCreator non-serializable.

Binary compatibility:

- Keep nested PredefTypeCreator class to avoid backward binary
  incompatible change.
- Keep `var` modifiers on the class parameters of
  SerializedTypeTag for backward binary compatibility.
- Adds filter rules to forward binary compatibility whitelist:
  - `TypeCreator`, `PredefTypeCreator`, and `TreeCreator` must now
    extend from `Serializable`.
  - Must have new class `scala.reflect.api.PredefTypeCreator`
    to avoid problematic outer reference.

1 files changed, 2 insertions, 2 deletions

diff --git a/test/files/run/abstypetags_serialize.check b/test/files/run/abstypetags_serialize.check
index bddc4523e6..1b5e2ebddf 100644
--- a/test/files/run/abstypetags_serialize.check
+++ b/test/files/run/abstypetags_serialize.check
@@ -1,2 +1,2 @@
-java.io.NotSerializableException: Test$$typecreator1$1
-java.io.NotSerializableException: Test$$typecreator2$1
+WeakTypeTag[T]
+WeakTypeTag[U[String]]