Depending on what your goals are, I don't think it's enough to allow any class that
is assignable to Serializable.

Two problem cases:

 - The entire hierarchy of Exceptions is Serializable, and I'm sure there's a
subclass of RuntimeException that's in java.something that is not emulated by
GWT--such an Exception could be thrown by the server by accident and not be
deserialized by the client

 - Dead code elimination seems to erase class definitions for things like
NullPointerException unless they're explicitly declared as thrown, or explicitly
caught, which means that the client can't deserialize a server-side
NullPointerException (for example) even though NullPointerException is both
assignable to Serializable _and_ emulated by GWT

Ian

Updated the summary to clarify the goal.

"I will grant that this won't be fool proof because the checks performed by the
server are not as extensive as those of the client but I think that it will reduce
the number incidents significantly."

The cases that you point out are examples of why it won't be fool proof.  However,
there are a lot of common cases like Arrays.asList which it will catch that people
run into very often.

The fool proof solution requires that the server to know exactly what types the
client can handle and not send any types outside of that set.  For that to work, the
server would need to get that information from the proxy generation system.  That
plumbing is not in place today and it won't be in place for 1.4.

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Hello. I think this is related:
I was returning an object from RPC service. That object contained one property of 
type Serializable. On the client side the AsyncCallback.onFailure was invoked with 
Throwable caught.toString() + ":" + caught.getMessage() 
containing: "java.lang.ClassCastException: null". This was only in non-hosted mode. 
In hosted mode I got no exception but AsyncCallback.onSuccess was invoked.

GWT 1.4 RC1

This can behavior can be explained by that fact that none of our JRE emulation
classes actually implement java.io.Serializable.  I suspect that that this is part of
the problem.  This would aslo explain why it works in hosted mode but not in web
mode.  See  Issue 1139  for more details.

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This has been committed as r1302.  Miguel, I'll let you add any notes you'd like to add.

1.4 RC2 now released.