An GWT engineer and former Scala guy, Lex Spoon, proved a similar theorem several years ago when we were upgrading generics for GWT-RPC. The GWT RPC code genera

An GWT engineer and former Scala guy, Lex Spoon, proved a similar theorem several years ago when we were upgrading generics for GWT-RPC. The GWT RPC code generator has to reason about return types of methods and which classes could possibly be deserialized say if you return a List of Foo. Lex showed how to construct the peano arithmetic in Java generics and how to formulate problems such that solving the question of what types could be returned from An Rpc call with List would amount to deciding these small theorems or problems and therefore the GWT SerializationTypeOracleBuilder class would have to solve the halting theorem. That's probably a wrong paraphrasing and maybe lex will see this and chime in but I think the original discussion was lost forever ironically when Wave, which was built with GWT, was shut down.