Serialization (de-)serializes types incorrectly for out-of-date bindings

[topolarity]

struct Foo
    x::Int32
end

x = Foo(1)

struct Foo end

using Serialization
Serialization.serialize("test.jls", x)
println(x) # prints "@world(Foo, 38778:38781)(1)"
println(Serialization.deserialize("test.jls")) # prints "Foo()"

In the the other order, this even breaks the serialization format:

struct Foo end

x = Foo()

struct Foo
    x::Int32
end

using Serialization
Serialization.serialize("test.jls", x)
println(x) # prints "@world(Foo, 38775:38777)()"
println(Serialization.deserialize("test.jls")) # ERROR: EOFError: read end of file

This errors:

ERROR: EOFError: read end of file
Stacktrace:
  [1] read(s::IOStream, ::Type{UInt8})
    @ Base ./iostream.jl:424

[JeffBezanson]

Yes, Serialization has a baked-in assumption that types don't change. This is a more general issue, since it has always been possible to serialize a value, then change the definition of its type, then recompile and reload the package, then try to deserialize the value, which will not work and is explicitly not supported. Now the same is possible within a process, but I'm not sure we need to do anything about it other than documentation.