[oaiey]

A protocol which is cumbersome to implement in many languages. Hmmm what can't go wrong. Partial support, late support of extensions, bugs,...

IMHO: a very bad choice. Complicated basic and higher level elements of protocols are the death of them (remember SOAP). I follow the train of thoughts to not restrict yourself too much but if (eg) java or C++ cannot implement it easy, not a good idea.

[samprotas]

Protobuf supports "oneof" which is also cumbersome to implement in these same languages but all of them support it (with some extra LOC and no exhaustiveness checking watching your back).

Java/Go/C++ are perfectly capable of parsing a "type" key and conditionally parsing different shaped data. If you make a programming mistake here, you'll get a parse error (bad, but not a security problem). The pushback seems to be that a Java/Go/C++ implementation adds LOCS and won't gain much by doing this extra step so lets make the protocol itself match match their (less precise) data representation.

FWIW there is work towards improving Java in this regard: https://cr.openjdk.java.net/~briangoetz/amber/pattern-match....

[oaiey]

But is not that elementary OOP polymorphism? It all depends on the fact whether the type is annotated or whether it needs to be analyzed from the data by probing. And types annotation are present in protobuf parts I remember :).