My use case: handling an IPC stream of arbitrary object messages, specifically with msgpack. I desire to use Structs for custom object serializations that can be passed between memory boundaries.

My presumptions were originally:

• a top level decoder is used to process msgpack data over an IPC channel
• by default, i'd expect that decoder will decode using the python type set to be able to accept arbitrary msgpack bytes and tagged msgspec.Structs
• if a custom tagged struct was placed inside some std python type (aka embedded), i'd expect this decoder (if enabled as such) to be able to detect the tagged object field (say {"type": "CustomStruct", "field0": "blah"}) and automatically know that the embedded msgpack object is one of our custom tagged structs and should be decoded as a CustomStruct.

Conclusions

Based on below thread:

• you can't easily define the std type set and a custom tagged struct using Union
• Decoder(Any | Struct) won't work even for top level Structs in the msgpack frame

This took me a (little) while to figure out because the docs didn't have an example for this use case, but if you want to create a Decoder that will handle a Union of tagged structs and it will still also process the standard built-in type set, you need to specify the subset of the std types that don't conflict with Struct as per @jcrist's comment in the section starting with:

This is not possible, for the same reason as presented above. msgspec forbids ambiguity.

So Decoder(Any | MyStructType) will not work.

I had to dig into the source code to figure this out and it's probably worth documenting this case for users?

Alternative solutions:

It seems there is no built-in way to handle an arbitrary serialization encode-stream that you wish to decode into the default set as well as be able to decode embedded tagged Struct types.

But, you can do a couple other things inside custom codec routines to try and accomplish this:

• create a custom boxed Any struct type, as per @jcrist's comment under the section starting with:

  Knowing nothing about what you're actually trying to achieve here, why not just define an extra Struct type in the union that can wrap Any.

• consider creating a top-level boxing Msg type and then using msgspec.Raw and a custom decoder table to decode payload msgpack data as in my example below