The Observable macro does not play well with the standard Codable protocol

Motivation

Hello,

When a developer declares a type both @Observable and Codable, there is a bad surprise: the Observable macro transforms regular properties into computed ones, and adds new properties, prefixed by underscores. Codable then only deals with underscored properties.

In the end, the type gets a "corrupted" Codable facet due to Observable:

No one expects coding keys to be prefixed with underscores.
Adding @Observable to a working Codable type breaks its Codable facet, because it can no longer decode previously encoded data. This is pretty bad, actually, because this breaks the strong compatibility contract of Codable.

I was aware of this problem from two sources:

In both cases, developers were completed puzzled.

And workarounds are not pretty. Basically they ruin the most valuable aspect of Codable, which is the convenience of compiler-synthesized code.

Please consider solving this issue. Standard components such as @Observable and Codable should not conflict together.

Proposed solution

I don't have any ready-made solution. I expect that we'd need to give much more powers to macros.

Alternatives considered

No response

Additional information

I submitted the same report as FB13756604 (The Observable macro does not play well with the standard Codable protocol)

swiftlang / swift