[Bug]: OpenAPI spec using the `Read` model for the `requestBody` of an `Update` operation

[abes-xpv]

Describe the bug

Given a model A with read-only properties, I would expect the openapi emitter to emit a dedicated AUpdate type to use for the requestBody of an op decorated with @parameterVisibility(Lifecycle.Update) but in some cases it will use the A model as-is.

The correct types are emitted if one of the model properties is decorated with @visibility(Lifecycle.Update) (i.e. a visibility that includes Update and excludes Read)

At face value this appears to be similar to #6917 but the issues are not exactly the same:

• In [Bug]: Create and Update transforms emit original models in OpenAPI when specified in op parameters #6917 the XUpdate type is emitted and used for the requestBody but the emitted type incorrectly include fields that should not be visible for update
• Here the XUpdate type is not emitted and it incorrectly uses X as the type for requestBody

Reproduction

https://typespec.io/playground/?c=bW9kZWwgQSB7CiAgQHZpc2liaWxpdHkoTGlmZWN5Y2xlLlJlYWQpxB5kb2MoInJlYWRPbmx5RmllbGQgc2hvdWxkIG5vdCBhcHBlYXIgaW4gdGhlIHNjaGVtYSBmb3LFD2B1cGRhdGVgIHJlcXVlc3RCb2R5IsRVzU86IHN0cmluZzsKCiAgLy8gRW5hYmxlxUFsaW5lIGJlbG93IHRvxFdjxhhjcmVhdGlvbiBvZiBhIGRlZGljYXRlZCBgQVXHcuUA4cZQ9gDgxinkAI1zb21lT3RoZXLlAI4%2F6gCPfQoKQHBhcmFtZXRlclbcRW9wIOYA6yguLi5BKTogQTsK&e=%40typespec%2Fopenapi3&options=%7B%7D&version=1.0.0-rc.0

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.

[timotheeguerin]

Our current approach with the openapi3 emitter is that it tries to produce as little models as possible. This means if the update visibility model would be able to just be represented by the same as the read then it will reuse that.

In your example I believe it is the case as the visibility read property can be represented with readonly attribute in openapi3.

[abes-xpv]

Thanks, I agree this is not a bug. After some reflection I can see that a single type with readOnly properties for both request and response bodies does align with the documented intent of the readOnly property in the OpenAPI spec.