[Bug]: getDiscrminator return undefined while TCGC can find the discriminator

[archerzz]

Describe the bug

Check the TSP spec for OpenAI.Assitants: https://github.com/Azure/azure-rest-api-specs/blob/c4e661cdf92c8f579574008d0cd11874cc303da0/specification/ai/OpenAI.Assistants/runs/models.tsp#L171-L188

RequiredAction is the base model, and SubmitToolOutputsAction is the child model.

getDiscriminator() on RequiredAction will return undefined, while TCGC can find the discriminator.

Here modelType is the TCGC SdkModelType, and modelType.__raw is the compiler type.

Reproduction

Just try to parse the Tsp spec in the link above.

Checklist

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• 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]

@archerzz is this an issue with tcgc or typespec I don't really get here?

[archerzz]

@archerzz is this an issue with tcgc or typespec I don't really get here?

@timotheeguerin I think it's in typespec, because modelType (which is TCGC SDKModelType) has the correct discriminator property, but getDiscriminator() (which is typespec API) failed.

[timotheeguerin]

how is tcgc getting it if the compiler is not returning it? can you make a smaller repro with exactly what is not working?

[archerzz]

how is tcgc getting it if the compiler is not returning it? can you make a smaller repro with exactly what is not working?

I'll try, I actually just copy&paste those two models into a smaller tsp project, and I could not reproduce it.

Some interesting finding, it looks like RequiredAction is parsed as Union in typespec...

[archerzz]

@tadelesh Is that possible that TCGC assigns a wrong instance to __raw? I don't think compiler will parse a model into a union.

[tadelesh]

See: https://github.com/Azure/azure-rest-api-specs/blob/c4e661cdf92c8f579574008d0cd11874cc303da0/specification/ai/OpenAI.Assistants/runs/models.tsp#L40.
The property type is a union of nullable type. For TCGC, we just return the type and set the property to nullable, but the raw will be the union type.

[timotheeguerin]

so is the issue you are calling getDiscriminator on RequiredAction | null but should go down to RequiredAction.

Can this issue be closed?

[archerzz]

so is the issue you are calling getDiscriminator on RequiredAction | null but should go down to RequiredAction.

Can this issue be closed?

@timotheeguerin sure, thank you all for the investigation