flatten-polymorphism

Id

@azure-tools/typespec-client-generator-core/flatten-polymorphism

Severity: error

This diagnostic is issued when @flattenProperty is applied to a model property whose type is polymorphic (a discriminated type). Flattening a polymorphic type is not supported, because most languages cannot represent it.

Impact

• Area: SDK model flattening. Blocks applying @flattenProperty because flattening a discriminated model would break generated polymorphic model structure.
• Not affected: The discriminator and inheritance described by the service model remain unchanged.

❌ Incorrect Usage

@discriminator("kind")
model Pet {
  kind: string;
}
model Cat extends Pet {
  kind: "cat";
}

model Owner {
  @flattenProperty // `pet` has polymorphic type `Pet`
  pet: Pet;
}

Diagnostic Message

TCGC reports:

Cannot flatten property of polymorphic type.

✅ How to Fix

Remove @flattenProperty from the polymorphic property, or change the property to a non-polymorphic type.

@discriminator("kind")
model Pet {
  kind: string;
}
model Cat extends Pet {
  kind: "cat";
}

model Owner {
  pet: Pet;
}