no-override-props

Id

@azure-tools/typespec-azure-resource-manager/no-override-props

Disallow redefining properties already defined in a base type.

Repeating inherited properties in a child model is an anti-pattern for Azure APIs and can cause problems with OpenAPI tooling and some language representations of the models.

Overriding an inherited property is allowed when:

• The overriding property has the same type as the inherited one (by identity). Type aliases resolve to the same underlying type, and template instantiations are cached, so this naturally covers cases such as redefining systemData: SystemData (alias) where the parent uses systemData: Foundations.SystemData, or the implicit tags: Record<string> cloned into a model that uses is TrackedResource<...>.
• Both the inherited property and the overriding property are compatible scalars — that is, both reduce to the same scalar family (string, numeric, or boolean). For example, an inherited property of type string may be overridden by a string, by a scalar that extends string, by a string literal, by a string-valued enum, or by an open or closed string union.

Impact

• Area: SDK, Tooling

Overridden properties can crash the breaking-change tool and are unsupported by most languages.

❌ Incorrect

@armProviderNamespace
namespace MyService;

model InnerBase {
  a: string;
}

model InnerDerived extends InnerBase {
  b: string;
}

model Base {
  nested: InnerBase;
}

model Child extends Base {
  nested: InnerDerived; // narrows the inherited "nested" to a derived model
}

✅ Correct

@armProviderNamespace
namespace MyService;

model Base {
  id: string;
  commonProp: string;
}

model Child extends Base {
  otherProp: string;
}

✅ Correct (compatible scalar override)

@armProviderNamespace
namespace MyService;

@discriminator("kind")
model Pet {
  kind: string;
  weight: int32;
}

model Cat extends Pet {
  kind: "cat"; // allowed: string and string literal are compatible scalars
  meow: boolean;
}

Suppression

Requires SDK sign-off. Remove overridden properties from the base type - they should be represented only in leaf types.