[Bug]: OpenAPI - Discriminator properties are not consistently marked as required

[johngallardo]

Describe the bug

AutoRest requires that discriminator properties are marked as required in their schemas:
DiscriminatorNotRequired | Semantic violation: Discriminator must be a required property.

However, schemas emitted by OpenAPI3 emitter do not consistently appear in the set of required properties for both @createsResource and @updatesResource decorated operations.

Reproduction

import "@typespec/http";
import "@typespec/rest";
import "@typespec/openapi";
import "@typespec/openapi3";

using TypeSpec.Http;
using TypeSpec.OpenAPI;
using TypeSpec.Rest;

@service({
    title: "A Zoo Service"
})

namespace Zoo;

@discriminator("kind")
model Animal {
    @visibility("read", "create") id: string;
    name?: string;
}

model Cat extends Animal {
    kind: "cat";
    whiskerCount?: integer;
}

model Penguin extends Animal {
    kind: "penguin";
}

@route("/zoo")
interface Zoo {
    @listsResource(Animal)
    read(): Animal[];

    @operationId("getAnimalById")
    readById(@path id: string): Animal;

    @createsResource(Animal)
    add(@body animal: Animal): Animal;

    @updatesResource(Animal)
    patch(@path id: string, @body animal: Animal): Animal;
}

Produces:

openapi: 3.0.0
info:
  title: A Zoo Service
  version: 0.0.0
tags: []
paths:
  /zoo:
    get:
      operationId: Zoo_read
      parameters: []
      responses:
        '200':
          description: The request has succeeded.
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/Animal'
    post:
      operationId: Zoo_add
      parameters: []
      responses:
        '200':
          description: The request has succeeded.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Animal'
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Animal'
  /zoo/{id}:
    get:
      operationId: getAnimalById
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: string
      responses:
        '200':
          description: The request has succeeded.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Animal'
    patch:
      operationId: Zoo_patch
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: string
      responses:
        '200':
          description: The request has succeeded.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Animal'
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/AnimalUpdate'
components:
  schemas:
    Animal:
      type: object
      required:
        - id
      properties:
        id:
          type: string
        name:
          type: string
        kind:
          type: string
          description: Discriminator property for Animal.
      discriminator:
        propertyName: kind
        mapping:
          cat: '#/components/schemas/Cat'
          penguin: '#/components/schemas/Penguin'
    AnimalUpdate:
      type: object
      properties:
        name:
          type: string
        kind:
          type: string
          description: Discriminator property for Animal.
      discriminator:
        propertyName: kind
        mapping:
          cat: '#/components/schemas/CatUpdate'
          penguin: '#/components/schemas/PenguinUpdate'
    Cat:
      type: object
      required:
        - kind
      properties:
        kind:
          type: string
          enum:
            - cat
        whiskerCount:
          type: integer
      allOf:
        - $ref: '#/components/schemas/Animal'
    CatUpdate:
      type: object
      properties:
        kind:
          type: string
          enum:
            - cat
        whiskerCount:
          type: integer
      allOf:
        - $ref: '#/components/schemas/AnimalUpdate'
    Penguin:
      type: object
      required:
        - kind
      properties:
        kind:
          type: string
          enum:
            - penguin
      allOf:
        - $ref: '#/components/schemas/Animal'
    PenguinUpdate:
      type: object
      properties:
        kind:
          type: string
          enum:
            - penguin
      allOf:
        - $ref: '#/components/schemas/AnimalUpdate'

When attempting to generate autorest:

PS C:\src\typespectest> autorest --input-file='.\tsp-output\@typespec\openapi3\openapi.yaml' --typescript
AutoRest code generation utility [cli version: 3.7.1; node: v18.18.0]
(C) 2018 Microsoft Corporation.
https://aka.ms/autorest
info    | AutoRest core version selected from configuration: ^3.7.0.
info    |    Loading AutoRest core      'C:\Users\jgalla\.autorest\@[email protected]\nodemodules\@autorest\core\dist' (3.10.2)
info    |    Loading AutoRest extension '@autorest/typescript' (latest->6.0.27)
info    |    Loading AutoRest extension '@autorest/modelerfour' (4.26.2->4.26.2)
error   | DiscriminatorNotRequired | Semantic violation: Discriminator must be a required property. (components > schemas > Animal)
  discriminator: {
  propertyName: 'kind',
  mapping: {
    cat: 'file:///C:/src/typespectest/tsp-output/@typespec/openapi3/openapi.yaml#/components/schemas/Cat',
    penguin: 'file:///C:/src/typespectest/tsp-output/@typespec/openapi3/openapi.yaml#/components/schemas/Penguin'
  }
}
    - file:///C:/src/typespectest/tsp-output/@typespec/openapi3/openapi.yaml:75:5
fatal   | Process() cancelled due to failure
error   | DiscriminatorNotRequired | Semantic violation: Discriminator must be a required property. (components > schemas > AnimalUpdate)
  discriminator: {
  propertyName: 'kind',
  mapping: {
    cat: 'file:///C:/src/typespectest/tsp-output/@typespec/openapi3/openapi.yaml#/components/schemas/CatUpdate',
    penguin: 'file:///C:/src/typespectest/tsp-output/@typespec/openapi3/openapi.yaml#/components/schemas/PenguinUpdate'
  }
}
    - file:///C:/src/typespectest/tsp-output/@typespec/openapi3/openapi.yaml:92:5
error   |   Error: Semantic validation failed. There was some errors
error   | Autorest completed with an error. If you think the error message is unclear, or is a bug, please declare an issues at https://github.com/Azure/autorest/issues with the error message you are seeing.

One can attempt to work around by modifying the Animal model as follows:

@discriminator("kind")
model Animal {
    @visibility("read", "create") id: string;
    kind: string;
    name?: string;
}

This fixes the schema that is used for Create and Read, but the schema for update loses kind as a required property:

    AnimalUpdate:
      type: object
      properties:
        kind:
          type: string
        name:
          type: string
      discriminator:
        propertyName: kind
        mapping:
          cat: '#/components/schemas/CatUpdate'
          penguin: '#/components/schemas/PenguinUpdate'

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• The provided reproduction is a minimal reproducible example of the bug.