Allow discriminator to be optional

src/oas.md

@@ -2832,7 +2832,7 @@ JSON Schema implementations MAY choose to treat keywords defined by the OpenAPI
 
 | Field Name | Type | Description |
 | ---- | :----: | ---- |
-| <a name="schema-discriminator"></a>discriminator | [Discriminator Object](#discriminator-object) | Adds support for polymorphism. The discriminator is used to determine which of a set of schemas a payload is expected to satisfy. See [Composition and Inheritance](#composition-and-inheritance-polymorphism) for more details. |
+| <a name="schema-discriminator"></a>discriminator | [Discriminator Object](#discriminator-object) | The discriminator provides a "hint" for which of a set of schemas a payload is expected to satisfy. See [Composition and Inheritance](#composition-and-inheritance-polymorphism) for more details. |
 | <a name="schema-xml"></a>xml | [XML Object](#xml-object) | This MAY be used only on property schemas. It has no effect on root schemas. Adds additional metadata to describe the XML representation of this property. |
 | <a name="schema-external-docs"></a>externalDocs | [External Documentation Object](#external-documentation-object) | Additional external documentation for this schema. |
 | <a name="schema-example"></a>example | Any | A free-form field to include an example of an instance for this schema. To represent examples that cannot be naturally represented in JSON or YAML, a string value can be used to contain the example with escaping where necessary.<br><br>**Deprecated:** The `example` field has been deprecated in favor of the JSON Schema `examples` keyword. Use of `example` is discouraged, and later versions of this specification may remove it. |
@@ -2870,9 +2870,15 @@ The OpenAPI Specification allows combining and extending model definitions using
 `allOf` takes an array of object definitions that are validated _independently_ but together compose a single object.
 
 While composition offers model extensibility, it does not imply a hierarchy between the models.
-To support polymorphism, the OpenAPI Specification adds the [`discriminator`](#schema-discriminator) field.
-When used, the `discriminator` indicates the name of the property that hints which schema definition is expected to validate the structure of the model.
-As such, the `discriminator` field MUST be a required field.
+
+JSON Schema also provides the `anyOf` and `oneOf` keywords, which allow defining multiple schemas where at least one or exactly one of them must be valid, respectively.
+As is the case with `allOf`, the schemas are validated _independently_.
+These keywords can be used to describe polymorphism, where a single field can accept multiple types of values.
+
+The OpenAPI specification extends the JSON Schema support for polymorphism by adding the [`discriminator`](#schema-discriminator) field.
+When used, the `discriminator` indicates the name of the property that hints which schema of an `anyOf` or `oneOf` is expected to validate the structure of the model.
+The `discriminator` property may be defined as required or optional, but when defined as an optional property the `discriminator` field must include a `default` field that specifies which schema of the `anyOf` or `oneOf` is expected to validate the structure of the model.
+
 There are two ways to define the value of a discriminator for an inheriting instance.
 
 * Use the schema name.
@@ -3135,116 +3141,108 @@ components:
 
 ###### Models with Polymorphism Support
 
-```json
-{
-  "components": {
-    "schemas": {
-      "Pet": {
-        "type": "object",
-        "discriminator": {
-          "propertyName": "petType"
-        },
-        "properties": {
-          "name": {
-            "type": "string"
-          },
-          "petType": {
-            "type": "string"
-          }
-        },
-        "required": ["name", "petType"]
-      },
-      "Cat": {
-        "description": "A representation of a cat. Note that `Cat` will be used as the discriminating value.",
-        "allOf": [
-          {
-            "$ref": "#/components/schemas/Pet"
-          },
-          {
-            "type": "object",
-            "properties": {
-              "huntingSkill": {
-                "type": "string",
-                "description": "The measured skill for hunting",
-                "default": "lazy",
-                "enum": ["clueless", "lazy", "adventurous", "aggressive"]
-              }
-            },
-            "required": ["huntingSkill"]
-          }
-        ]
-      },
-      "Dog": {
-        "description": "A representation of a dog. Note that `Dog` will be used as the discriminating value.",
-        "allOf": [
-          {
-            "$ref": "#/components/schemas/Pet"
-          },
-          {
-            "type": "object",
-            "properties": {
-              "packSize": {
-                "type": "integer",
-                "format": "int32",
-                "description": "the size of the pack the dog is from",
-                "default": 0,
-                "minimum": 0
-              }
-            },
-            "required": ["packSize"]
-          }
-        ]
-      }
-    }
-  }
-}
+The following example describes a `Pet` model that can represent either a cat or a dog, as distinguished by the `petType` property. Each type of pet has other properties beyond those of the base `Pet` model. An instance without a `petType` property, or with a `petType` property that does not match either `cat` or `dog`, is invalid.
+
+```yaml
+components:
+  schemas:
+    Pet:
+      type: object
+      properties:
+        name:
+          type: string
+      required:
+        - name
+        - petType
+      oneOf:
+        - $ref: '#/components/schemas/Cat'
+        - $ref: '#/components/schemas/Dog'
+    Cat:
+      description: A pet cat
+      type: object
+      properties:
+        petType:
+          const: 'cat'
+        huntingSkill:
+          type: string
+          description: The measured skill for hunting
+          enum:
+            - clueless
+            - lazy
+            - adventurous
+            - aggressive
+      required:
+        - huntingSkill
+    Dog:
+      description: A pet dog
+      type: object
+      properties:
+        petType:
+          const: 'dog'
+        packSize:
+          type: integer
+          format: int32
+          description: the size of the pack the dog is from
+          default: 0
+          minimum: 0
+      required:
+        - packSize
 ```
 
+###### Models with Polymorphism Support and a Discriminator field
+
+The following example extends the example of the previous section by adding a `discriminator` field to the `Pet` model. Note that the `discriminator` is only a hint to the consumer of the API, and does not change the validation outcome of the schema.
+
 ```yaml
 components:
   schemas:
     Pet:
       type: object
       discriminator:
         propertyName: petType
+        mapping:
+          cat: '#/components/schemas/Cat'
+          dog: '#/components/schemas/Dog'
       properties:
         name:
           type: string
+      required:
+        - name
+        - petType
+      oneOf:
+        - $ref: '#/components/schemas/Cat'
+        - $ref: '#/components/schemas/Dog'
+    Cat:
+      description: A pet cat
+      type: object
+      properties:
         petType:
+          const: 'cat'
+        huntingSkill:
           type: string
+          description: The measured skill for hunting
+          enum:
+            - clueless
+            - lazy
+            - adventurous
+            - aggressive
+      required:
+        - huntingSkill
+    Dog:
+      description: A pet dog
+      type: object
+      properties:
+        petType:
+          const: 'dog'
+        packSize:
+          type: integer
+          format: int32
+          description: the size of the pack the dog is from
+          default: 0
+          minimum: 0
       required:
-        - name
         - petType
-    Cat: # "Cat" will be used as the discriminating value
-      description: A representation of a cat
-      allOf:
-        - $ref: '#/components/schemas/Pet'
-        - type: object
-          properties:
-            huntingSkill:
-              type: string
-              description: The measured skill for hunting
-              enum:
-                - clueless
-                - lazy
-                - adventurous
-                - aggressive
-          required:
-            - huntingSkill
-    Dog: # "Dog" will be used as the discriminating value
-      description: A representation of a dog
-      allOf:
-        - $ref: '#/components/schemas/Pet'
-        - type: object
-          properties:
-            packSize:
-              type: integer
-              format: int32
-              description: the size of the pack the dog is from
-              default: 0
-              minimum: 0
-          required:
-            - packSize
+        - packSize
 ```
 
 ###### Generic Data Structure Model
@@ -3362,7 +3360,9 @@ components:
 
 #### Discriminator Object
 
-When request bodies or response payloads may be one of a number of different schemas, a Discriminator Object gives a hint about the expected schema of the document.
+When request bodies or response payloads may be one of a number of different schemas, these should use the JSON Schema `anyOf` or `oneOf` keywords to describe the possible schemas (see [Composition and Inheritance](#composition-and-inheritance-polymorphism)).
+
+A polymorphic schema MAY include a `discriminator` field, which defines the name of the property that may be used as a hint for which schema of the `anyOf` or `oneOf` is expected to validate the structure of the model.
 This hint can be used to aid in serialization, deserialization, and validation.
 The Discriminator Object does this by implicitly or explicitly associating the possible values of a named property with alternative schemas.
 
@@ -3372,8 +3372,9 @@ Note that `discriminator` MUST NOT change the validation outcome of the schema.
 
 | Field Name | Type | Description |
 | ---- | :----: | ---- |
-| <a name="property-name"></a>propertyName | `string` | **REQUIRED**. The name of the property in the payload that will hold the discriminating value. This property SHOULD be required in the payload schema, as the behavior when the property is absent is undefined. |
+| <a name="property-name"></a>propertyName | `string` | **REQUIRED**. The name of the property in the payload that will hold the discriminating value. This property may be defined as required or optional, but when defined as an optional property the `discriminator` field must include a `default` field that specifies which schema is expected to validate the structure of the model. |
 | <a name="discriminator-mapping"></a> mapping | Map[`string`, `string`] | An object to hold mappings between payload values and schema names or URI references. |
+| <a name="default"></a> default | `string` | The schema name or URI reference to a schema that is expected to validate the structure of the model when the `discriminator` property is not present in the payload. |
 
 This object MAY be extended with [Specification Extensions](#specification-extensions).
 
@@ -3395,12 +3396,29 @@ The behavior of any configuration of `oneOf`, `anyOf`, `allOf` and `discriminato
 The value of the property named in `propertyName` is used as the name of the associated schema under the [Components Object](#components-object), _unless_ a `mapping` is present for that value.
 The `mapping` entry maps a specific property value to either a different schema component name, or to a schema identified by a URI.
 When using implicit or explicit schema component names, inline `oneOf` or `anyOf` subschemas are not considered.
-The behavior of a `mapping` value that is both a valid schema name and a valid relative URI reference is implementation-defined, but it is RECOMMENDED that it be treated as a schema name.
+The behavior of a `mapping` value or `default` value that is both a valid schema name and a valid relative URI reference is implementation-defined, but it is RECOMMENDED that it be treated as a schema name.
 To ensure that an ambiguous value (e.g. `"foo"`) is treated as a relative URI reference by all implementations, authors MUST prefix it with the `"."` path segment (e.g. `"./foo"`).
 
 Mapping keys MUST be string values, but tooling MAY convert response values to strings for comparison.
 However, the exact nature of such conversions are implementation-defined.
 
+##### Optional `discriminator` property
+
+When the `discriminator` property is defined as optional, the `discriminator` field must include a `default` field that specifies a schema that is expected to validate the structure of the model when the `discriminator` property is not present in the payload. This allows the schema to still be validated correctly even if the discriminator property is missing.
+
+The primary use case for an optional `discriminator` property is to allow a schema to be extended with a discriminator without breaking existing clients that do not provide the discriminator property.
+
+Typically the schema specified in the `default` field will specify that the `discriminator` property is not present, e.g.
+
+```yaml
+OtherPet:
+  type: object
+  not:
+    required: ['petType']
+```
+
+This will prevent the default schema from validating a payload that includes the `discriminator` property, which would cause a validation of the payload to fail when polymorphism is described using the `oneOf` JSON schema keyword.
+
 ##### Examples
 
 For these examples, assume all schemas are in the [entry document](#openapi-description-structure) of the OAD; for handling of `discriminator` in referenced documents see [Resolving Implicit Connections](#resolving-implicit-connections).
@@ -3458,6 +3476,28 @@ Here the discriminating value of `dog` will map to the schema `#/components/sche
 
 When used in conjunction with the `anyOf` construct, the use of the discriminator can avoid ambiguity for serializers/deserializers where multiple schemas may satisfy a single payload.
 
+When the `discriminator` property is defined as optional, the `discriminator` field must include a `default` field that specifies a schema of the `anyOf` or `oneOf` is expected to validate the structure of the model when the `discriminator` property is not present in the payload. This allows the schema to still be validated correctly even if the discriminator property is missing.
+
+For example:
+
+```yaml
+MyResponseType:
+  oneOf:
+    - $ref: '#/components/schemas/Cat'
+    - $ref: '#/components/schemas/Dog'
+    - $ref: '#/components/schemas/Lizard'
+    - $ref: '#/components/schemas/OtherPet'
+  discriminator:
+    propertyName: petType
+    default: OtherPet
+OtherPet:
+  type: object
+  not:
+    required: ['petType']
+```
+
+In this example, if the `petType` property is not present in the payload, the payload should validate against the `OtherPet` schema.
+
 This example shows the `allOf` usage, which avoids needing to reference all child schemas in the parent:
 
 ```yaml