How to require form parameters on the value of another form parameter in OpenAPI?

Question

I have the following OpenAPI (Swagger) definition for a POST request with form data. How can I vary the required form parameters based on the value of the type parameter?

If type="email" only the email is required and if type="phone" only country and phone parameters are required.

  /login:
    post:
      required:
      - type
      - password
      responses:
        200:
          description: Successful response
          schema:
            $ref: '#/definitions/SomeResponse'
      parameters:
        - name: type
          type: string
          in: formData
          enum: [email, phone]
        - name: email
          type: string
          in: formData
        - name: country
          type: string
          in: formData
        - name: phone
          type: string
          in: formData
        - name: password
          type: string

Answer 1

Conditional dependencies in form data can be expressed in OpenAPI 3, but not in OpenAPI 2.0 (swagger: 2.0).

OpenAPI 3.1

This example uses if..then, a new construct in OAS 3.1.

openapi: 3.1.0
...

paths:
  /login:
    post:
      requestBody:
        required: true
        content:
          application/x-www-form-urlencoded:
            schema:
              $ref: '#/components/schemas/LoginRequest'
      responses:
        '200':
          description: Successful response

components:
  schemas:
    LoginRequest:
      type: object
      required:
        - type
        - password
      properties:
        type:
          type: string
          enum: [email, phone]
        email:
          type: string
        country:
          type: string
        phone:
          type: string
        password:
          type: string

      # Conditional dependencies
      allOf:
        # If type="email", then the `email` field is required
        - if:
            properties:
              type:
                const: email
          then:
            required: [email]

        # If type="phone", then the `country` and `phone` fields are required
        - if:
            properties:
              type:
                const: phone
          then:
            required: [country, phone]

OpenAPI 3.0.x

In OAS 3.0, you can use the following oneOf construct to express these conditions.

openapi: 3.0.3
...

paths:
  /login: ... # The path definition is the same as in the previous example


components:
  schemas:
    LoginRequest:
      type: object
      required:
        - type
        - password
      properties:
        type:
          type: string
          enum: [email, phone]
        email:
          type: string
        country:
          type: string
        phone:
          type: string
        password:
          type: string

      # Conditional dependencies
      oneOf:
        # If type="email" ...
        - properties:
            type:
              enum: [email]
          # ... then the `email` field is required
          required: [email]

        # If type="phone" ...
        - properties:
            type:
              enum: [phone]
          # ...then the `country` and `phone` fields are required
          required: [country, phone]

OpenAPI 2.0

In OpenAPI 2.0, the most you can do is define the conditionally required parameters (email, country and phone in your example) as optional and mention the dependencies in the operation description and/or parameter descriptions.

swagger: '2.0'
...

paths:
  /login:
    post:
      ...
      parameters:
        - name: type
          type: string
          in: formData
          enum: [email, phone]
          required: true
        - name: email
          type: string
          in: formData
          description: Required if type=email
        - name: country
          type: string
          in: formData
          description:  Required if type=phone
        - name: phone
          type: string
          in: formData
          description:  Required if type=phone
        - name: password
          type: string
          in: formData
          required: true