WARNING: definition is not used anywhere if a definition is used in composition using allOf.

[folago]

Problem statement

swagger validate returns WARNING: definition [...] is not used anywhere if a definition is used in composition using allOf.

Is this the expected behaviour?
We are getting more strict against warnings in our CI and as far as I understood this is the right way to do things. But then why there is this warning?

Swagger specification

swagger: "2.0"

info:
  title: Sample API
  description: 'API description in Markdown.'
  version: '1.0.0'

host: 'api.example.com'

basePath: '/v1'

schemes:
  - https
    
paths:
  /pets:
    get:
      summary: Returns pets.
      description: Optional extended description in Markdown.
      produces:
        - application/json
      responses:
        200:
          description: OK
          schema:
            type: object
            required: 
              - data
            properties:
              data:
                $ref: '#/definitions/Pet'


definitions:
  Pet:
    type: object
    required:
      - pet_type
    properties:
      pet_type:
        type: string
    discriminator: pet_type
  Dog:     # "Dog" is a value for the pet_type property (the discriminator value)
    allOf: # Combines the main `Pet` schema with `Dog`-specific properties 
      - $ref: '#/definitions/Pet'
      - type: object
        # all other properties specific to a `Dog`
        properties:
          bark:
            type: boolean
          breed:
            type: string
            enum: [Dingo, Husky, Retriever, Shepherd]
  Cat:     # "Cat" is a value for the pet_type property (the discriminator value)
    allOf: # Combines the main `Pet` schema with `Cat`-specific properties 
      - $ref: '#/definitions/Pet'
      - type: object
        # all other properties specific to a `Cat`
        properties:
          hunts:
            type: boolean
          age:
            type: integer

Steps to reproduce

Run swagger validate on a file containing the specs above.

It gives two warnings:

2020/06/08 13:52:07 - WARNING: definition "#/definitions/Dog" is not used anywhere
2020/06/08 13:52:07 - WARNING: definition "#/definitions/Cat" is not used anywhere

Environment

swagger version: v0.23.0

go version: go1.12.17 darwin/amd64

OS: MacOS 10.15.4

[fredbi]

@folago yes the validator (github.com/go-openapi/validate) is not aware of special rules with discriminated types.
This is indeed a shortcoming and we shouldn't have this warning (since subtypes are indirectly used through their base type).
There are a handful of other validation rules specifically for discriminated types that should be implemented.
A PR fixing this would definitely be welcome.