Using discriminator annotation for union types with not unique discriminator generates schema that is logically inconsistent

[mvanniekerkSQ]

Using a discriminator annotation for union types with a non-unique discriminator generates a schema that is logically inconsistent.

For example:

/**
 * @discriminator type
 */
export type Union
    = A | B;

export type A = {
    type: "A",
    a: string,
}

export type B = {
    type: "A",
    b: string,
}

Gives the following output:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "$ref": "#/definitions/Union",
  "definitions": {
    "Union": {
      "allOf": [
        {
          "if": {
            "properties": {
              "type": {
                "type": "string",
                "const": "A"
              }
            }
          },
          "then": {
            "$ref": "#/definitions/A"
          }
        },
        {
          "if": {
            "properties": {
              "type": {
                "type": "string",
                "const": "A"
              }
            }
          },
          "then": {
            "$ref": "#/definitions/B"
          }
        }
      ]
    },
    "A": {
      "type": "object",
      "properties": {
        "type": {
          "type": "string",
          "const": "A"
        },
        "a": {
          "type": "string"
        }
      },
      "required": [
        "type",
        "a"
      ],
      "additionalProperties": false
    },
    "B": {
      "type": "object",
      "properties": {
        "type": {
          "type": "string",
          "const": "A"
        },
        "b": {
          "type": "string"
        }
      },
      "required": [
        "type",
        "b"
      ],
      "additionalProperties": false
    }
  }
}

In this schema, having type 'A' requires both schema A and B to be valid. However, they are mutually exclusive. A requires fields type and a and no additional properties. B requires fields type and b and no additional properties. So the schema cannot be satisfied.

The simplest solution for this is requiring that each discriminator key is unique. This issue is related to #1492 .

[github-actions]

🚀 Issue was released in v1.2.0-next.4 🚀