Using discriminator annotation for union types generates schema that accepts too much

[mvanniekerkSQ]

Generating a schema for a union type creates a schema that accepts anything if the value for the discriminator is not one of the values in the union. For example:

Typescript types:

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

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

export type B = {
    type: "B",
    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": {
            "$ref": "#/definitions/A"
          }
        },
        {
          "if": {
            "properties": {
              "type": {
                "type": "string",
                "const": "B"
              }
            }
          },
          "then": {
            "$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": "B"
        },
        "b": {
          "type": "string"
        }
      },
      "required": [
        "type",
        "b"
      ],
      "additionalProperties": false
    }
  }
}

Currently the schema would validate against for example this JSON while it should not be accepted:

{
	"type": "C",
	"c": "this is a string"
}

Note that the type Union does not require the discriminator value to be one of the constants defined in the type and expected by the if keyword. This problem can be fixed by requiring the Union type to have a type that is either "a" or "b" like so:

  "properties": {
    "type": {
      "enum": ["a", "b"]
    }
  },

[github-actions]

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