fix: recursively check for validity of discriminator

[AmadeusK525]

The old implementation was only checking for a single level of validity of a schema using the discriminator extension, without accounting for "unions of unions". So, for example:

{
  "discriminator": {
    "propertyName": "field"
  }
  "required": ["field"],
  "oneOf": [
    {
      "oneOf": [
        {
          "properties": {
            "field": { "const": "a" }
          },
          "required": ["field"]
        },
        {
          "properties": {
            "field": { "const": "b" }
          },
          "required": ["field"]
        }
      ],
      "required": ["field"]
    },
    {
      "properties": {
        "field": { "const": "c" }
      },
      "required": ["field"]
    }
  ]
}

This schema would fail because the compiler would complain about the first schema in oneOf not having the field property, which was defined as the discriminator. But it does have that property, just in a subschema inside another oneOf definition!

[epoberezkin]

It's debatable it's correct, given how discriminator is specified I think.

[AmadeusK525]

It's debatable it's correct, given how discriminator is specified I think.

@epoberezkin Sorry, but why would this be debatable? The spec may not mention recursion explicitly, but there's no reason to not cover this case, as it's in line with the reasoning behind the discriminator keyword (making sure that every object in the discriminated union has the discriminator property). I really don't see how an implementation that doesn't cover the recursion "edge" case would be more correct than one that doesn't