InvariantFailedException on swagger inheritance

[dsilvasc]

Hi,

I have a swagger spec where the definitions include inheritance like this minimal example:

---
swagger: "2.0"
info:
  version: "1.0.1"
basePath: "/v1"
paths:
definitions:
  A:
    type: "object"
  B:
    allOf:
      - $ref: "#/definitions/A"
      - type: "object"
  C:
    allOf:
    - $ref: "#/definitions/B"
    - type: "object"

Running guardrail 0.43.0 with that spec crashes with:

Error: Error attempting to process swagger.yaml:

org.scalameta.invariants.InvariantFailedException: invariant failed:
when verifying value.!=(null)
found that value is equal to null
where value = null
        at com.twilio.guardrail.generators.CirceProtocolGenerator$PolyProtocolTermInterp$.apply(CirceProtocolGenerator.scala:395)
        at com.twilio.guardrail.generators.CirceProtocolGenerator$PolyProtocolTermInterp$.apply(CirceProtocolGenerator.scala:364)
          ...
        at com.twilio.guardrail.generators.AkkaHttp$.apply(AkkaHttp.scala:32)
        at com.twilio.guardrail.generators.AkkaHttp$.apply(AkkaHttp.scala:14)
          ...
        at com.twilio.guardrail.core.CoreTermInterp$$anon$1.$anonfun$apply$37(CoreTermInterp.scala:129)
          ...
        at com.twilio.guardrail.ReadSwagger$.readSwagger(ReadSwagger.scala:24)
        at com.twilio.guardrail.CLICommon$.$anonfun$run$8(CLI.scala:69)
          ...
        at com.twilio.guardrail.CLICommon$.run(CLI.scala:67)
        at com.twilio.guardrail.CLICommon$$anonfun$handleLanguage$1.$anonfun$applyOrElse$1(CLI.scala:128)
        at com.twilio.guardrail.CLICommon$$anonfun$handleLanguage$1.$anonfun$applyOrElse$1$adapted(CLI.scala:128)
        at com.twilio.guardrail.CLICommon.main(CLI.scala:133)
        at com.twilio.guardrail.CLICommon.main$(CLI.scala:131)
        at com.twilio.guardrail.CLI$.main(CLI.scala:137)
        at com.twilio.guardrail.CLI.main(CLI.scala)

[blast-hardcheese]

@dsilvasc Thanks for the report!

This seems to be because the allOf handling assumes a type hierarchy (Pet >: Cat >: Siamese) based on the discriminator field.

The required change would be to wrap https://github.com/twilio/guardrail/blob/aee9798633ad9f9be55f7a3445e63744233b41b2/modules/codegen/src/main/scala/com/twilio/guardrail/ProtocolGenerator.scala#L128 in Option, then follow the compiler errors, eventually getting to the point where https://github.com/twilio/guardrail/blob/aee9798633ad9f9be55f7a3445e63744233b41b2/modules/codegen/src/main/scala/com/twilio/guardrail/generators/CirceProtocolGenerator.scala#L406-L414 can just be reduced to the underlying single object decoder.

[blast-hardcheese]

CC @stanislav-chetvertkov, as this will likely impact #54