[Go][Experimental] OneOf Implementation does not work without explicit mapping defined in spec

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• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
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Description

As per the discriminator section in the OAS spec (https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#discriminatorObject), the mapping part should be optional and required only when an implicit mapping is not possible.

I do not see implicit mapping derivation working with go-experimental generator today. The Unmarshal code does not generate any type specific unmarshal code for each of the OneOf types.

This should be enough

  ┆ ntp.Policy.Response:
  ┆ ┆ description: test
  ┆ ┆ oneOf:
  ┆ ┆ ┆ - $ref: '#/components/schemas/mo.DocumentCount'
  ┆ ┆ ┆ - $ref: '#/components/schemas/ntp.Policy.List'
  ┆ ┆ discriminator:
  ┆ ┆ ┆ propertyName: ObjectType

But current generator seem to require this,

  ┆ ntp.Policy.Response:
  ┆ ┆ description: test
  ┆ ┆ oneOf:
  ┆ ┆ ┆ - $ref: '#/components/schemas/mo.DocumentCount'
  ┆ ┆ ┆ - $ref: '#/components/schemas/ntp.Policy.List'
  ┆ ┆ discriminator:
        mapping:
  ┆ ┆ ┆ ┆ mo.DocumentCount: '#/components/schemas/mo.DocumentCount'
  ┆ ┆ ┆ ┆ ntp.Policy.List: '#/components/schemas/ntp.Policy.List'
  ┆ ┆ ┆ propertyName: ObjectType

openapi-generator version

latest master

OpenAPI declaration file content or url

Command line used for generation

Steps to reproduce

Related issues/PRs

Suggest a fix

Start with the go-experimental generator's OneOf and Unmarshal generation for the provided OpenAPI schema, comparing output with and without an explicit discriminator mapping. Done means generated Go code derives implicit mappings and emits type-specific unmarshal behavior without requiring the mapping section.