[v2] Empty object body schema generated when using @Accept

[kreed]

Summary

When using @Accept json with @Param body in swaggo v2.0.0-rc5, the generated OpenAPI 3.1 spec incorrectly wraps the request body schema in a oneOf with an empty object schema.

Reproduction

Code

package main

// @title Bug Repro API
// @version 1.0
// @BasePath /api

type CreateRequest struct {
	Name  string `json:"name"`
	State string `json:"state"`
}

type Response struct {
	ID int `json:"id"`
}

// CreateThing creates a thing
// @Summary Create thing
// @Accept json
// @Produce json
// @Param body body CreateRequest true "Request"
// @Success 200 {object} Response
// @Router /things [post]
func CreateThing() {}

func main() {}

Generate with rc5

go run github.com/swaggo/swag/v2/cmd/swag@v2.0.0-rc5 init --v3.1 -g main.go -o docs

Actual Output (rc5 - INCORRECT)

paths:
  /things:
    post:
      requestBody:
        content:
          application/json:
            schema:
              oneOf:
              - type: object
              - $ref: '#/components/schemas/main.CreateRequest'
                description: Request
                summary: body
        description: Request
        required: true

Expected Output (rc4 - CORRECT)

paths:
  /things:
    post:
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/main.CreateRequest'
        description: Request
        required: true

Root Cause

In operationv3.go, when @Accept json is processed, it initializes the media type with an empty schema:

// In fillRequestBody or similar
mediaType := spec.NewMediaType()
// This creates mediaType.Spec.Schema with an empty object
o.RequestBody.Spec.Spec.Content["application/json"] = mediaType

Later, when @Param body is processed, it checks if mediaType.Spec.Schema != nil. Since the schema was already initialized (albeit empty), it creates a oneOf to combine the existing empty schema with the body param schema.

Workaround

Remove @Accept json annotations from handlers that have @Param body. When @Param body is present without @Accept, swaggo defaults to application/json content type, so this doesn't change the generated API spec.

Impact

This breaks code generators like ogen that expect a direct $ref to the schema, not a oneOf with an empty object.

[theqeeqez]

Having the same issue

[kreed]

#2086 looks like the same issue. Sorry for the duplicate.