When generating server code using oapi-codegen for an OpenAPI specification that includes responses with the content type "text/plain", the generated code contains invalid type assertions.

Steps to Reproduce:

1. Create the OpenAPI Specification (spec.yaml):

   openapi: 3.0.3
   info:
     title: Example API
     version: 1.0.0
   
   paths:
     /ping:
       get:
         responses:
           '200':
             $ref: "#/components/responses/200_status_str"
           '201':
             $ref: "#/components/responses/201_status_int"
   
   components:
     responses:
       201_status_int:
         description: status
         content:
           text/plain:
             schema:
               type: integer
       200_status_str:
         description: status
         content:
           text/plain:
             schema:
               type: string

2. Create the Configuration File (oapi-cfg.yaml):

   # yaml-language-server: $schema=https://raw.githubusercontent.com/deepmap/oapi-codegen/HEAD/configuration-schema.json
   package: main
   output: api.gen.go
   generate:
     models: true
     gorilla-server: true
     strict-server: true
   output-options:
     skip-prune: true

3. Create main.go:

   package main
   
   import (
   	"context"
   	"fmt"
   	"net/http"
   	"strconv"
   
   	"github.com/gorilla/mux"
   )
   
   type StrictHandler struct{}
   
   func (w StrictHandler) GetPing(ctx context.Context, request GetPingRequestObject) (GetPingResponseObject, error) {
   	return GetPing200TextResponse{
   		N200StatusStrTextResponse(strconv.Itoa(200)),
   	}, nil
   }
   
   func main() {
   	var myHandler StrictHandler
   
   	router := HandlerWithOptions(NewStrictHandler(myHandler, nil), GorillaServerOptions{}).(*mux.Router)
   
   	fmt.Println("Starting server at port 8080")
   	if err := http.ListenAndServe(":8080", router); err != nil {
   		fmt.Println(err)
   	}
   }

4. Create gen.go:

   package main
   
   //go:generate oapi-codegen --config=oapi-cfg.yaml spec.yaml

Expected Behavior:

The generated code should correctly use GetPing200TextResponse struct:

func (response GetPing200TextResponse) VisitGetPingResponse(w http.ResponseWriter) error {
    w.Header().Set("Content-Type", "text/plain")
    w.WriteHeader(200)

    _, err := w.Write([]byte(response.N200StatusStrTextResponse)) // Correct line
    return err
}

Actual Behavior:

The generated code contains the following invalid line:

func (response GetPing200TextResponse) VisitGetPingResponse(w http.ResponseWriter) error {
    w.Header().Set("Content-Type", "text/plain")
    w.WriteHeader(200)

    _, err := w.Write([]byte(response)) // Invalid line
    return err
}

Handling other types

Generated code for struct GetPing201TextResponse looks the same:

type N201StatusIntTextResponse int

type GetPing201TextResponse struct{ N201StatusIntTextResponse }

func (response GetPing201TextResponse) VisitGetPingResponse(w http.ResponseWriter) error {
	w.Header().Set("Content-Type", "text/plain")
	w.WriteHeader(201)

	_, err := w.Write([]byte(response)) // Invalid line
	return err
}

I expected to see something like this:

_, err := w.Write([]byte(strconv.Itoa(int(response.N201StatusIntTextResponse))))

Additional Information:

• Version of oapi-codegen: v2.4.1
• Go Version: 1.22.8
• Dependencies:
  • github.com/gorilla/mux v1.8.1
  • github.com/oapi-codegen/runtime v1.1.1