A oneOf/anyOf branch that is an array of an inline enum gets the same type name as its items #2640

When a union branch is an array whose items are an inline enum, the branch's type and the items' enum type get the same name (...Roles1). In a request body or a parameter, generation succeeds and the output doesn't compile. In a component schema, generation fails with "duplicate typename", and the suggested x-go-name doesn't apply, since both names are generated.

"A string or a list of strings" (oneOf: [string, array of string]) is a common shape; adding an enum to the items is enough to trigger this.

Reproduction

openapi.yaml:

openapi: 3.0.3
info: {title: t, version: "1"}
paths:
  /items:
    post:
      requestBody:
        content:
          application/json:
            schema:
              type: object
              properties:
                roles:
                  oneOf:
                    - {type: string}
                    - {type: array, items: {type: string, enum: [admin, viewer]}}
      responses:
        "204": {description: ok}

$ oapi-codegen -generate types -package api -o api.gen.go openapi.yaml
$ go build ./...
./api.gen.go:21:6: invalid recursive type: PostItemsJSONBodyRoles1 refers to itself
./api.gen.go:24:6: PostItemsJSONBodyRoles1 redeclared in this block
	./api.gen.go:21:6: other declaration of PostItemsJSONBodyRoles1

No configuration file; the module requires github.com/oapi-codegen/runtime v1.8.0. The generated code:

// PostItemsJSONBodyRoles1 defines parameters for PostItems.
type PostItemsJSONBodyRoles1 = []PostItemsJSONBodyRoles1

// PostItemsJSONBodyRoles1 defines parameters for PostItems.
type PostItemsJSONBodyRoles1 string

On main the first one is type PostItemsJSONBodyRoles1 []PostItemsJSONBodyRoles1 and only the "redeclared" error remains.

The same roles property as a query parameter's schema gives GetItemsParamsRoles1 redeclared. Under components.schemas.Item (referenced from a response) generation fails instead:

error generating code: error generating code for type definitions: duplicate typename 'ItemRoles1' detected, can't auto-rename, please use x-go-name to specify your own name for one of them

It also happens with a single-branch oneOf, with anyOf, with an enum on the string branch too, and with resolve-type-name-collisions: true. It reproduces on v2.4.1 as well. Setting x-go-type-name on the items (items: {type: string, enum: [admin, viewer], x-go-type-name: Role}) works around it.

Expected

Two distinct types, for example PostItemsJSONBodyRoles1 []PostItemsJSONBodyRoles1Item and PostItemsJSONBodyRoles1Item string, and code that compiles.

Versions

v2.8.1 and main (15f5714, v2.8.1-0.20261028140811-15f571472ff8), Go 1.27.3, linux/amd64.

Likely cause

An array's items are generated at the array's own path (generateGoSchema(ctx.at(itemHint), schema.Items, path), pkg/codegen/schema.go:1729 on main), and an inline enum is named from its path (schema.go:1693), so the items' enum takes the array's name. A plain array property never gets a type of its own, so this doesn't show there. But generateUnionElement (pkg/codegen/union.go:89) names the array branch from that same path. #1878 had a similar shape: an array with an enum of its own and an enum on its items, both called ApprovalConfigurationExclude.