A oneOf/anyOf branch that is a whole-file $ref generates Go that doesn't parse #2639

When a oneOf or anyOf branch is a $ref to a whole file ($ref: ./user.yaml, no # fragment), the union's As*/From*/Merge* accessors are named after the branch's inline Go type, so the generated code has methods like AsStruct {\n Name *string ... }() and formatting fails.

Whole-file refs are otherwise generated inline (IsGoTypeReference returns false for them, #308): the same file used directly as a body schema or as a property works, and an inline branch (- {type: object, properties: ...}) in the same union gets its own type, GetItems200JSONResponseBody0.

Reproduction

openapi.yaml:

openapi: 3.0.3
info: {title: t, version: "1"}
paths:
  /items:
    get:
      responses:
        "200":
          description: ok
          content:
            application/json:
              schema:
                oneOf:
                  - $ref: ./user.yaml
                  - $ref: ./group.yaml

user.yaml:

type: object
properties:
  name: {type: string}

group.yaml:

type: object
properties:
  id: {type: integer}

$ oapi-codegen -generate types -package api -o api.gen.go openapi.yaml
error generating code: error formatting Go code at line 53: api.go:53:5: expected declaration, found Name (and 2 more errors)

No configuration file. The unformatted output (with output-options: skip-fmt: true) shows where it breaks:

        // AsStruct {
    Name *string`json:"name,omitempty"`
} returns the union data inside the GetItems200JSONResponseBody as a struct {
    Name *string`json:"name,omitempty"`
}
        func (t GetItems200JSONResponseBody) AsStruct {
    Name *string`json:"name,omitempty"`
}() (struct {

The same happens with anyOf, with only one branch being a whole-file ref, with openapi: 3.1.0, for a property's oneOf, and for a component schema (components.schemas.Item: {oneOf: [$ref: ./user.yaml, $ref: ./group.yaml]}, referenced from the response). The component form fails the same way on v2.4.1 through v2.6.0; the inline body form has failed since v2.7.0, when inline body unions started getting accessors.

Expected

The branches are generated like inline ones: GetItems200JSONResponseBody0 and GetItems200JSONResponseBody1, with AsGetItems200JSONResponseBody0() and so on, and the code compiles. Or, if whole-file refs aren't meant to be supported as union branches, an error that says so.

Versions

v2.8.1 and main (15f5714, v2.8.1-0.20261028140811-15f571472ff8, error at line 56 there), Go 1.27.3, linux/amd64.

Likely cause

generateUnionElement (pkg/codegen/union.go:88 on main) names and defines a branch only when element.Ref == "". A whole-file ref has a non-empty Ref, but generateGoSchema inlined it, so its GoType is the struct literal. Checking !IsGoTypeReference(element.Ref) there instead makes both forms above generate the ...0/...1 types, and the output compiles; I haven't run the test suite on that change.