OneOf Type in Streaming Payload Generates Invalid Code

[RedMarcher]

When using a OneOf type within a Payload for a gRPC streaming method (StreamingPayload/StreamingResult), Goa attempts to map the Payload fields to gRPC metadata (headers). However, for OneOf types, the generated decoding logic is incomplete/invalid, resulting in compilation errors.

Steps to Reproduce

1. Define a OneOf Type

var VersionRef = Type("VersionRef", func() {
    Description("Reference to a version - either by UUID or by branch/tag name.")
    OneOf("ref_type", func() {
        Field(1, "version_id", String, "UUID of the specific version", func() {
            Format(FormatUUID)
        })
        Field(2, "ref_name", String, "Name of a branch or tag.")
    })
    Required("ref_type")
})

2. Use it in a Streaming Method Payload

Method("downloadVersion", func() {
    // ...
    Payload(func() {
        // ...
        Field(3, "version_ref", VersionRef, "Version reference")
        // ...
    })
    StreamingResult(VersionStreamPayload)
    GRPC(func() {
        Response(CodeOK)
    })
})

3. Run goa gen
Resulting Error
The generated file
gen/grpc/repository/server/encode_decode.go
contains invalid Go code in DecodeDownloadVersionRequest.

Compiler Output:

gen/grpc/repository/server/encode_decode.go:464:4: declared and not used: versionRefRaw
gen/grpc/repository/server/encode_decode.go:472:9: undefined: VersionRef_VersionID
gen/grpc/repository/server/encode_decode.go:475:9: undefined: VersionRef_RefName
gen/grpc/repository/server/types.go:240:17: cannot use versionRef (variable of type struct{RefType interface{refTypeVal()}}) as *repository.VersionRef value in assignment

Generated Code Snippet
The generator leaves a comment seemingly, but it does not block compilation nor return an error.

// gen/grpc/repository/server/encode_decode.go
if vals := md.Get("version_ref"); len(vals) == 0 {
    err = goa.MergeErrors(err, goa.MissingFieldError("version_ref", "metadata"))
} else {
    versionRefRaw := vals[0]
    // unsupported type VersionRef for var versionRef  <-- GENERATED COMMENT
}