no error when type enum is an interface without a function

[matt2e]

Steps to repro:

• Define an ftl type enum, but with an empty interface

Expected:

• This should be an error on the enum, and not produce errors everywhere else

What actually happens:

• Errors appear every else because everything is now an enum variant because it conforms to the interface

//ftl:enum
type OpenEnum interface{}

// innocent bystander
type DAL struct {
  db *sql.DB
}

error:

unsupported type "*database/sql.DB" for field "db"

[alecthomas]

😂

[worstell]

we should also validate that the interface function is unexported, i don't think we're doing that right now