alias fails to compile

[Arun Chandrasekaran]

On Monday, 22 April 2019 at 14:07:11 UTC, Alex wrote:
> On Monday, 22 April 2019 at 08:02:06 UTC, Arun Chandrasekaran 
> wrote:
>> What am I doing wrong here?
>>
>> struct A
>> {
>>     union B
>>     {
>>         int bb;
>>     }
>>     B b;
>>     alias aa = b.bb;
>> }
>>
>> void main()
>> {
>>     A a = A();
>>     // a.b.bb = 4; // works
>>     a.aa = 4; // fails
>> }
>>
>>
>> https://run.dlang.io/is/kXaVy2
>
> aa is a "static" semantic.
>
> your statement a.aa does not get translated in to a.b.bb like 
> you think it does.
>
> You are doing nothing any different than A.aa = 4.
>
> which, when you realize this you'll understand the "need this" 
> error.
>
>
> It's as if you are doing
>
>> struct A
>> {
>>     union B
>>     {
>>         int bb;
>>     }
>>     B b;
>> }
>      alias aa = A.b.bb;
>
> aa = 4;
>
> which is like trying to say
>
> A.b.bb = 4;
>
>
> In D we can access "static" stuff using the object and so this 
> can make things look like the mean something they are not.
>
> If you could magically pass the this to it using something like 
> UFCS then it could work.
>
> import std.stdio;
> struct A
> {
>     union B
>     {
>         int bb;
>     }
>     B b;
>     alias aa = (ref typeof(this) a) { return &a.b; };
> }
>
> void main()
> {
>     A a = A();
>     a.b.bb = 4;
>     a.aa(a).bb = 5;
>     writeln(a.aa(a).bb);
> }
>
> But here a.aa is not aa(a) and UFCS does not work so one must 
> first access the alias and then pass the this.
>
> Basically you are not going to get it to work. Just not how the 
> semantics works.
>
> It would be nice if a.aa(a) could reduce to a.aa as is UFCS was 
> smart enough to realize it could pass it to the alias as this, 
> but it doesn't.
>
> else one might could do
>
> alias aa = this.b.bb;
>
> and
>
> a.aa = 4;

I see. This is a simplified use case in /usr/include/net/if.h on 
Linux, where struct ifreq has something like this:

struct ifreq
   {
# define IFHWADDRLEN    6
# define IFNAMSIZ   IF_NAMESIZE
     union
       {
     char ifrn_name[IFNAMSIZ];   /* Interface name, e.g. "en0".  */
       } ifr_ifrn;

     union
       {
     struct sockaddr ifru_addr;
     struct sockaddr ifru_dstaddr;
     struct sockaddr ifru_broadaddr;
     struct sockaddr ifru_netmask;
     struct sockaddr ifru_hwaddr;
     short int ifru_flags;
     int ifru_ivalue;
     int ifru_mtu;
     struct ifmap ifru_map;
     char ifru_slave[IFNAMSIZ];  /* Just fits the size */
     char ifru_newname[IFNAMSIZ];
     __caddr_t ifru_data;
       } ifr_ifru;
   };
# define ifr_name   ifr_ifrn.ifrn_name  /* interface name   */
# define ifr_hwaddr ifr_ifru.ifru_hwaddr    /* MAC address      */
# define ifr_addr   ifr_ifru.ifru_addr  /* address      */
# define ifr_dstaddr    ifr_ifru.ifru_dstaddr   /* other end of 
p-p lnk */
# define ifr_broadaddr  ifr_ifru.ifru_broadaddr /* broadcast 
address    */
# define ifr_netmask    ifr_ifru.ifru_netmask   /* interface net 
mask   */
# define ifr_flags  ifr_ifru.ifru_flags /* flags        */
# define ifr_metric ifr_ifru.ifru_ivalue    /* metric       */
# define ifr_mtu    ifr_ifru.ifru_mtu   /* mtu          */
# define ifr_map    ifr_ifru.ifru_map   /* device map       */
# define ifr_slave  ifr_ifru.ifru_slave /* slave device     */
# define ifr_data   ifr_ifru.ifru_data  /* for use by interface */
# define ifr_ifindex    ifr_ifru.ifru_ivalue    /* interface 
index      */
# define ifr_bandwidth  ifr_ifru.ifru_ivalue    /* link bandwidth 
   */
# define ifr_qlen   ifr_ifru.ifru_ivalue    /* queue length     */
# define ifr_newname    ifr_ifru.ifru_newname   /* New name     */
# define _IOT_ifreq _IOT(_IOTS(char),IFNAMSIZ,_IOTS(char),16,0,0)
# define _IOT_ifreq_short 
_IOT(_IOTS(char),IFNAMSIZ,_IOTS(short),1,0,0)
# define _IOT_ifreq_int 
_IOT(_IOTS(char),IFNAMSIZ,_IOTS(int),1,0,0)


Macro magic has been doing the job and luckily there is no 
ifr_name, etc defined in the user code. Looks like there is no 
way but to name the elements of this struct as ifr_name, etc?