[Bug c++/21510] Possible bug

[sven at clio dot in-berlin dot de]

------- Additional Comments From sven at clio dot in-berlin dot de  2005-05-16 10:22 -------
(In reply to comment #7) 
> > Is there a way to distinguish between unions (which are not usable as base  
> > classes) and classes? If not, the standard is incomplete. 
>  
> You should know that 10 years ago people didn't even imagine the kind of 
> template usages that you are assuming as obvious. Indeed, everyone wants 
> to tell unions from classes now and you bet, it will be possible sometime 
> in the future, likely not using SFINAE at all. 
 
I know and accept this. SFINAE is a possible dangerous feature, anyway. It may 
be better if the ISO standard declares something like the following 
(incomplete, feel free to propose it to the standard comitee): 
 
The pragma statements 
 
_Pragme(set, "sfinae", on)   // enables SFINAE 
_Pragma(set, "sfinae", off)  // disables SFINAE 
_Pragma(reset, "sfinae")     // sets SFINAE to the previous state 
 
control the substitution-failure-is-not-an-error-feature. When setting to "on" 
any substitution of a template argument of a class or function declared or 
defined in the context may fail without an error, if 
- the statement is syntactical correct 
- there is a less specific template which could be instantiated without an 
error. 
 
When leaving the context, _Pragma(reset, "sfinae") is called automatical. The 
user is complete responsible for using SFINAE, no further compiler support is 
standardized. 
 
This would localize the feature and alert the user that this is a potential 
dangerous construct. And it is much more general, possible easier to 
implement. 
 

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21510