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