http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51336
--- Comment #3 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2011-11-29 07:08:45 UTC --- (In reply to comment #1) > When you instantiate A<double>, the declaration of the template constructor > is > also instantiated, but at that point A<double> is an incomplete type. I just recognize that the last part of this is a bit misleading, please read this as: "When you instantiate A<double>, the declaration of the template constructor is also instantiated, but this turns out to be an ill-formed signature" The argument in regard to type-completeness was intended to point to the library specification, which requires A<double> to be complete, which means that A<double> may be instantiated including it's member declarations, which again is the cause of the error, because we have no sfinae situation here. In regard to the __is_abstract builtin question it seems that in simple cases like these (no dependent base classes involved), it may be possible to implement the intrinsic without instantiating A<double>, but maybe the current behavior is actually better, because it makes portable programming easier;-)
