Daniel Walker wrote:
Is there any other option? I mean if t<x> is ill-formed, because the type x doesn't support some expression used in the template t, then is there some context where t could be instantiated with x without making the whole program ill-formed? I can't think of one.
SFINAE for expressions, available in C++0x (GCC 4.4 implements it, albeit only with sizeof and not with decltype).
If there were such a context, then you might be able associate t<x>'s ill-formation with some boolean constant, and you would have a way to implement tests based on concept checking templates - a sort of concept-based type introspection for C++03, which would be way cool.
That thing is mostly already doable in C++03. See this, for example: <http://neoscientists.org/~tschwinger/boostdev/concept_traits/libs/concept_traits/doc/>
This brings up an interesting question regarding the proposed concept language extensions for C++0x: If a function in an overload set requires a concept that its argument doesn't model but some other function in the set accepts the argument, does overload resolution succeed?
Of course.