Non-existing identifier used as default argument in non-instantiated function template

Question

Today I found a piece of code equivalent to the following:

enum X
{
    x1 = 0,
    x2 = 1
};

template<typename T>
void bar(T obj, X x = x3) { }
//                    ^^
//                    This identifier is bogus!

int main() { }

Both VC10 and VC12 happily compile it. Both clang 3.4 and GCC 4.8.1 reject it (which is what I would expect).

Is this a bug, or is VC's behavior actually allowed by the Standard? If so, which are the relevant paragraphs?

Answer 1

It's a well-known fact that VC doesn't have two-phase lookup. That means it accepts all sorts of bogosity in templates, as long as it at least looks like syntactically valid C++ and it is not actually instantiated.

This is just one more of those instances.

As you can see in their conformance roadmap, two-phase lookup is planned for sometime after the post-RTM CTP, which I guess would mean you will have access to it after you pay for the next iteration of the Visual Studio suite.

As for the Standard references, 14.6/9-10 say:

When looking for the declaration of a name used in a template definition, the usual lookup rules (3.4.1, 3.4.2) are used for non-dependent names. [...]

If a name does not depend on a template-parameter (as defined in 14.6.2), a declaration (or set of declarations) for that name shall be in scope at the point where the name appears in the template definition.