Why do I keep on getting a syntax error when using a base class typedef indirectly, when it's fine to use it directly?

Question

template<typename T>
struct self
{
    typedef T type;
};

template<class T>
class A
{
    struct Type { int x; };
};

template<class T>
class B1 : A<T>
{
    Type insert();     // OK
};

template<class T>
class B2 : self< A<T> >::type
{
    Type insert();     // syntax error? Why?
};

In conjunction with http://stackoverflow.com/questions/1643035/propagating-typedef-from-based-to-derived-class-for-template. Here's a working sample, the reason explained in the question link: http://ideone.com/Zk5vt — chris, Jul 24 '12 at 04:05
possible duplicate of [Inheritance and templates in C++ - why are methods invisible?](http://stackoverflow.com/questions/1567730/inheritance-and-templates-in-c-why-are-methods-invisible) — iammilind, Jul 24 '12 at 04:14
@chris: I made an edit. That doesn't really seem to explain what's happening. — user541686, Jul 24 '12 at 04:18
@Mehrdad, [ideone](http://ideone.com/4zH9W) gives an error for your OK line, and so does GCC 4.7.1. — chris, Jul 24 '12 at 04:22
@Mehrdad, I honestly wouldn't be surprised. It's clearly stated in the standard why it shouldn't work. — chris, Jul 24 '12 at 04:23
@chris: Ugh, it seems like something so basic that I *am* surprised a compiler would mess it up... although in their defense, I guess compilers don't have an obligation to reject invalid code... :\ — user541686, Jul 24 '12 at 04:24

score 1 · Accepted Answer · edited May 23 '17 at 12:21

Visual C++ has non-standard name lookup rules for templates; it's one of the three non-compliances they document here (third paragraph).

I believe all lookups are deferred until the template is instantiated. At that time, all dependent names are available, and so the usual template and typename keywords and explicit member access are often not necessary; and this compiler doesn't bother to enforce them.

In both of your derived classes, Type is a dependent name and so must (when using a compiler that uses standard two-stage lookup) be qualified with typename:

 typename Type insert();

Why do I keep on getting a syntax error when using a base class typedef indirectly, when it's fine to use it directly?

1 Answers1