Overload resolution behaviour difference between GCC and clang (SFINAE)

Question

GCC accepts the following code:

template 
struct meta
{
    typedef typename T::type type;
};

struct S {};

template 
ty         


        

Answer 1

C++03 uses this wording as part of the specification for what is usually referred to as SFINAE (14.8.2 Template argument deduction [temp.deduct], paragraph 2):

[...] If a substitution in a template parameter or in the function type of the function template results in an invalid type, type deduction fails. [...]

By contrast, C++11 uses this wording (14.8.2 Template argument deduction [temp.deduct], paragraph 8):

If a substitution results in an invalid type or expression, type deduction fails. [...] Only invalid types and expressions in the immediate context of the function type and its template parameter types can result in a deduction failure. [...]

Emphasis is mine. As I understand it the wording was improved in C++11 to unambiguously outline what should result in SFINAE (so-called soft errors) and what shouldn't (hard errors). This 2008 paper is an example of the discussion that was going on at the time and led to the current rules.

With this in mind it may be the case that according to C++03 an implementation may be right to accept your code (and even perhaps it should). I suspect that a C++11 implementation should reject it however: the error (int::type) is in the context of meta, not of foo.