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template aliases and sfinae

In the case of a substitution failure involving a template alias (e.g. a template alias on a missing member typename, as in the code snippet below), should an error be triggered ?

Clang and gcc seem to disagree on this:

// some types
struct bar { };

struct foo {
    typedef void member_type;
};


// template alias
template<class T>
using member = typename T::member_type;


template<class T>
void baz(... ) { }

// only works for gcc, clang fails with: no type named 'member_type'
// in 'bar'
template<class T>
void baz( member<T>* ) { }


int main(int, char** ) {

    baz<bar>(0);            // picks first
    baz<foo>(0);            // picks second

    return 0;
}

So the question is: who is correct, and why ?

Thanks :-)

like image 1000
max Avatar asked Dec 05 '12 16:12

max


1 Answers

According to the Standards, it is clearly GCC that is correct, because the alias template must immediately be replaced and then normal/usual SFINAE is applied to typename T::member_type later when T is known.

But there currently is an issue for this, see http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1554.

According to the meetings outcome, it appears that clangs behavior is desired: The substitution of T will be done in the context of the alias template (even though at the time of substitution in typename T::member_type, there is no reference to the alias template anymore - it will still need to be referenced as the source of where the parameter type pattern originated from, if this is how it's implemented).


This is similar to another situation where patterns are thrown away on definition time that could influence instantiation semantics

template<int I>
void f(int x[I]);

int main() {
  f<0>(nullptr);
}

In this case too, in my opinion the Standard normatively is clear that the parameter is immediately being replaced by int* and thus the instantiation works. See http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1322 .

like image 114
Johannes Schaub - litb Avatar answered Sep 30 '22 20:09

Johannes Schaub - litb