Question

Following on from this answer, it seems these constructors:

template<class U, class V> pair(pair<U, V>&& p); 
template<class U, class V> pair(const pair<U, V>& p);

are forbidden from participating in overload resolution when they would require an explicit conversion.

From C++11 (§20.3.2, n3290):

Remark: This constructor shall not participate in overload resolution unless U is implicitly convertible to first_type and V is implicitly convertible to second_type.

An interesting SFINAE workaround has been suggested, but this digresses from the text of the standard.

How can a conforming implementation possibly exclude this from overload resolution, short of some special internal compiler magic? I.e. can an implementation do this and can I duplicate it for my own type perhaps? There doesn’t seem to be anyway of conforming with this! Is it a hangover from the removal of concepts from C++11?

I did wonder about using a private constructor to do the SFINAE part and delegating from the public constructor, but it doesn’t look like constructor delegation participates in SFINAE in such a way as to make that work.

Answer 1

but this digresses from the text of the standard

An implementation is allowed to add default arguments to any non-virtual library member function. This seems to permit precisely this kind of SFINAE tricks.