Hello,

as the title says, qHash fails to work with nested std::pair. Given this code:

#include <QHash>
#include <utility>
using namespace std;
int main()
{
  auto p{make_pair(1, make_pair(2, 3))};
  auto q{qMakePair(1, qMakePair(2, 3))};
  ::qHash(p);
  return 0;
}

I get an error on qHash(p) saying there is no candidate for qHash(T) with T=std::pair<int, int>, ie. the "inner" pair. However, qHash(q) is without problems.

Cause

The reason for this is that name lookup during the instantiation of qHash(const std::pair<T1, T2>&, uint) can't find the same function, either in the noexcept clause or in QtPrivate::QHashCombine::operator()(..., std::pair<A, std::pair<B, C>>) which comes earlier in the header file and is needed for instantiation of qHash(p). This is because normal lookup only sees declarations before the current declaration and the qHash<std::pair> overload comes after QtPrivate::QHashCombine. And Koenig lookup can't find that overload, because it is i Qt's (or global) namespace, but it would need to be in std namespace.

Solution

The best solution would be to move qHash(const std::pair<T1, T2> &key, uint seed = 0) out of QT_BEGIN_NAMESPACE into the std namespace. This allows it to work without problems using Koenig lookup.

Note that if future Qt versions use std::pair as QPair, this problem should be considered for them as well.