https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86852
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to Michael Veksler from comment #0) > According to > https://en.cppreference.com/w/cpp/container/unordered_map/deduction_guides > > Deduction guides for unodrered_map: > template<class Key, class T, class Hash = std::hash<Key>, > class Pred = std::equal_to<Key>, > class Alloc = std::allocator<std::pair<const Key, T>>> > unordered_map(std::initializer_list<std::pair<const Key, T>>, > typename /*see below*/::size_type = /*see below*/, > Hash = Hash(), Pred = Pred(), Alloc = Alloc()) > -> unordered_map<Key, T, Hash, Pred, Alloc>; > > > Note that the guide is for std::pair<const Key, T>, i.e., the key is const. > In libstdc++'s unodered_map: > > unordered_map(initializer_list<pair<_Key, _Tp>>, > typename unordered_map<int, int>::size_type = {}, > _Hash = _Hash(), _Pred = _Pred(), _Allocator = > _Allocator()) > -> unordered_map<_Key, _Tp, _Hash, _Pred, _Allocator>; > > > Note that pair<_Key, _Tp>, i.e., the key is not const. > > This breaks unordered_map and map deduction guides: > #include <unordered_map> > #include <initializer_list> > int main() { > std::unordered_map m1(std::initializer_list< > std::pair<const int, int>>({{1, 2}, {2, 3}})); > } This is a ridiculous example, and I see no advantage to supporting it. Ideally it would work as: std::unordered_map m{{1,2}, {3,4}}; Having to write std::initializer_list<std::pair<const int, int>> is ridiculous, and giving the std::pair type explicitly isn't much better. You might as well just avoid argument deduction entirely at that point: std::unordered_map<int, int> m{{1,2}, {3,4}}; The point of class template argument deduction is to simplify code, not so you can make it far more verbose. > The only way to make this work in gcc is > std::unordered_map m{std::pair<int, int>{1,2}, {3,4}}; You can deduce the pair type: std::unordered_map m{std::pair{1,2}, {3,4}};
