std::array arr{ 1, 2, 3, 4, 5 }; // with C++17 template argument deduction
std::array<int, 5> arr{ 1, 2, 3, 4, 5 }; // just a normal C++11 std::array

std::array arr{ 1, 2, 3.f, 4, 5 };

main.cpp:7:26: error: class template argument deduction failed:
     std::array arr{2,4.,5}; // will not compile

std::array<float> arr{ 1, 2, 3.f, 4, 5 }; // will not compile as well

main.cpp:7:21: error: wrong number of template arguments (1, should be 2)
     std::array<float> arr{2,4.,5};
                    ^

std::tuple t(1, 2, 3);              // OK: deduction
std::tuple<int,int,int> t(1, 2, 3); // OK: all arguments are provided
std::tuple<int> t(1, 2, 3);         // Error: partial deduction