c++ - Why passing constexpr object by const reference works, but by value doesn't compile -

i have code below, maps std::integer_sequence<> std::array<> @ compile time:

#include <iostream> #include <utility> #include <array>  template<int...is> constexpr auto make_array(const std::integer_sequence<int, is...>& param) // works */ // constexpr auto make_array(std::integer_sequence<int, is...> param) // doesn't compile {     return std::array<int, sizeof...(is)> {is...}; }  int main() {     constexpr std::integer_sequence<int, 1,2,3,4> iseq;      // if pass value, error: value of 'iseq' not usable in constant expression     constexpr auto arr = make_array(iseq);        for(auto elem: arr)         std::cout << elem << " "; } 

the code works fine whenever make_array takes argument const-reference. whenever try passing value, in commented line, spits error:

error: value of 'iseq' not usable in constant expression

    constexpr auto arr = make_array(iseq);   

why this? parameter iseq constant expression, why cannot pass make_array?

for example, code below works expected when passing value:

#include <iostream> #include <utility>  struct foo {     int _m;     constexpr foo(int m): _m(m){}; };  constexpr foo factory_foo(int m) {     return foo{m}; }  constexpr foo copy_foo(foo foo) {     return foo; }  int main() {     constexpr foo cxfoo = factory_foo(42);     constexpr foo cpfoo = copy_foo(cxfoo); } 

edit

i'm using g++5.1 macports. using clang++ 3.5, error message code compiles g++ (with const reference):

error: default initialization of object of const type 'const std::integer_sequence' requires user-provided default constructor

so guess there issue lack of user-provided default constructor, @ point don't understand what's going on.

you missing initializer on iseq. have add it:

constexpr std::integer_sequence<int, 1,2,3,4> iseq{};                                                   ^^ 

from [dcl.constexpr]:

a constexpr specifier used in object declaration declares object const. such object shall have literal type , shall initialized. if initialized constructor call, call shall constant expression (5.20). otherwise, or if constexpr specifier used in reference declaration, every fullexpression appears in initializer shall constant expression. [ note: each implicit conversion used in converting initializer expressions , each constructor call used initialization part of such full-expression. —end note ]
[ example:

struct pixel {     int x, y; }; constexpr pixel ur = { 1294, 1024 };  // ok constexpr pixel origin;               // error: initializer missing 

—end example ]

moreover, columbo suggests in comment , answer, being initialized insufficent. user-provided constructor required, per [dcl.init]:

if program calls default initialization of object of const-qualified type t, t shall class type user-provided default constructor.

it's little odd have relevant section (dcl.constexpr) have incomplete description of requirements constepxr object declaration.