Why is constexpr required even though member function is constexpr?

c++, c++11, constexpr, initializer-list

Solution

std::initializer_list<int> il = rand() ? std::initializer_list<int>{1}
                                       : std::initializer_list<int>{1,2,3};

std::array<int, il.size()> a;

That's why.

A `constexpr` member function is a function that can be executed within a constant expression, it doesn't necessarily yield a result that is a compile-time constant. For example:

struct S
{
    int m;
    constexpr int foo() const { return m; }
};

S s{rand()};
int j = s.foo();     // only known at run-time

constexpr S cs{42};
int arr[cs.foo()];   // compile-time constant

Problem

The following does not compile unless I put constexpr before initializer_list: ``` constexpr std::initializer_list<int> il = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }; std::array<int, il.size()> a; ``` But initializer_list size is constexpr: ``` constexpr size_type size() const; ```

Original source