Why does the C++ compiler makes it possible to declare a function as constexpr, which can not be constexpr?

Question

Why does the C++ compiler makes it possible to declare a function as constexpr, which can not be constexpr?

For example: http://melpon.org/wandbox/permlink/AGwniRNRb

Answer 1

Why does the C++ compiler makes it possible to declare a function as constexpr, which can not be constexpr?

It doesn't. But you are not defining a function constexpr. You are defining a template.

Let's set up:

struct Is_constexpr {
  constexpr Is_constexpr() = default;
  constexpr auto bar() {
    return 24;
  }
};

struct Not_constexpr {
  auto bar() {
    return 24;
  }
};

Now if you try to define a function (not a template) as constexpr that uses Not_constexpr the compiler won't let you:

constexpr auto foo_function(Not_constexpr v)
{
  return v.bar();
  // error: call to non-constexpr function 'auto Not_constexpr::bar()'
}

You are however defining a template. Let's see how this goes:

template 
constexpr auto foo(T v)
{
  return v.bar();
}

The compiler lets you do this. No error. Why? Because it's a template. Some instantiations may be constexpr, some not, depending on T:

int main() {
  constexpr Is_constexpr is_c;
  constexpr Not_constexpr not_c;

  std::integral_constant a; // OK
  //std::integral_constant a; // ERROR

}