can have definition variable of non-literal type in constexpr function body c++14?

Question

I think that in C++14 more restrictives are removed from constexpr. But according N3797 7.1.5 3-punct:

---

The definition of a contexpr function shall satisfy the following constraints:

• it shall not be virtual
• its return type shall be a literal type;
• each of its parameters types shall be a literal type;
• its function-body shall be = delete, = default, or a compound-statement that does not contain:
• an asm-definition,
• a goto statement,
• a try-block, or
• a definition of a variable of non-literal type or of static or thread storage duration or for which no initialization is performed.

---

I know why static, thread storage duration variables are disallowed, but I don't see any reason, why ONLY allowed definition of a variable of literal type ?

Or I'm not understand the standard.

I'm not sure, but according standard following errors should be created even C++14:

struct point{
constexpr point(): x(0), y(0){}
constexpr point(int x_, int y_): x(x_),y(y_){}
constexpr int hypot()const { return x*x + y*y; }
int x,y;
};

constexpr int hypot(int x, int y) {  
   point p{x,y}; //error, because p - is not literal type.
   return p.hypot();
}

// error, because return type is not literal.
constexpr point  getPoint(int x, int y) { return {x,y}; }

// error, because parameter is not literal.
constexpr int hypot(point p) { return p.hypot(); }

Q: If really above errors would happen, why these restrictions are not removed?

Answer 1

literal type is defined in 3.9/10:

A type is a literal type if it is:

• void; or

• a scalar type; or

• a reference type; or

• an array of literal type; or

• a class type (Clause 9) that has all of the following properties:

  • it has a trivial destructor,

  • it is an aggregate type (8.5.1) or has at least one constexpr constructor or constructor template that is not a copy or move constructor, and

  • all of its non-static data members and base classes are of non-volatile literal types

So your struct point is a literal type and your sample code is valid C++1y.

As to why constexpr functions are restricted to variables of literal type, they are the only types that are guaranteed to be interpretable at compile time.