c++ - constexpr void 函数被拒绝

Question

我有一个非常简单但无法编译的函数。

constexpr void func()
{
}

我得到的错误是:

error: invalid return type 'void' of constexpr function 'constexpr void func()'     constexpr void func()

在 C++14 中，void 是文字类型 [§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.

有人可以解释为什么这是无效的吗？

Answer 1

使 void 成为文字类型的提案是 n3652 Relaxing constraints on constexpr functions . G++ 决定将此功能推送到版本 5 (我使用的是 4.9.2):

G++ now supports C++14 extended constexpr.

constexpr int f (int i)
{
  int j = 0;
  for (; i > 0; --i)
    ++j;
  return j;
}

constexpr int i = f(42); // i is 42

Clang 从 3.4 版开始实现了这一点。