c++ - constexpr 和函数体 = 删除 : what's the purpose?

Question

根据[dcl.constexpr/3] :

The definition of a constexpr function shall satisfy the following requirements:
[...]
- its function-body shall be = delete, = default, or [...]

这意味着以下类片段是有效的:

struct S {
    constexpr void f() = delete;
};

删除 constexpr 函数的目的是什么？
定义它 constexpr 如果有的话，有什么好处？

我无法找出任何原因，但事实上，在标准中允许它可能比禁止它更容易。

Answer 1

这是基于CWG 1199 。丹尼尔·克鲁格勒写道:

it could be useful to allow this form in a case where a single piece of code is used in multiple configurations, in some of which the function is constexpr and others deleted; having to update all declarations of the function to remove the constexpr specifier is unnecessarily onerous.