c – 据我所知,下面的函数不是constexpr,但是代码在clang和g中编译.我失踪了什么

constexpr int incr(int &n) {
    return ++n;
}

在C14中,constexpr函数的规则放松了,文章 N3597: Relaxing constraints on constexpr functions.本文介绍了理论和效果,包括以下(重点是)：

As in C++11,the constexpr keyword is used to mark functions which the implementation is required to evaluate during translation,if they are used from a context where a constant expression is required. Any valid C++ code is permitted in constexpr functions,including the creation and modification of local variables,and almost all statements,with the restriction that it must be possible for a constexpr function to be used from within a constant expression. A constant expression may still have side-effects which are local to the evaluation and its result.

和：

A handful of syntactic restrictions on constexpr functions are
retained:

• asm-declarations are not permitted.
• try-blocks and function-try-blocks are not permitted.
• Declarations of variables with static and thread storage duration have some restrictions (see below).

我们可以在N4140第7.1.5节[dcl.constexpr]中找到这个内容：

The deFinition of a constexpr function shall satisfy the following constraints:

• it shall not be virtual (10.3);

• its return type shall be a literal type;

• each of its parameter 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.

最后一个例子显示了如何在constexpr中使用incr：

constexpr int h(int k) {
  int x = incr(k); // OK: incr(k) is not required to be a core
                   // constant expression
  return x;
}

constexpr int y = h(1); // OK: initializes y with the value 2
                        // h(1) is a core constant expression because
                        // the lifetime of k begins inside h(1)

而涵盖k的寿命的规则从h(1)开始就是：

• modification of an object (5.17,5.2.6,5.3.2) unless it is applied to a non-volatile lvalue of literal type
  that refers to a non-volatile object whose lifetime began within the evaluation of e;

7.1.5 [dcl.constexpr]中的措词告诉我们为什么incr是一个有效的constexpr：

For a non-template,non-defaulted constexpr function or a non-template,non-defaulted,non-inheriting
constexpr constructor,if no argument values exist such that an invocation of the function or constructor
Could be an evaluated subexpression of a core constant expression (5.19),the program is ill-formed; no
diagnostic required.

作为T.C的修改示例：

constexpr int& as_lvalue(int&& i){ return i; }

constexpr int x = incr(as_lvalue(1)) ;

显示,我们确实可以使用incr作为核心常数表达式的子表达式,因此它不是不成形的.