为什么约束函数允许未定义的行为? [英] Why does a consteval function allow undefined behavior?
问题描述
C ++中的常量表达式具有非常整洁的特性:它们的求值不能具有未定义的行为(
There is a very neat property of constant expressions in C++: their evaluation cannot have undefined behavior (7.7.4.7):
表达式e是核心常量表达式，除非按照抽象机([intro.execution])的规则对e求值将对以下值之一进行求值:
An expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine ([intro.execution]), would evaluate one of the following:
-
...
...
一种操作，该操作将具有本文档[intro]至[cpp]中指定的未定义行为[注意:例如，包括带符号整数溢出([expr.prop])，某些指针算术([ expr.add])，除以零或某些移位操作-尾注];
an operation that would have undefined behavior as specified in [intro] through [cpp] of this document [ Note: including, for example, signed integer overflow ([expr.prop]), certain pointer arithmetic ([expr.add]), division by zero, or certain shift operations — end note ] ;
尝试将13!的值存储在constexpr int实际上产生一个不错的编译错误:
Trying to store the value of 13! in a constexpr int indeed yields a nice compile error:
constexpr int f(int n)
{
int r = n--;
for (; n > 1; --n) r *= n;
return r;
}
int main()
{
constexpr int x = f(13);
return x;
}
输出:
9:19: error: constexpr variable 'x' must be initialized by a constant expression
constexpr int x = f(13);
^ ~~~~~
4:26: note: value 3113510400 is outside the range of representable values of type 'int'
for (; n > 1; --n) r *= n;
^
9:23: note: in call to 'f(3)'
constexpr int x = f(13);
^
1 error generated.
(顺便说一句，为什么错误是调用'f(3)'"，而它却是对f(13)的调用?.)
(BTW why does the error say "call to 'f(3)'", while it is a call to f(13)?..)
然后，我从x中删除constexpr，但是将f设置为consteval.根据文档:
Then, I remove constexpr from x, but make f a consteval. According to the docs:
consteval-指定一个函数为立即函数，也就是说，每次对该函数的调用都必须产生一个编译时常量
consteval - specifies that a function is an immediate function, that is, every call to the function must produce a compile-time constant
我确实希望这样的程序会再次导致编译错误.但是，相反，该程序可以与UB一起编译并运行.
I do expect that such a program would again cause a compile error. But instead, the program compiles and runs with UB.
那是为什么?
UPD::评论者建议这是一个编译器错误.我报告了它: https://bugs.llvm.org/show_bug.cgi?id=43714
UPD: Commenters suggested that this is a compiler bug. I reported it: https://bugs.llvm.org/show_bug.cgi?id=43714
推荐答案
这是一个编译器错误.或者，更确切地说，这是未实现的"解决方案.功能(请参见 bugzilla中的注释):
This is a compiler bug. Or, to be more precise, this is an "underimplemented" feature (see the comment in bugzilla):
(可能已添加了关键字，但未添加实际的实现支持)
(the keyword's probably been added but not the actual implementation support)
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