如果是非布尔条件，则为Constexpr [英] Constexpr if with a non-bool condition

问题描述

我似乎找到了Clang和GCC不一致的东西.这是代码:

I seem to have found something that Clang and GCC disagree on. Here's the code:

int main() {
  if constexpr (2) {}
}

这可以在GCC 7.4.0上成功编译，但是在Clang 7.0.0上却失败，并显示以下错误消息:

This successfully compiles with GCC 7.4.0, but it fails with Clang 7.0.0 with this error message:

test.cpp:3:17: error: constexpr if condition evaluates to 2, which cannot be narrowed to type 'bool'
      [-Wc++11-narrowing]
  if constexpr (2) {}
                ^
1 error generated.

cppreference 似乎并没有提到缩小"像Clang错误一样，但我不确定.如果这是任何一个编译器的错误，是否已经报告过?

cppreference doesn't seem to mention "narrowing", so this seems like a Clang bug, but I'm not entirely certain. If this is a bug with either compiler, has it already been reported?

推荐答案

Clang正在根据以下段落进行诊断

Clang is diagnosing under these paragraphs

[stmt.if] (强调我的意思)

[stmt.if] (emphasis mine)

2 如果if语句的形式为if constexpr，的值 条件应为的上下文转换常量表达式 输入bool ；这种形式称为constexpr if语句.

2 If the if statement is of the form if constexpr, the value of the condition shall be a contextually converted constant expression of type bool; this form is called a constexpr if statement.

[expr.const]

4 类型T的转换常量表达式是一个表达， 隐式转换为T类型，其中转换后的表达式为 常量表达式，并且隐式转换序列仅包含

4 A converted constant expression of type T is an expression, implicitly converted to type T, where the converted expression is a constant expression and the implicit conversion sequence contains only

• 积分转化，而不是缩小转化率，

现在，当涉及整数转换时，将转换为 bool

Now, when it comes to integral conversions, a conversion to bool is listed as an integral conversion. And it is narrowing, in the strictest sense of the word, since a bool cannot represent all the values of an int. So the diagnostic is not without grounds.

但是，我认为考虑转换为bool通常是为了检查真实性"这一事实也是很合理的，因此，其狭义性质并不重要.它看起来像是标准 ¹ 中的一个小错误，GCC采用了常识性路线，而Clang在最严格的意义上坚持了法律的枯燥乏味.

But I think it's also quite reasonable to consider the fact a conversion to bool is usually intended to check for "truthiness", and so the narrowing nature of it shouldn't matter. It looks like a minor bug in the standard¹, with GCC taking the common-sense route, and Clang adhering to the dry letter of the law in the strictest sense.

_{¹ -和}存在一项更改提案的提案.

_{¹ - And a proposal exists to change it.}

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