clang vs gcc:可变参数lambda捕获 [英] clang vs gcc: variadic lambda captures

问题描述

我试图在内部lambda中捕获一个可变参数的lambda参数，并在其中使用它. 例如，请考虑以下代码:

I am trying to capture a variadic lambda argument inside a inner lambda and use it there. As an example, consider this code:

int main () {
    auto first = [&] (auto&&... one) {
        auto second = [&] (auto&&... two) {
            return ((one * two) + ...);
        };
        return second(one...);
    };
    return first(5);
}

这适用于gcc9，但不适用于clang8( https://godbolt.org/z/i2K9cK ).

This works with gcc9 but fails with clang8 (https://godbolt.org/z/i2K9cK).

使代码编译的一种方法是显式捕获[&one...]，但是我想知道这是否是clang中的错误.

A way to make the code compile is to explicitly capture [&one...], but i was wondering whether this is a bug in clang.

也很有趣:将return语句更改为直接扩展one的内容(与two组合在一起)，然后再次编译: return (((one * ...) * two) + ...);

Also interesting: Changing the return statement to something where one is directly expanded (before combining with two), this compiles again: return (((one * ...) * two) + ...);

我发现了与此相关的帖子，但是声明的错误似乎已在clang8中修复.

I have found this related post, but the bug declared there seems to be fixed in clang8.

推荐答案

这是Clang中的错误.已已报告.根据评论:

This is a bug in Clang. It has been reported. Per comment:

已在r362358中修复.

Fixed in r362358.

(附带说明:Clang似乎在捕获中通常难以扩展包.让我们推出为通用lambda first生成的闭包类型的自己版本:

(Side note: Clang seems to have difficulty with pack-expansions in captures in general. Let's roll out our own version of the closure type generated for the generic lambda first:

struct __closure_first {
    template <typename... Args>
    auto operator()(Args&&... one) const
    {
        auto second = [&] (auto&&... two) {
            return ((one * two) + ...);
        };
        return second(one...);
    }
};

很明显，这不是真正的闭包类型，并且非闭包本地类不能具有成员函数模板.将其放到全球范围内， GCC仍然有效和

Clearly, this is not a real closure type, and non-closure local classes cannot have member function templates. Putting this in the global scope, GCC still works and Clang still fails.)

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