在为操作员()显式指定模板参数时，对Clang前端ICE的C ++快速修复? [英] C++ quickfix to Clang frontend ICE when explicitly specifying template parameter to operator()?

问题描述

• 关于LLVM错误报告的已报告问题: https://bugs.llvm.org/show_bug.cgi?id = 49881

考虑以下最小再现案例:

Considering the following minimal reproduction case example :

#include <tuple>

int main()
{
    // KO, explicit template parameter

    const auto lambda_1 = []<typename T>(){};

    [&lambda_1]<typename ... Ts>(std::tuple<Ts...>){
        ((lambda_1.template operator()<Ts>()), ...);
    }(std::tuple<int>{});

    // OK, template deduced using parameter

    const auto lambda_2 = []<typename T>(T){};

    [&lambda_2]<typename ... Ts>(std::tuple<Ts...>){
        ((lambda_2(Ts{})), ...);
    }(std::tuple<int>{});
}

这里的问题是，这样的解决方案要求 Ts ... 是默认可构造的.

The issue here is, that such solution requires Ts... to be default constructible.

因此，它不适用于非类型，例如带有存储的lambda，
例如:

Thus, it does not work with types which are not, such as lambdas with storage,
for instance :

// KO, Ts... is not default constructible

const auto lambda = []<typename T>(T){};

    int i{0};
    [&lambda]<typename ... Ts>(std::tuple<Ts...>){
        ((lambda(Ts{})), ...);
    }(std::tuple{
        [i]() {}
    });

问题:

• 我们是否应谴责暂时禁用对项目的Clang支持，并等待补丁，
  还是在下一个版本(将修复此ICE的版本)之前，我们可以使用任何快速修复程序进行这项工作?

Question :

• Are we condemned to temporarily disable Clang support to our projects, and wait for a patch,
  or is there any quick-fix we can use to make this work until the next release(s) (that will fix this ICE) ?

推荐答案

"struct可能有助于传递类型并允许推导:

A "tag" struct might help to pass type and allow deduction:

template <typename T>
struct Tag
{
    using type = T;
};
// or
// template <typename T> using Tag = std::type_identity<T>;

然后而不是:

const auto lambda_1 = []<typename T>(){/*...*/};
lambda_1.template operator()<SomeT>();

您可能有:

const auto lambda_2 = []<typename T>(Tag<T>){ /*...*/};
lambda_2(Tag<SomeT>{});

// C++14: No constraints on tag
const auto lambda_3 = [](auto tag){ using T = typename decltype(tag)::type; /*...*/ };
lambda_3(Tag<SomeT>{});

这篇关于在为操作员()显式指定模板参数时，对Clang前端ICE的C ++快速修复?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！