推导第一个模板参数与其他默认模板参数 [英] Deducing first template argument with other template parameters defaulted

问题描述

Gcc和clang在此代码是否应该编译方面似乎有分歧:

Gcc and clang seem to disagree on whether this code should compile or not:

#include <type_traits>

template <typename Signature, int N = 0>
struct MyDelegate { };

template <typename D>
struct signature_traits;

template <template <typename> class Delegate, typename Signature>
struct signature_traits<Delegate<Signature>>
{
    using type = Signature;
};

static_assert(std::is_same_v<
    void(int, int),
    signature_traits<MyDelegate<void(int, int)>>::type
>);

请参见 godbolt输出并尝试.我在这里使用clang，但是C ++标准对此有何看法?

See godbolt output here and try it. I'm siding with clang here, but what does the C++ standard say about this?

一个后续问题-可以使之适用于clang吗?

A follow-up question - can this be made to work in clang?

推荐答案

这是完全有效的代码，gcc是正确的. 功能"是C ++ 17中引入的 .这不是真正的功能，因为它是缺陷报告. MyDelegate与signature_traits的部分专业化匹配，因此应该像gcc一样正确地对待它.请注意，它之所以有效是因为默认使用第二个模板参数.

This is perfectly valid code, and gcc is right. The "feature" was introduced in C++17. It's not really a feature because it is a defect report. MyDelegate matches the partial specialization of signature_traits, and so it should be taken as gcc correctly does. Note that it works because the second template parameter is defaulted.

clang不编译的原因是因为该缺陷报告具有缺陷:P.它未在部分排序中引入适当的更改，不太好，使先前的有效代码又变得模棱两可.

The reason why clang doesn't compile it is because that defect report has a defect :P. It doesn't introduce the appropriate change in partial ordering, which is not really nice and makes previousy valid code ambiguous again.

预计它将很快修复，但是与此同时，clang决定隐藏"标志-frelaxed-template-template-args后面的功能.

It is expected to be fixed soon, but in the meanwhile, clang decided to "hide" the feature behind a flag, -frelaxed-template-template-args.

因此，只需在启用该标志的情况下进行编译，就可以了.

So, just compile with that flag enabled and you should be fine.

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