Clang不会编译gcc将要使用的模板专业化 [英] Clang won't compile a template specialization that gcc will

问题描述

Gcc可以很好地进行编译，但是Clang(trunk)拒绝了以下消息:

Gcc compiles this fine, but Clang (trunk) refuses with the message:

<source>:7:8: error: class template partial specialization is not more specialized than the primary template [-Winvalid-partial-specialization]

https://godbolt.org/g/h8rsWC

template<class T, T x>
struct S{};

template<int& x>
struct S<int&, x> { };

此代码正确吗?

推荐答案

这仅在 -std = c ++ 17 及更高版本中体现.更专业"的确定需要从类模板中合成一对功能模板，为模板参数合成唯一类型，值和模板，以及最后在两个方向上执行模板参数推导.如果推导在一个方向上成功而在另一方向上不成功，则模板是更专业的".

This only manifests in -std=c++17 and later. The "more specialized" determination requires synthesizing a pair of function templates from the class templates, synthesizing unique types, values, and templates for the template parameters, and finally performing template argument deduction in both directions. A template is "more specialized" if deduction succeeds in one direction but not the other.

在这里，Clang从两个来源推导 T 并得到不同的结果:

Here, Clang is deducing T from two sources and getting divergent results:

从显式指定的 int& 中
• 推断出 T:= int& .
• 从非类型参数 x 中，按照通常的推导规则(通常不推导引用类型)推导 T:= int .在C ++ 17中添加了从非类型模板参数的类型推论的功能.

• from the explicitly specified int&, it deduces T := int&.
• from the non-type argument x, it deduces T := int per the usual deduction rules (which generally do not deduce a reference type). This ability to deduce from the type of a non-type template argument was added in C++17.

这使得推论在两个方向上均失败，因此部分专业化未通过更专业化"的测试.

This makes the deduction fail in both directions, so the partial specialization fails the "more specialized" test.

这看起来像是标准中的缺陷.解决方法是在非推导上下文中将 T 包裹在原始模板中.

This looks like a defect in the standard. The workaround is to wrap the T in the original template in a non-deduced context.

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