是否必须在模板中指定typename？ [英] Does typename have to be specified inside template?

问题描述

在Clang中，似乎GCC不需要在模板中指定类型名。

It seems that GCC does not require a typename to be specified inside the template while Clang does.

请考虑以下代码段：

template<
    typename T,
    typename value_type = T::value_type
>
void func(T t) {}

当Clang报告时，以上代码在GCC 10.0.1上成功编译以下错误消息：

The code above compiles successfully on GCC 10.0.1 when Clang reports the following error message:

❯ clang++ -std=c++20 asdf.cpp
asdf.cpp:3:27: error: missing 'typename' prior to dependent type name 'T::value_type'
    typename value_type = T::value_type
                          ^~~~~~~~~~~~~
                          typename
1 error generated.

为什么会这样？

UPDATE 1：

如@Language Lawyer所述，关于 typename 的第一个怀疑是 P0634R3 当前不受Clang支持。

As noted by @Language Lawyer, the first suspicion about typename is a feature of P0634R3 not currently supported by Clang.

更新2：

我将分开讨论该问题，因为它包含两个不同的主题。

I'm going to separate this question as it contains two different topics.

有关详细信息，请参见编辑历史记录。

See edit history for more details.

推荐答案

C ++ 20无需在特定情况下为依赖模板名称指定 typename 显而易见，必须指定的是类型名。例如，已指定模板参数的默认值它是 typename ：

C++20 removes the need for specifying typename for dependent template names in specific cases where it's obvious that what is being specified must be a typename. Like, for example, a template parameter's default value when that parameter has already specified that it is a typename:

一个合格的名称被称为仅类型ID上下文，如果它出现在模板的类型参数的[...] 默认参数，[...]

Clang 尚未实现此功能，但是GCC已

Clang has not yet implemented this feature, but GCC has.

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