is_pair_v<std::iterator_traits<typename Container::iterator>::value_type>

应该

is_pair_v<typename std::iterator_traits<typename Container::iterator>::value_type>

因为 value_type 是一种类型.如果没有 typename，它将被解析为一个值并且使 enable_if 失败，从而回退到主模板.

because value_type is a type. Without the typename, it will be parsed as a value and fail the enable_if, thus falling back to the primary template.

您在 main 中的测试产生正确值的原因是因为那里的模板已经实例化并且 value_type 是类型还是值没有歧义.

The reason your tests in main yield the correct value is because the templates there have already been instantiated and there is no ambiguity whether value_type is a type or value.

第二个错误是您的主要模板

The second error is your primary template

template<typename...>

应该是

template<typename, typename = void>

否则，is_mapping 永远不会是具有两个参数的特化，因为参数计数不匹配.

Otherwise, is_mapping<T> will never be the specialization with two arguments because the argument count mismatch.

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