c++ - 在 C++20 中是否不再允许在 std 中为程序定义类型专门化函数模板？

Question

引自 cppreference.com :

Adding template specializations

It is allowed to add template specializations for any standard library |class (since C++20)| template to the namespace std only if the declaration depends on at least one program-defined type and the specialization satisfies all requirements for the original template, except where such specializations are prohibited.

这是否意味着，从 C++20 开始，将不再允许将函数模板的特化添加到用户定义类型的 std 命名空间？如果是这样，这意味着许多现有代码可能会中断，不是吗？ (在我看来，这是一种“激进”的改变。)此外，它会在此类代码中注入(inject)未定义的行为，这不会触发编译错误(希望会发出警告)。

Answer 1

就目前而言，它肯定是那样的。以前包含 [namespace.std]

A program may add a template specialization for any standard library template to namespace std only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.

而 the current draft states

Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace std provided that (a) the added declaration depends on at least one program-defined type and (b) the specialization meets the standard library requirements for the original template.

^强调我的

它看起来像论文 Thou Shalt Not Specialize std Function Templates!由 Walter E. Brown 负责。在其中，他详细说明了应更改此设置的一些原因，例如:

• Herb Sutter: “specializations don’t participate in overloading. [...] If you want to customize a function base template and want that customization to participate in overload resolution (or, to always be used in the case of exact match), make it a plain old function, not a specialization. And, if you do provide overloads, avoid also providing specializations.”
• David Abrahams: “it’s wrong to use function template specialization [because] it interacts in bad ways with overloads. [...] For example, if you specialize the regular std::swap for std::vector<mytype>&, your specialization won’t get chosen over the standard’s vector specific swap, because specializations aren’t considered during overload resolution.”
• Howard Hinnant: “this issue has been settled for a long time. . . . Disregard Dave’s expert opinion/answer in this area at your own peril.”
• Eric Niebler: “[because of] the decidedly wonky way C++ resolves function calls in templates. . . , [w]e make an unqualified call to swap in order to find an overload that might be defined in [...] associated namespaces[...] , and we do using std::swap so that, on the off-chance that there is no such overload, we find the default version defined in the std namespace.”
• High Integrity C++ Coding Standard: “Overload resolution does not take into account explicit specializations of function templates. Only after overload resolution has chosen a function template will any explicit specializations be considered.”