英文:
Can we name an ill-formed specialization as long as we don't instantiate it?
问题
This code is ill-formed, no diagnostic required.
英文:
template <typename T, int N>
struct array {
T data[N];
};
using bad_array = array<void, 0>;
In the above code, we are referring to a specialization of the array class template which is not instantiated. If it was instantiated, then the program would be ill-formed, because we would generate a void data[0] member.
Is this code ill-formed, no diagnostic required, or is it well-formed?
答案1
得分: 4
是的，它是格式良好的。我无法提供标准参考，因为它根本不违反其中任何规定。
没有规定类型别名的形成需要类型是完整的，也没有规定类型的完整性会影响别名声明的行为，因此根据temp.inst/2没有隐式实例化。因此，特定特化的实例化是否不正确无关紧要，直到您以某种其他方式隐式或显式实例化它。
而且，由于temp.res.general/8列出的项目中也没有适用于您的模板，因此根据temp.res.general/8的最后一句所要求的，编译器不应为模板本身发出任何诊断。
在编写除函数模板之外的任何模板标识符时，无论上下文如何，模板标识符必须是“有效”的，否则程序是不正确的。这方面的要求在temp.names/6中，并包括验证模板参数的数量和种类是否正确，以及对非相关模板标识符满足约束条件，即与模板的声明而不是定义相关的所有与语法有关的要求。但模板标识符array<void, 0>也不违反这些要求。
英文:
Yes, it is well-formed. I can't really provide you a standard reference, because it simply doesn't violate anything in it.
Nothing says that forming a type alias requires the type to be complete, nor does completeness of the type affect the behavior of the alias declaration and so there is no implicit instantiation according to [temp.inst]/2. So, that the instantiation of the particular specialization would be ill-formed is irrelevant until you instantiate it implicitly or explicit in some other way.
And since also none of the items listed in [temp.res.general]/8 apply to your template, the compiler shall not issue any diagnostics for the template itself as required by the last sentence of [temp.res.general]/8.
When writing a template-id of anything but a function template, then regardless of context, the template-id must be valid, otherwise the program is ill-formed. The requirements for this are in [temp.names]/6 and include verifying that the number and kinds of template arguments are correct as well as that constraints are satisfied for non-dependent template-ids, i.e. all the syntactical requirements connected to the declaration of the template rather than its definition. But none of these requirements are violated by the template-id array<void, 0> either.
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