c++ - 使用不完整类型显式实例化函数模板

Question

以下内容:

template< typename >
struct S;

template< typename T >
S< T >& f (S< T >& s) {
    const typename S< T >::nested ignore;
    return s;
}

template S< char >& f (S< char >&);

template< typename >
struct S {
    struct nested { };
};

使用 gcc 编译，但不使用 clang:

$ clang -c /tmp/t.cpp
/tmp/t.cpp:6:20: error: implicit instantiation of undefined template 'S<char>'
    const typename S< T >::nested ignore;
                   ^
/tmp/t.cpp:10:21: note: in instantiation of function template specialization 'f<char>' requested here
template S< char >& f (S< char >&);
                    ^
/tmp/t.cpp:2:8: note: template is declared here
struct S;
       ^
1 error generated.

我相信 clang 是正确的，因为在实例化时，函数 f 指的是 S 的不完整定义。OTOH，S 的后期特化可能会提供正确的定义，使相关的“嵌套”格式正确.有什么意见吗？

Answer 1

两个编译器都是正确的。

[温度点]/p6, 8:

6 An explicit instantiation definition is an instantiation point for the specialization or specializations specified by the explicit instantiation.

8 A specialization for a function template [...] may have multiple points of instantiations within a translation unit, and in addition to the points of instantiation described above, for any such specialization that has a point of instantiation within the translation unit, the end of the translation unit is also considered a point of instantiation. [...] If two different points of instantiation give a template specialization different meanings according to the one definition rule (3.2), the program is ill-formed, no diagnostic required.

f<char>有两个实例化点:在显式实例化定义和 TU 的末尾。因为这两个实例化点会产生不同的含义(因为查找 S<T>::nested 会产生不同的结果)，所以该程序是格式错误的 NDR。