g++ 和 clang++ 与在模板中定义的友元模板函数的不同行为 class

Question

C++ 标准专家的另一个 "who's right between g++ and clang++?" 类型的问题。

如下代码

template <int>
struct foo
 {
   template <typename>
   friend void bar ()
    { }
 };

int main ()
 {    
   foo<0>  f0;
   foo<1>  f1;
 }

使用 clang++ 编译没有问题（只有两个 "unused variable" 警告）但给出以下错误

tmp_002-11,14,gcc,clang.cpp: In instantiation of ‘struct foo<1>’:
tmp_002-11,14,gcc,clang.cpp:27:12:   required from here
tmp_002-11,14,gcc,clang.cpp:20:16: error: redefinition of ‘template<class> void bar()’
    friend void bar ()
                ^~~
tmp_002-11,14,gcc,clang.cpp:20:16: note: ‘template<class> void bar()’ previously defined here

使用 g++ 编译。

像往常一样，问题是：谁是对的？ g++ 或 clang++ ?

在我的 Debian 平台上使用 clang++ 3.9.1 和 g++ 6.3.0 检查。但是，在 Wandbox 中尝试，似乎与更新的版本相同。

Answer 1

GCC 在这种情况下是正确的。

相关标准写法在[temp.inst]/2:

The implicit instantiation of a class template specialization causes
— the implicit instantiation of the declarations, but not of the definitions, of the non-deleted class member functions, member classes, scoped member enumerations, static data members, member templates, and friends; and
[...]
However, for the purpose of determining whether an instantiated redeclaration is valid according to 6.2 and 12.2, a declaration that corresponds to a definition in the template is considered to be a definition. [ Example: [...]
template<typename T> struct Friendly {
   template<typename U> friend int f(U) { return sizeof(T); }
};
Friendly<char> fc;
Friendly<float> ff; // ill-formed: produces second definition of f(U)
— end example ]

与朋友有关的部分由DR2174添加到本段并在C++17中发布（这是一个缺陷报告，因此编译器也应将其应用于以前的标准版本）。

最新版本的 MSVC 和 EDG 在严格模式下也拒绝代码，抱怨重新定义。

[temp.inject]/1有点关系，但只讲友元函数，不谈友元函数模板：

Friend classes or functions can be declared within a class template. When a template is instantiated, the names of its friends are treated as if the specialization had been explicitly declared at its point of instantiation.

g++ 和 clang++ 与在模板中定义的友元模板函数的不同行为 class

g++ and clang++ different behaviour with friend template function defined inside a template class

c++

templates

friend

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