为什么我不能在模板化的类中内联定义非模板化的朋友？

Question

MCVE's speak louder than words:

// int bar();
template <bool B> class Foo {
    friend int ::bar() { return 123; }
};

int main()
{
    Foo<false> f1;
    Foo<true> f2;
}

with GCC 6 and --std=c++14 , this gives me:

a.cpp: In instantiation of ‘class Foo<true>’:
a.cpp:9:12:   required from here
a.cpp:3:13: error: redefinition of ‘int bar()’
  friend int ::bar() { return 123; }
             ^~
a.cpp:3:13: note: ‘int bar()’ previously defined here

Now, I'm not sure what the standard says; but I know that the compiler knows that the friend is not templated on B , nor does its definition use B . So why can't it apply the "oh, all inline copies of the same definition of a function are the same" rule?

Answer 1

Now, I'm not sure what the standard says;

This case has in fact been clarified with an example in the upcoming C++17

[temp.inst]/2 The implicit instantiation of a class template specialization ... [snip] ... for the purpose of determining whether an instantiated redeclaration of a member is valid according to 3.2 [basic.def.odr] and 9.2 [class.mem], a declaration that corresponds to a definition in the template is considered to be a definition. [ Example:

... [snip (another 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 ]

Admittedly as you point out, the example of the standard does produce a different definition for each instantiation, but that is not necessary for the example to be ill-formed according to that rule.

So why can't it apply the "oh, all inline copies of the same definition of a function are the same" rule?

This question seems to apply to the a much simpler situation as well:

inline void foo(){}
inline void foo(){}

Surely a compiler can see that the definitions are identical, just as much as a compiler can see that the definition of your ::bar depends not on the template argument of Foo .

Yet, odr says that the re-definition is ill-formed. This is true for definitions outside a class template, as well as definitions that are caused by instantiation of a class template.

---

Perhaps odr could be relaxed for the case that you demonstrate, but that would require complicating the standard with a special case rule, and complicate the compilers that then would have to analyse whether template arguments are used within the definition, so such relaxation certainly isn't without compromise.