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

Another question of type "who's right between g++ and clang++?" for C++ standard gurus.

The following code

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

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

compile without problem with clang++ (only two "unused variable" warnings) but give a the following error

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

compiling with g++.

The question, as usual, is: who's right ? g++ or clang++ ?

Checked with clang++ 3.9.1 and g++ 6.3.0 in my Debian platform. But, trying in Wandbox, seems equals with more recent versions.

Solution

  • GCC is right in this case.

    The relevant standard wording is in [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 ]

    The parts related to friends were added to this paragraph by DR2174 and published in C++17 (it's a defect report, so compilers should apply it to previous standard versions as well).

    Recent versions of MSVC and EDG in strict mode also reject the code, complaining about a redefinition.

    [temp.inject]/1 is somewhat related, but it only talks about friend functions, not friend function templates:

    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.