[cfe-dev] Templated operator overloading

[antoni at buszta.info]

Hi all,

I have some small problem with clang behavior in such code:

struct stream
{
     stream() {}
     template<typename T>
     inline stream& operator<<(T& t)
     {
         return *this;
     }
};

template<typename Stream>
inline Stream& operator<<(Stream& stream, int& t)
{
     return stream;
}

int main(int argc, char *argv[])
{
     int i = 42;
     stream a;
     a << i;

     return 0;
}

This code compiles fine under clang (I have checked with 3.4 and current 
trunk) and gcc. However when I add -std=c++11 it still compiles fine 
under gcc but it stops compiling under clang. Error is:

/home/abuszta/Development/llvm/bin/clang++ -std=c++11    operator.cpp   
-o operator
operator.cpp:21:7: error: use of overloaded operator '<<' is ambiguous 
(with operand types 'stream' and 'int')
     a << i;
     ~ ^  ~
operator.cpp:5:20: note: candidate function [with T = int]
     inline stream& operator<<(T& t)
                    ^
operator.cpp:12:16: note: candidate function [with Stream = stream]
inline Stream& operator<<(Stream& stream, int& t)
                ^
1 error generated.

In order to answer the question which behavior is good and which is bad 
I tried to find appropriate section in C++ standard. After analyzing 
14.5.6.2 (section about Partial ordering of function templates in N3337) 
and related I think there should not be any ambiguity after substitution 
and function should be choosed...

What is more, when I make const stream a; this code compiles fine even 
with -std=c++11 and chooses the standalone function in favor to member 
function which suggests that there may be some problem with template 
resolution here.

What is your interpretation of this behavior? Should I report bug?

Best regards,
Antoni Buszta.