Does an lvalue argument prefer an lvalue reference

问题:

While playing with universal references, I came across this instance where clang and gcc disagree on overload resolution.

#include <iostream>

struct foo {};

template<typename T>
void bar(T&) { std::cout << "void bar(T&)\n"; }

template<typename T>
void bar(T&&) { std::cout << "void bar(T&&)\n"; }

int main()
{
    foo f;
    bar(f);  // ambiguous on gcc, ok on clang
}

gcc reports the call above is ambiguous. However, clang selects the T& overload and compiles successfully.

Which compiler is wrong, and why?

Edit:
Tested the same code on VS2013 Preview, and it agrees with clang; except Intellisense, which is on gcc's side :-)

回答1:

The "universal reference" deduces the parameter to foo&. The first template also deduces the parameter to foo&.

C++ has a partial ordering rule for function templates that makes T& be more specialized than T&&. Hence the first template must be chosen in your example code.