Why does the following code compile with MSVC++?

Question

struct X{};

template<class T>
decltype(X() == int()) f(T const&){ return true; }

int main(void) {
  X x;
  f(x);
}

Why, just why? There is no operator== defined anywhere!

I really want to understand what's going on here, to provide a detailed bug report on MS Connect. My journey to insanity began around here in the Lounge<C++> chat room...

(Note: Neither GCC nor Clang accept this code.)

Oh, and btw, adding a private X(int) ctor causes the compilation to fail:

struct X{
    X(){}
private:
    X(int);
};

template<class T>
decltype(X() == int()) f(T const&){ return true; }

int main(void) {
  X x;
  f(x);
}

Output:

1>src\main.cpp(12): error C2248: 'X::X' : cannot access private member declared in class 'X'
1>          src\main.cpp(4) : see declaration of 'X::X'
1>          src\main.cpp(1) : see declaration of 'X'

Answer 1

What version of MS VC++ are you using?

For whatever it may be worth, VC++11 Beta rejects your code with:

trash.cpp(8): error C2893: Failed to specialize function template ''unknown-type' f(const T &)'
          With the following template arguments:
          'X'

I'm not sure that's what I'd call the most helpful or informative error message ever, but it is rejecting the code.

Under the circumstances, I'd guess filing a bug report is probably going to accomplish little (if anything). The response I'd expect would be essentially: "Already fixed in VC++11. Upgrade when you can."