[cfe-dev] Apparently valid C++ code rejected as ambiguous

[Douglas Gregor]

On Nov 12, 2010, at 11:26 AM, Csaba Raduly wrote:

> Hi all,
> clang 2.8 fails to compile the following code:
> 
> template <typename T_type>
> class O {
> T_type *val;
> 
> public:
>  O();
>  O(T_type); // t
> 
>  O(const O& other_value); // copy ctor
> 
>  template <typename T_tmp>
>  explicit O(const O<T_tmp>& other_value); // other O
> 
>  template <typename T_tmp>
>  explicit O(const T_tmp& other_value); // other t
> 
>  inline bool operator==(const O& other_value) const;
> 
>  inline bool operator!=(const O& other_value) const;
> 
>  template <typename T_tmp>
>  inline bool operator==(const T_tmp& other_value) const; // line 22
> 
>  template <typename T_tmp>
>  inline bool operator!=(const T_tmp& other_value) const;
> 
> };
> 
> class C{
> public:
>  C();
> };
> 
> class U_C{
> public:
>  U_C();
>  U_C(const U_C&); // copy ctor
>  /*explicit*/ U_C(const C&);
> };
> 
> template<typename T_left, typename T_right>
> inline bool operator==(const T_left& left_value, // line 42
>  const O<T_right>& right_value)
> { return true; }
> 
> 
> template<typename T_left, typename T_right>
> inline bool operator!=(const T_left& left_value,
>  const O<T_right>& right_value)
> { return false; }
> 
> int main()
> {
>  C cs;
>  U_C us;
>  O<C> ocs;
>  O<U_C> ous;
> 
>  if (ocs == ous) {
>    return 0;
>  }
> }
> - - - - -
> wtf.cc:59:11: error: use of overloaded operator '==' is ambiguous
>  if (ocs == ous) {
>      ~~~ ^  ~~~
> wtf.cc:22:15: note: candidate function [with $0 = O<U_C>]
>  inline bool operator==(const T_tmp& other_value) const;
>              ^
> wtf.cc:42:13: note: candidate function [with T_left = O<C>, T_right = U_C]
> inline bool operator==(const T_left& left_value,

This *might* be related to

	http://llvm.org/bugs/show_bug.cgi?id=8130

but I haven't looked at it closely enough to be sure.

> The code is accepted by GCC 4.6, the online version of Comeau and even
> the online version of LLVM 2.7 !

The online version of LLVM 2.7 is LLVM-GCC, which is based on GCC 4.2.

> This appears to be a regression. I had a brief look in Bugzilla but I
> didn't spot anything close enough.

I'm fairly certain that it is not a regression. I looked back quite a ways, and no version of Clang has handled this.

	- Doug