http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46719
--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> 2010-12-01
01:51:06 UTC ---
(In reply to comment #6)
> I think I did a bad job describing the issue, and I think that it's still a
> problem. I'd like to clarify it if I can. The following code works:
>
> template <typename Return, typename... Args>
> int curry(function<Return(double)> func) // NOTE: double
Here, Args can never be deduced, so must be given explicitly.
> {
> return 0;
> }
>
> int f(double x)
> {
> return 0;
> }
>
> int main()
> {
> curry<int, double>(f);
Args... must be {double}
It is not possible to call curry() without giving all template args, so the
compiler knows that the only viable specialisation is where Args...=double
> }
>
> While this does not:
>
> template <typename Return, typename... Args>
> int curry(function<Return(Args...)> func) // NOTE: Args...
Here Args... appears in a parameter and can be deduced.
> {
> return 0;
> }
>
> int f(double x)
> {
> return 0;
> }
>
> int main()
> {
> curry<int, double>(f);
> }
>
> Even though the template parameter pack Args expands to double, GCC is unable
> to match f to the type function<int(double)> if I use Args..., where it was
How does it know the template parameter pack expands to double? Maybe it
expands to {double,T} and you want T to be deduced. Or maybe {double,T,U}
If you passed a function<int(double)> it could deduce Args, but passing f
requires a user-defined conversion to an unknown type, so deduction cannot
succeed.
