[Bug c++/117410] Ambiguous overload with variadic arguments

[ddvamp007 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117410

--- Comment #5 from Artyom Kolpakov  ---
Here is a slightly different example:

template 
void foo(int = 0) {}

template 
void foo(...) requires true {}

int main() {
foo();
}

The current standard contains ambiguity regarding ellipsis. I suggested opening
a new core issue (https://cplusplus.github.io/CWG/issues/2949.html) that can be
tracked for this "bug".

[Bug c++/117410] Ambiguous overload with variadic arguments

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117410

--- Comment #4 from Jonathan Wakely  ---
(In reply to Jonathan Wakely from comment #2)
> I think the reason GCC rejects the original one is because it's the only
> compiler that correctly implements P2113.

Clang apparently supports P2113 since version 16.

[Bug c++/117410] Ambiguous overload with variadic arguments

[ddvamp007 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117410

--- Comment #3 from Artyom Kolpakov  ---
(In reply to Jonathan Wakely from comment #2)
> But without the variadic arguments, the functions have equivalent function
> parameter lists, so the constraints are checked. With the ellipsis, they do
> not have equivalent function parameter lists, so the constraints are ignored.

I could not find any mention in the standard of how exactly length of function
parameter list is determined. It is unclear whether and how an ellipsis or a
function parameter pack are included in this length ([dcl.fct#4]). It is also
unclear which parameter list is mentioned in [temp.func.order#6.1].

Is this a new core Issue?

[Bug c++/117410] Ambiguous overload with variadic arguments

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117410

--- Comment #2 from Jonathan Wakely  ---
But without the variadic arguments, the functions have equivalent function
parameter lists, so the constraints are checked. With the ellipsis, they do not
have equivalent function parameter lists, so the constraints are ignored.

That was specified by https://wg21.link/P2113 and implemented by
r11-1571-g57b4daf8dc4ed7

Since the constraints are not checked, we can remove them without changing the
meaning of the program:


template 
bool foo() { return false; }

template 
bool foo(...) { return true; }

template 
bool bar() { return false; }

template 
bool bar(...) { return true; }

int main() {
(foo());
(bar());
}


All compilers agree that this is ambiguous.

I think the reason GCC rejects the original one is because it's the only
compiler that correctly implements P2113.