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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/102257] call of overloaded 'tuple' is ambiguous
Date: Thu, 09 Sep 2021 17:18:36 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102257

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=3D102247

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Take:
#include <tuple>
#include <string>

int main() {
  std::tuple<std::string, int, int> t{{}, 0};
}
----- CUT ----
GCC does reject this though with the following error message:
<source>: In function 'int main()':
<source>:6:44: error: converting to 'std::allocator_arg_t' from initializer
list would use explicit constructor 'constexpr
std::allocator_arg_t::allocator_arg_t()'
    6 |   std::tuple<std::string, int, int> t{{}, 0};
      |                                            ^

So the question becomese does the conversion happen while chosing the overl=
oad
for the tuple constructor?
Looks like GCC is the only one which rejects this as being ambiguous too.

I did try:
struct tag{};
struct alloc {explicit alloc(int){};};

template<typename T1, typename T2>
struct tuple {
    tuple(tag, const alloc&);
    tuple(const T1&, const T2&);
};

class t2{};

tuple<t2,int> tt{{},0};

and GCC accepts this so is suprising based on the error message of the other
one.

Also you don't need std::string, a simple class like this will fail:

#include <tuple>

class t2{};
int main() {
  std::tuple<t2, int> t{{}, 0};
}=