[Bug c++/102928] New: Template argument deduction when mixing variadic template with C-style variadic function

[Bug c++/102928] New: Template argument deduction when mixing variadic template with C-style variadic function

[cerrigno at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102928

            Bug ID: 102928
           Summary: Template argument deduction when mixing variadic
                    template with C-style variadic function
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cerrigno at gmail dot com
  Target Milestone: ---

GCC fails to deduce template arguments when mixing variadic template with
C-style variadic function.

This code compiles fine on clang, MSVC and ICC.

```
template <typename... Args>
constexpr auto foo(Args&& ...args, ...) noexcept {
    return sizeof...(args);
}

constexpr auto bar() noexcept {
        return foo<int>(1, 2);
}

static_assert(bar() == 2, "");
```

but on GCC bar() returns 1.

Even if not so clear on the standard, the correct output should be 2. More
information can be found on this answer on StackOverflow:
https://stackoverflow.com/a/69700603/3287591

See also this sandbox on Compiler Explorer: https://godbolt.org/z/e5v74n6fE

[Bug c++/102928] Template argument deduction when mixing variadic template with C-style variadic function

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102928

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-11-06
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
temp.deduct.call

For a function parameter pack that occurs at the end of the
parameter-declaration-list,
deduction is performed for each remaining argument of the call, taking the type
P of the declarator-id of the
function parameter pack as the corresponding function template parameter type.
Each deduction deduces
template arguments for subsequent positions in the template parameter packs
expanded by the function
parameter pack.


parameter-declaration-list does not include the ... variadic arguments part.

So Confirmed.

As far as I read in the standard, the template variadic deduction should be
greedy (deduction is performed for each remaining argument of the call).