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From: "milasudril at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107123] New: Size deduction for vector size in template
 fails
Date: Sun, 02 Oct 2022 10:59:32 +0000
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X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.2.1
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107123

            Bug ID: 107123
           Summary: Size deduction for vector size in template fails
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: milasudril at gmail dot com
  Target Milestone: ---
              Host: x86-64_linux_gnu
            Target: x86-64_linux_gnu

I tried to write a generic inner product implementation, accepting vectoriz=
ed
arguments:

```c++
#include <type_traits>
#include <cstddef>

template<class T>
concept arithmetic =3D std::is_arithmetic_v<T>;

template<arithmetic T, size_t N>
using native_vector [[gnu::vector_size(sizeof(T)*N)]] =3D T;

template<class T, size_t N>
auto inner_product(native_vector<T, N> a, native_vector<T, N> b)
{
    auto const prod =3D a * b;
    T ret{};
    for(size_t k =3D 0; k !=3D N; ++k)
    { ret +=3D prod[k]; }

    return ret;
}

auto test(native_vector<int, 4> a, native_vector<int, 4> b)
{
    return inner_product(a, b);
}
```

Apparently, it is not possible to deduce N here:

```
<source>: In function 'auto test(native_vector<int, 4>, native_vector<int,
4>)':
<source>:23:25: error: no matching function for call to
'inner_product(native_vector<int, 4>&, native_vector<int, 4>&)'
   23 |     return inner_product(a, b);
      |            ~~~~~~~~~~~~~^~~~~~
<source>:11:6: note: candidate: 'template<class T, long unsigned int N> auto
inner_product(native_vector<T, N>, native_vector<T, N>)'
   11 | auto inner_product(native_vector<T, N> a, native_vector<T, N> b)
      |      ^~~~~~~~~~~~~
<source>:11:6: note:   template argument deduction/substitution failed:
<source>:23:25: note:   couldn't deduce template parameter 'N'
   23 |     return inner_product(a, b);
```

I would appreciate if size deduction worked like for std::array:

```c++
#include <array>

template<class T, size_t N>
auto inner_product(std::array<T, N> a, std::array<T, N> b)
{
    T ret{};
    for(size_t k =3D 0; k !=3D N; ++k)
    { ret +=3D a[k]*b[k]; }

    return ret;
}

auto test(std::array<int, 4> a, std::array<int, 4> b)
{
    return inner_product(a, b);  // N deduced to 4
}
```

The problem is present on gcc 10-trunk.=