[PATCH v1] libstdc++: Refactor tests for mdspan related accessors.

[Tomasz Kaminski]

On Sun, Jul 27, 2025 at 3:38 PM Luc Grosheintz <luc.grosheintz@gmail.com>
wrote:

> Versions 1, 2 and 3 of the patch for adding aligned_accessor had a
> bug in the constraints that allowed conversion of
>
>   aligned_accessor<T, N> a = aligned_accessor<const T, N>{};
>
> and prevented the reverse.
>
> The file mdspan/accessors/generic.cc already contains code that checks
> all variation of the constraint. This commit allows passing in two
> different accessors. Enabling it to be reused more widely.
>
> libstdc++-v3/ChangeLog:
>
>         * testsuite/23_containers/mdspan/accessors/generic.cc: Refactor
>         test_ctor.
>
> Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> ---
>
LGTM, with one note about duplicated assertions. I will remove it locally,
and push
the commit.

>  .../23_containers/mdspan/accessors/generic.cc | 61 ++++++++++++-------
>  1 file changed, 39 insertions(+), 22 deletions(-)
>
> diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/accessors/generic.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/generic.cc
> index c3350353aae..70c3bd0751c 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/accessors/generic.cc
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/generic.cc
> @@ -29,44 +29,61 @@ class Base
>  class Derived : public Base
>  { };
>
> -template<template<typename T> typename Accessor>
> +template<typename RhsAccessor, typename LhsAccessor, bool
> ExpectConvertible>
> +  constexpr void
> +  check_convertible()
> +  {
> +    RhsAccessor rhs;
> +    [[maybe_unused]] LhsAccessor lhs(rhs);
> +    static_assert(std::is_nothrow_constructible_v<LhsAccessor,
> RhsAccessor>);
> +    static_assert(std::is_convertible_v<RhsAccessor, LhsAccessor> ==
> ExpectConvertible);
> +  }
> +
> +template<template<typename T> typename LhsAccessor,
> +        template<typename T> typename RhsAccessor = LhsAccessor,
> +        bool ExpectConvertible = true>
>    constexpr bool
>    test_ctor()
>    {
>      // T -> T
> -    static_assert(std::is_nothrow_constructible_v<Accessor<double>,
> -                                                 Accessor<double>>);
> -    static_assert(std::is_convertible_v<Accessor<double>,
> Accessor<double>>);
> +    static_assert(std::is_nothrow_constructible_v<LhsAccessor<double>,
> +                                                 RhsAccessor<double>>);
>
This static assert seems to be redundant to one in check_convertible, so I
will remove it and the push th changes.

> +    check_convertible<RhsAccessor<double>, LhsAccessor<double>,
> +                     ExpectConvertible>();
>
>      // T -> const T
> -    static_assert(std::is_convertible_v<Accessor<double>,
> -                                       Accessor<const double>>);
> -    static_assert(std::is_convertible_v<Accessor<Derived>,
> -                                       Accessor<const Derived>>);
> +    check_convertible<RhsAccessor<double>, LhsAccessor<const double>,
> +                     ExpectConvertible>();
> +    check_convertible<RhsAccessor<Derived>, LhsAccessor<const Derived>,
> +                     ExpectConvertible>();
>
>      // const T -> T
> -    static_assert(!std::is_constructible_v<Accessor<double>,
> -                                          Accessor<const double>>);
> -    static_assert(!std::is_constructible_v<Accessor<Derived>,
> -                                          Accessor<const Derived>>);
> +    static_assert(!std::is_constructible_v<LhsAccessor<double>,
> +                                          RhsAccessor<const double>>);
> +    static_assert(!std::is_constructible_v<LhsAccessor<Derived>,
> +                                          RhsAccessor<const Derived>>);
>
>      // T <-> volatile T
> -    static_assert(std::is_convertible_v<Accessor<int>, Accessor<volatile
> int>>);
> -    static_assert(!std::is_constructible_v<Accessor<int>,
> -                                          Accessor<volatile int>>);
> +    check_convertible<RhsAccessor<int>, LhsAccessor<volatile int>,
> +                     ExpectConvertible>();
> +    static_assert(!std::is_constructible_v<LhsAccessor<int>,
> +                                          RhsAccessor<volatile int>>);
>
>      // size difference
> -    static_assert(!std::is_constructible_v<Accessor<char>,
> Accessor<int>>);
> +    static_assert(!std::is_constructible_v<LhsAccessor<char>,
> +                                          RhsAccessor<int>>);
>
>      // signedness
> -    static_assert(!std::is_constructible_v<Accessor<int>,
> -                                          Accessor<unsigned int>>);
> -    static_assert(!std::is_constructible_v<Accessor<unsigned int>,
> -                                          Accessor<int>>);
> +    static_assert(!std::is_constructible_v<LhsAccessor<int>,
> +                                          RhsAccessor<unsigned int>>);
> +    static_assert(!std::is_constructible_v<LhsAccessor<unsigned int>,
> +                                          RhsAccessor<int>>);
>
>      // Derived <-> Base
> -    static_assert(!std::is_constructible_v<Accessor<Base>,
> Accessor<Derived>>);
> -    static_assert(!std::is_constructible_v<Accessor<Derived>,
> Accessor<Base>>);
> +    static_assert(!std::is_constructible_v<LhsAccessor<Base>,
> +                                          RhsAccessor<Derived>>);
> +    static_assert(!std::is_constructible_v<LhsAccessor<Derived>,
> +                                          RhsAccessor<Base>>);
>      return true;
>    }
>
> --
> 2.50.0
>
>
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