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

[Luc Grosheintz]

On 7/28/25 10:15, Tomasz Kaminski wrote:
> 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.

Indeed, nicely spotted!

> 
>> +    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
>>
>>
>