Re: [PATCH v1] libstdc++: Fix class mandate for extents.

[Luc Grosheintz]

To make review easier, I'd like to provide links to the
sections in the standard I used.

mandate: https://eel.is/c++draft/mdspan.extents#overview-1.1

signed integer: https://eel.is/c++draft/basic.fundamental#1
unsigned integer: https://eel.is/c++draft/basic.fundamental#2

integral: https://eel.is/c++draft/basic.fundamental#11
          https://eel.is/c++draft/meta#tab:meta.unary.cat-row-4

On 5/14/25 9:13 PM, Luc Grosheintz wrote:

The standard states that the IndexType must be a signed or unsigned
integer. This mandate was implemented using `std::is_integral_v`. Which
also includes (among others) char and bool, which neither signed nor
unsigned integers.

libstdc++-v3/ChangeLog:

        * include/std/mdspan: Implement the mandate for extents as
        signed or unsigned integer and not any interal type.
        * testsuite/23_containers/mdspan/extents/class_mandates_neg.cc: Check
        that extents<char,...> and extents<bool,...> are invalid.
        * testsuite/23_containers/mdspan/extents/misc.cc: Update
        tests to avoid `char` and `bool` as IndexType.
---
  libstdc++-v3/include/std/mdspan                        |  3 ++-
  .../23_containers/mdspan/extents/class_mandates_neg.cc | 10 +++++++---
  .../testsuite/23_containers/mdspan/extents/misc.cc     |  8 ++++----
  3 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index aee96dda7cd..22509d9c8f4 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -163,7 +163,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    template<typename _IndexType, size_t... _Extents>
      class extents
      {
-      static_assert(is_integral_v<_IndexType>, "_IndexType must be integral.");
+      static_assert(__is_standard_integer<_IndexType>::value,
+                   "_IndexType must be a signed or unsigned integer.");
        static_assert(
          (__mdspan::__valid_static_extent<_Extents, _IndexType> && ...),
          "Extents must either be dynamic or representable as _IndexType");
diff --git 
a/libstdc++-v3/testsuite/23_containers/mdspan/extents/class_mandates_neg.cc 
b/libstdc++-v3/testsuite/23_containers/mdspan/extents/class_mandates_neg.cc
index b654e3920a8..63a2db77c08 100644
--- a/libstdc++-v3/testsuite/23_containers/mdspan/extents/class_mandates_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/extents/class_mandates_neg.cc
@@ -1,8 +1,12 @@
  // { dg-do compile { target c++23 } }
  #include<mdspan>

-std::extents<char, size_t(1) << 9> e1; // { dg-error "from here" }

-std::extents<double, 1> e2;            // { dg-error "from here" }
+#include <cstdint>
+
+std::extents<uint8_t, size_t(1) << 9> e1; // { dg-error "from here" }
+std::extents<char, 1> e2;                 // { dg-error "from here" }
+std::extents<bool, 1> e3;                 // { dg-error "from here" }
+std::extents<double, 1> e4;               // { dg-error "from here" }
  // { dg-prune-output "dynamic or representable as _IndexType" }
-// { dg-prune-output "must be integral" }
+// { dg-prune-output "signed or unsigned integer" }
  // { dg-prune-output "invalid use of incomplete type" }
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc 
b/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
index 16204aaaa75..e71fdc54230 100644
--- a/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
@@ -1,6 +1,7 @@
  // { dg-do run { target c++23 } }
  #include <mdspan>

+#include <cstdint>

  #include <testsuite_hooks.h>

constexpr size_t dyn = std::dynamic_extent;

@@ -20,7 +21,6 @@ static_assert(std::is_same_v<std::extents<int, 1, 2>::rank_type, 
size_t>);
  static_assert(std::is_unsigned_v<std::extents<int, 2>::size_type>);
  static_assert(std::is_unsigned_v<std::extents<unsigned int, 2>::size_type>);

-static_assert(std::is_same_v<std::extents<char, 2>::index_type, char>);

  static_assert(std::is_same_v<std::extents<int, 2>::index_type, int>);
  static_assert(std::is_same_v<std::extents<unsigned int, 2>::index_type,
              unsigned int>);
@@ -49,7 +49,7 @@ static_assert(check_rank_return_types<int, 1>());

// Check that the static extents don't take up space.

  static_assert(sizeof(std::extents<int, 1, dyn>) == sizeof(int));
-static_assert(sizeof(std::extents<char, 1, dyn>) == sizeof(char));
+static_assert(sizeof(std::extents<short, 1, dyn>) == sizeof(short));

template<typename Extents>

  class Container
@@ -58,7 +58,7 @@ class Container
    [[no_unique_address]] std::extents<size_t> b0;
  };

-static_assert(sizeof(Container<std::extents<char, 1, 2>>) == sizeof(int));

+static_assert(sizeof(Container<std::extents<short, 1, 2>>) == sizeof(int));
  static_assert(sizeof(Container<std::extents<size_t, 1, 2>>) == sizeof(int));

// operator=

@@ -103,7 +103,7 @@ test_deduction_all()
    test_deduction<0>();
    test_deduction<1>(1);
    test_deduction<2>(1.0, 2.0f);
-  test_deduction<3>(int(1), char(2), size_t(3));
+  test_deduction<3>(int(1), short(2), size_t(3));
    return true;
  }