libstdc++: Harmonize mdspan operator[] and at methods.

This patch changes the implementation of operator[], to follow similar
pattern as at, where we normalize argument types and forward to single
overload. The conversion now uses __mdspan::__index_type_cast, instead
of static_assert. This introduces asserts assuring that conversion is
is value preserving.

Finally, __is_multi_index is implemented as local lambda, avoiding
the need to create a local array with copies of the arguments.

libstdc++-v3/ChangeLog:

	* include/std/mdspan (__mdspan::__is_multi_index): Removed,
	replaced with local lambda in mdspan::operator[].
	(__mdspan::operator[]): Implement in terms of single overload
	accepting index_type. Use __mdspan::__index_type_cast for type
	conversion.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index a6d2aadcb0cb3d459962e007a62cd64a31599428..f8847642c80e667f29b38c524ee80e0faa631c13 100644 (file)
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -2906,20 +2906,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     };
 #endif
 
-  namespace __mdspan
-  {
-    template<typename _Extents, typename _IndexType, size_t _Nm>
-      constexpr bool
-      __is_multi_index(const _Extents& __exts, span<_IndexType, _Nm> __indices)
-      {
-       static_assert(__exts.rank() == _Nm);
-       for (size_t __i = 0; __i < __exts.rank(); ++__i)
-         if (__indices[__i] >= __exts.extent(__i))
-           return false;
-       return true;
-      }
-  }
-
   template<typename _ElementType, typename _Extents,
           typename _LayoutPolicy = layout_right,
           typename _AccessorPolicy = default_accessor<_ElementType>>
@@ -3056,17 +3042,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        constexpr reference
        operator[](_OIndexTypes... __indices) const
        {
-         auto __checked_call = [this](auto... __idxs) -> index_type
-           {
-             if constexpr (sizeof...(__idxs) > 0)
-               __glibcxx_assert(__mdspan::__is_multi_index(extents(),
-                 span<const index_type, sizeof...(__idxs)>({__idxs...})));
-             return _M_mapping(__idxs...);
-           };
+         if constexpr (rank() == 0)
+           return _M_accessor.access(_M_handle, _M_mapping());
+         else if constexpr (!(is_same_v<_OIndexTypes, index_type> && ...))
+           return operator[](
+             __mdspan::__index_type_cast<index_type>(std::move(__indices))...);
+         else
+          {
+            auto __is_multi_index = [&]<size_t... _Counts>(index_sequence<_Counts...>)
+            { return ((__indices < extents().extent(_Counts)) && ...); };
 
-         auto __index = __checked_call(
-           static_cast<index_type>(std::move(__indices))...);
-         return _M_accessor.access(_M_handle, __index);
+            __glibcxx_assert(__is_multi_index(make_index_sequence<rank()>()));
+            return _M_accessor.access(_M_handle, _M_mapping(__indices...));
+          }
        }
 
       template<typename _OIndexType>
@@ -3076,7 +3064,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        {
          auto __call = [&]<size_t... _Counts>(index_sequence<_Counts...>)
            -> reference
-           { return (*this)[index_type(as_const(__indices[_Counts]))...]; };
+           {
+              return operator[](
+                __mdspan::__index_type_cast<index_type>(as_const(__indices[_Counts]))...);
+           };
          return __call(make_index_sequence<rank()>());
        }
 
@@ -3084,7 +3075,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        requires __mdspan::__valid_index_type<const _OIndexType&, index_type>
        constexpr reference
        operator[](const array<_OIndexType, rank()>& __indices) const
-       { return (*this)[span<const _OIndexType, rank()>(__indices)]; }
+       { return operator[](span<const _OIndexType, rank()>(__indices)); }
 
 #if __cplusplus > 202302L
       template<__mdspan::__valid_index_type<index_type>... _OIndexTypes>