Merge 2024-03 LWG Motion 18

[LWG motion 18] P3029R1 Better mdspan's CTAD

Author: tkoeppe

source/containers.tex

@@ -18228,6 +18228,20 @@
   // constants
   inline constexpr size_t @\libglobal{dynamic_extent}@ = numeric_limits<size_t>::max();
 
+  template<class T>
+    concept @\defexposconcept{integral-constant-like}@ =                    // \expos
+      is_integral_v<decltype(T::value)> &&
+      !is_same_v<bool, remove_const_t<decltype(T::value)>> &&
+      @\libconcept{convertible_to}@<T, decltype(T::value)> &&
+      @\libconcept{equality_comparable_with}@<T, decltype(T::value)> &&
+      bool_constant<T() == T::value>::value &&
+      bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;
+
+  template<class T>
+    constexpr size_t @\defexposconcept{maybe-static-ext}@ = dynamic_extent; // \expos
+  template<@\exposconcept{integral-constant-like}@ T>
+    constexpr size_t @\exposconcept{maybe-static-ext}@<T> = {T::value};
+
   // \ref{views.span}, class template \tcode{span}
   template<class ElementType, size_t Extent = dynamic_extent>
     class span;                                                             // partially freestanding
@@ -18343,7 +18357,8 @@
   };
 
   template<class It, class EndOrSize>
-    span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<It>>>;
+    span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<It>>,
+                                @\exposconcept{maybe-static-ext}@<EndOrSize>>;
   template<class T, size_t N>
     span(T (&)[N]) -> span<T, N>;
   template<class T, size_t N>
@@ -18635,7 +18650,8 @@
 \indexlibrary{\idxcode{span}!deduction guide}%
 \begin{itemdecl}
 template<class It, class EndOrSize>
-  span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<It>>>;
+  span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<It>>,
+                              @\exposconcept{maybe-static-ext}@<EndOrSize>>;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -19098,15 +19114,6 @@
       const mdspan<ElementType, Extents, LayoutPolicy, AccessorPolicy>& src,
       SliceSpecifiers... slices) -> @\seebelow@;
 
-  template<class T>
-    concept @\defexposconcept{integral-constant-like}@ =        // \expos
-      is_integral_v<decltype(T::value)> &&
-      !is_same_v<bool, remove_const_t<decltype(T::value)>> &&
-      @\libconcept{convertible_to}@<T, decltype(T::value)> &&
-      @\libconcept{equality_comparable_with}@<T, decltype(T::value)> &&
-      bool_constant<T() == T::value>::value &&
-      bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;
-
   template<class T, class IndexType>
     concept @\defexposconcept{index-pair-like}@ =               // \expos
       @\exposconcept{pair-like}@<T> &&
@@ -19458,7 +19465,7 @@
 
 \pnum
 \remarks
-The deduced type is \tcode{dextents<size_t, sizeof...(Integrals)>}.
+The deduced type is \tcode{extents<size_t, \exposconcept{maybe-static-ext}<Integrals>...>}.
 \end{itemdescr}
 
 \rSec4[mdspan.extents.obs]{Observers of the multidimensional index space}
@@ -22549,7 +22556,7 @@
   template<class ElementType, class... Integrals>
     requires ((is_convertible_v<Integrals, size_t> && ...) && sizeof...(Integrals) > 0)
     explicit mdspan(ElementType*, Integrals...)
-      -> mdspan<ElementType, dextents<size_t, sizeof...(Integrals)>>;
+      -> mdspan<ElementType, extents<size_t, @\exposconcept{maybe-static-ext}@<Integrals>...>>;
 
   template<class ElementType, class OtherIndexType, size_t N>
     mdspan(ElementType*, span<OtherIndexType, N>)