P2447R6 std::span over an initializer list

Author: burblebee

source/compatibility.tex

@@ -64,6 +64,31 @@
                                 // changed according to the global C locale
 \end{codeblock}
 
+\rSec2[diff.cpp23.containers]{\ref{containers}: containers library}
+
+\diffref{span.overview}
+\change
+\tcode{span<const T>} is constructible from \tcode{initializer_list<T>}.
+\rationale
+Permit passing a braced initializer list to a function taking \tcode{span}.
+\effect
+Valid \CppXXIII{} code that relies on the lack of this constructor
+may refuse to compile, or change behavior in this revision of \Cpp{}.
+For example:
+\begin{codeblock}
+void one(pair<int, int>);       // \#1
+void one(span<const int>);      // \#2
+void t1() { one({1,2}); }       // ambiguous between \#1 and \#2; previously called \#1
+
+void two(span<const int, 2>);
+void t2() { two({{1,2}}); }     // ill-formed; previously well-formed
+
+void *a[10];
+int x = span<void* const>{a, 0}.size();     // \tcode{x} is \tcode{2}; previously \tcode{0}
+any b[10];
+int y = span<const any>{b, b+10}.size();    // \tcode{y} is \tcode{2}; previously \tcode{10}
+\end{codeblock}
+
 \rSec1[diff.cpp20]{\Cpp{} and ISO \CppXX{}}
 
 \rSec2[diff.cpp20.general]{General}

source/containers.tex

@@ -18218,6 +18218,8 @@
 
 \indexheader{span}%
 \begin{codeblock}
+#include <initializer_list>     // see \ref{initializer.list.syn}
+
 namespace std {
   // constants
   inline constexpr size_t @\libglobal{dynamic_extent}@ = numeric_limits<size_t>::max();
@@ -18289,6 +18291,7 @@
       constexpr span(const array<T, N>& arr) noexcept;
     template<class R>
       constexpr explicit(extent != dynamic_extent) span(R&& r);
+    constexpr explicit(extent != dynamic_extent) span(std::initializer_list<value_type> il);
     constexpr span(const span& other) noexcept = default;
     template<class OtherElementType, size_t OtherExtent>
       constexpr explicit(@\seebelow@) span(const span<OtherElementType, OtherExtent>& s) noexcept;
@@ -18533,6 +18536,27 @@
 What and when \tcode{ranges::data(r)} and \tcode{ranges::size(r)} throw.
 \end{itemdescr}
 
+\indexlibraryctor{span}%
+\begin{itemdecl}
+constexpr explicit(extent != dynamic_extent) span(std::initializer_list<value_type> il);
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\constraints
+\tcode{is_const_v<element_type>} is \tcode{true}.
+
+\pnum
+\expects
+If \tcode{extent} is not equal to \tcode{dynamic_extent}, then
+\tcode{il.size()} is equal to \tcode{extent}.
+
+\pnum
+\effects
+Initializes \exposid{data_} with \tcode{il.begin()} and
+\exposid{size_} with \tcode{il.size()}.
+\end{itemdescr}
+
 \indexlibraryctor{span}%
 \begin{itemdecl}
 constexpr span(const span& other) noexcept = default;

source/support.tex

@@ -753,6 +753,7 @@
 #define @\defnlibxname{cpp_lib_smart_ptr_owner_equality}@          202306L // also in \libheader{memory}
 #define @\defnlibxname{cpp_lib_source_location}@                   201907L // freestanding, also in \libheader{source_location}
 #define @\defnlibxname{cpp_lib_span}@                              202002L // also in \libheader{span}
+#define @\defnlibxname{cpp_lib_span_initializer_list}@             202311L // also in \libheader{span}
 #define @\defnlibxname{cpp_lib_spanstream}@                        202106L // also in \libheader{spanstream}
 #define @\defnlibxname{cpp_lib_ssize}@                             201902L // freestanding, also in \libheader{iterator}
 #define @\defnlibxname{cpp_lib_sstream_from_string_view}@          202306L // also in \libheader{sstream}