libstdc++: add initializer_list constructor to std::span (P2447R6)

This commit implements P2447R6. The code is straightforward (just one
extra constructor, with constraints and conditional explicit).

I decided to suppress -Winit-list-lifetime because otherwise it would
give too many false positives. The new constructor is meant to be used
as a parameter-passing interface (this is a design choice, see
P2447R6/§2) and, as such, the initializer_list won't dangle despite
GCC's warnings.

The new constructor isn't 100% backwards compatible. A couple of
examples are included in Annex C, but I have also lifted some more
from R4. A new test checks for the old and the new behaviors.

libstdc++-v3/ChangeLog:

	* include/bits/version.def: Add the new feature-testing macro.
	* include/bits/version.h: Regenerate.
	* include/std/span: Add constructor from initializer_list.
	* testsuite/23_containers/span/init_list_cons.cc: New test.
	* testsuite/23_containers/span/init_list_cons_neg.cc: New test.

Signed-off-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

libstdc++-v3/include/bits/version.def

@@ -1869,6 +1869,14 @@ ftms = {
   };
 };
 
+ftms = {
+  name = span_initializer_list;
+  values = {
+    v = 202311;
+    cxxmin = 26;
+  };
+};
+
 ftms = {
   name = text_encoding;
   values = {

libstdc++-v3/include/bits/version.h

@@ -2075,6 +2075,16 @@
 #endif /* !defined(__cpp_lib_saturation_arithmetic) && defined(__glibcxx_want_saturation_arithmetic) */
 #undef __glibcxx_want_saturation_arithmetic
 
+#if !defined(__cpp_lib_span_initializer_list)
+# if (__cplusplus >  202302L)
+#  define __glibcxx_span_initializer_list 202311L
+#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_span_initializer_list)
+#   define __cpp_lib_span_initializer_list 202311L
+#  endif
+# endif
+#endif /* !defined(__cpp_lib_span_initializer_list) && defined(__glibcxx_want_span_initializer_list) */
+#undef __glibcxx_want_span_initializer_list
+
 #if !defined(__cpp_lib_text_encoding)
 # if (__cplusplus >  202302L) && _GLIBCXX_HOSTED && (_GLIBCXX_USE_NL_LANGINFO_L)
 #  define __glibcxx_text_encoding 202306L

libstdc++-v3/include/std/span

@@ -39,6 +39,7 @@
 #endif
 
 #define __glibcxx_want_span
+#define __glibcxx_want_span_initializer_list
 #include <bits/version.h>
 
 #ifdef __cpp_lib_span // C++ >= 20 && concepts
@@ -46,6 +47,9 @@
 #include <cstddef>
 #include <bits/stl_iterator.h>
 #include <bits/ranges_base.h>
+#ifdef __cpp_lib_span_initializer_list
+# include <initializer_list>
+#endif
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
@@ -228,6 +232,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	: _M_ptr(ranges::data(__range)), _M_extent(ranges::size(__range))
 	{ }
 
+#if __cpp_lib_span_initializer_list >= 202311L // >= C++26
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Winit-list-lifetime"
+	constexpr
+	explicit(extent != dynamic_extent)
+	span(initializer_list<value_type> __il)
+	requires (is_const_v<_Type>)
+	: _M_ptr(__il.begin()), _M_extent(__il.size())
+	{
+	}
+#pragma GCC diagnostic pop
+#endif
+
       constexpr
       span(const span&) noexcept = default;
 

libstdc++-v3/testsuite/23_containers/span/init_list_cons.cc

@@ -0,0 +1,65 @@
+// { dg-do compile { target c++26 } }
+
+#include <span>
+#include <type_traits>
+
+#if !defined(__cpp_lib_span_initializer_list)
+# error "__cpp_lib_span_initializer_list should be defined"
+#elif __cpp_lib_span_initializer_list < 202311L
+# error "Wrong value for __cpp_lib_span_initializer_list (should be >= 202311L)"
+#endif
+
+// Check the constraint on the initializer_list constructor
+static_assert( std::is_const_v<std::span<const int>::element_type>);
+static_assert(!std::is_const_v<std::span<      int>::element_type>);
+
+static_assert( std::is_constructible_v<std::span<const int    >, std::initializer_list<      int>>);
+static_assert( std::is_constructible_v<std::span<const int    >, std::initializer_list<const int>>);
+static_assert( std::is_constructible_v<std::span<const int, 42>, std::initializer_list<      int>>);
+static_assert( std::is_constructible_v<std::span<const int, 42>, std::initializer_list<const int>>);
+static_assert(!std::is_constructible_v<std::span<      int    >, std::initializer_list<      int>>);
+static_assert(!std::is_constructible_v<std::span<      int    >, std::initializer_list<const int>>);
+static_assert(!std::is_constructible_v<std::span<      int, 42>, std::initializer_list<      int>>);
+static_assert(!std::is_constructible_v<std::span<      int, 42>, std::initializer_list<const int>>);
+
+// Check the explicit-ness on the initializer_list constructor
+static_assert( std::is_convertible_v<std::initializer_list<      int>, std::span<const int    >>);
+static_assert( std::is_convertible_v<std::initializer_list<const int>, std::span<const int    >>);
+static_assert(!std::is_convertible_v<std::initializer_list<      int>, std::span<const int, 42>>);
+static_assert(!std::is_convertible_v<std::initializer_list<const int>, std::span<const int, 42>>);
+static_assert(!std::is_convertible_v<std::initializer_list<      int>, std::span<      int    >>);
+static_assert(!std::is_convertible_v<std::initializer_list<const int>, std::span<      int    >>);
+static_assert(!std::is_convertible_v<std::initializer_list<      int>, std::span<      int, 42>>);
+static_assert(!std::is_convertible_v<std::initializer_list<const int>, std::span<      int, 42>>);
+
+constexpr size_t fun1(std::span<const int> s)
+{
+  return s.size();
+}
+
+static_assert(fun1({}) == 0);
+static_assert(fun1({1, 2, 3}) == 3);
+static_assert(fun1(std::initializer_list<int>{1, 2, 3}) == 3);
+
+// Stress-test array->pointer decays
+struct decayer {
+  constexpr decayer() = default;
+  constexpr decayer(decayer *) {}
+};
+
+constexpr size_t fun2(std::span<const decayer> s)
+{
+  return s.size();
+}
+
+void test01()
+{
+  int intArray[42];
+  static_assert(fun1(intArray) == 42);
+
+  decayer decArray[42];
+  static_assert(fun2(decArray) == 42);
+  static_assert(fun2({decArray}) == 1); // decayer[] -> decayer* -> decayer(decayer*) -> init_list<decayer> of 1 element
+  static_assert(fun2({decArray, decArray + 42}) == 2); // does not select span(iterator, iterator)
+  static_assert(fun2({decArray, decArray, decArray}) == 3);
+}

libstdc++-v3/testsuite/23_containers/span/init_list_cons_neg.cc

@@ -0,0 +1,36 @@
+// { dg-do run { target { c++20 && c++23_down } } }
+// { dg-do compile { target c++26 } }
+
+#include <span>
+#include <utility>
+
+#include <testsuite_hooks.h>
+
+struct Any {
+  constexpr Any() { }
+  template<typename T> constexpr Any(T) { }
+};
+
+// Examples from P2447R4
+void one(std::pair<int, int>) {}
+void one(std::span<const int>) {}
+void two(std::span<const int, 2>) {}
+constexpr std::size_t three(std::span<void * const> v) { return v.size(); }
+constexpr std::size_t four(std::span<const Any> v) { return v.size(); }
+
+void *array3[10];
+Any array4[10];
+
+int main()
+{
+  one({1, 2}); // { dg-error "call of overloaded" "should be ambiguous with the one(std::pair) overload" { target c++26 } }
+  two({{1, 2}}); // { dg-error "would use explicit constructor" "should prefer the initializer_list constructor, which is explicit" { target c++26 } }
+
+#if __cpp_lib_span_initializer_list
+  static_assert(three({array3, 0}) == 2);
+  static_assert(four({array4, array4 + 10}) == 2);
+#else
+  static_assert(three({array3, 0}) == 0);
+  static_assert(four({array4, array4 + 10}) == 10);
+#endif
+}