https://gcc.gnu.org/g:5db068738469be8c2bf3cfbda4c54725bd9fe228
commit r15-6693-g5db068738469be8c2bf3cfbda4c54725bd9fe228
Author: Giuseppe D'Angelo <giuseppe.dang...@kdab.com>
Date: Fri Dec 20 12:09:10 2024 +0000
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/ยง2) 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.dang...@kdab.com>
Diff:
---
libstdc++-v3/include/bits/version.def | 8 +++
libstdc++-v3/include/bits/version.h | 10 ++++
libstdc++-v3/include/std/span | 17 ++++++
.../testsuite/23_containers/span/init_list_cons.cc | 65 ++++++++++++++++++++++
.../23_containers/span/init_list_cons_neg.cc | 36 ++++++++++++
5 files changed, 136 insertions(+)
diff --git a/libstdc++-v3/include/bits/version.def
b/libstdc++-v3/include/bits/version.def
index 2e7b6842cdb4..002e560dc0d2 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/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 = {
diff --git a/libstdc++-v3/include/bits/version.h
b/libstdc++-v3/include/bits/version.h
index 28d76d2385d8..70de189b1e0b 100644
--- a/libstdc++-v3/include/bits/version.h
+++ b/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
diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
index 4cee89002ad6..4b40bd0f6558 100644
--- a/libstdc++-v3/include/std/span
+++ b/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;
diff --git a/libstdc++-v3/testsuite/23_containers/span/init_list_cons.cc
b/libstdc++-v3/testsuite/23_containers/span/init_list_cons.cc
new file mode 100644
index 000000000000..1dc30ab1a50b
--- /dev/null
+++ b/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);
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/init_list_cons_neg.cc
b/libstdc++-v3/testsuite/23_containers/span/init_list_cons_neg.cc
new file mode 100644
index 000000000000..6b78156ad5df
--- /dev/null
+++ b/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
+}
