Hello,The attached patch adds the span(initializer_list) constructor, added by P2447R6 for C++26.
It seems that GCC is somewhat aggressive with its -Winit-list-lifetime warning, which actively interferes with this feature. The idea of the new constructor is to allow calls like:
void f(std::span<const int>);
f({1, 2, 3});
which is completely fine as the lifetime of the initializer_list
encompasses the one of the std::span parameter. However GCC complains
about the risk of dangling here. I've therefore disabled the warning for
the new constructor.
Thanks, -- Giuseppe D'Angelo
From bb1d537ee7b68883403127903834427c6787c505 Mon Sep 17 00:00:00 2001
From: Giuseppe D'Angelo <giuseppe.dang...@kdab.com>
Date: Tue, 3 Dec 2024 16:56:45 +0100
Subject: [PATCH] libstdc++: add initializer_list constructor to std::span
(P2447)
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: Added the new feature-testing macro.
* include/bits/version.h (defined): Regenerated.
* include/std/span: Added 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.
---
libstdc++-v3/include/bits/version.def | 8 +++
libstdc++-v3/include/bits/version.h | 10 +++
libstdc++-v3/include/std/span | 21 ++++++
.../23_containers/span/init_list_cons.cc | 65 +++++++++++++++++++
.../23_containers/span/init_list_cons_neg.cc | 31 +++++++++
5 files changed, 135 insertions(+)
create mode 100644 libstdc++-v3/testsuite/23_containers/span/init_list_cons.cc
create mode 100644 libstdc++-v3/testsuite/23_containers/span/init_list_cons_neg.cc
diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
index 8d4b8e9b383..cfa0469fb2d 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/libstdc++-v3/include/bits/version.def
@@ -1853,6 +1853,14 @@ 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 c556aca38fa..6a2c66bdf81 100644
--- a/libstdc++-v3/include/bits/version.h
+++ b/libstdc++-v3/include/bits/version.h
@@ -2055,6 +2055,16 @@
#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 f1c19b58737..b84ac87b657 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
@@ -226,6 +230,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
}
+#if __cpp_lib_span_initializer_list >= 202311L
+#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_extent(__il.size()), _M_ptr(__il.begin())
+ {
+ if constexpr (extent != dynamic_extent)
+ {
+ __glibcxx_assert(__il.size() == extent);
+ }
+ }
+#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 00000000000..7aabbf1e809
--- /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 initialier_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 initialier_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 00000000000..ef43541b769
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/init_list_cons_neg.cc
@@ -0,0 +1,31 @@
+// { dg-do run { target c++20 } }
+
+#include <span>
+#include <utility>
+#include <any>
+
+#include <testsuite_hooks.h>
+
+// 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 std::any> v) { return v.size(); }
+
+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 } }
+
+ void *array3[10];
+ std::any array4[10];
+
+#if __cpp_lib_span_initializer_list
+ static_assert(three({array3, 0}) == 2);
+ VERIFY(four({array4, array4 + 10}) == 2);
+#else
+ static_assert(three({array3, 0}) == 0);
+ VERIFY(four({array4, array4 + 10}) == 10);
+#endif
+}
--
2.34.1
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