Split clang-win job in appveyor.yml to avoid timeout

The macos-13 image is retierd.

Also, clang on macos-15 supports C++23.

.github/workflows/ci.yml

@@ -151,20 +151,6 @@ jobs:
               - g++-12
 
           # Linux, clang
-          - toolset: clang
-            compiler: clang++-3.5
-            cxxstd: "03,11"
-            os: ubuntu-latest
-            container: ubuntu:16.04
-            install:
-              - clang-3.5
-          - toolset: clang
-            compiler: clang++-3.6
-            cxxstd: "03,11,14"
-            os: ubuntu-latest
-            container: ubuntu:16.04
-            install:
-              - clang-3.6
           - toolset: clang
             compiler: clang++-3.7
             cxxstd: "03,11,14"
@@ -334,15 +320,26 @@ jobs:
             linkflags: -stdlib=libc++
           - toolset: clang
             compiler: clang++-20
-            cxxstd: "03,11,14,17,20,2b,2c"
+            cxxstd: "03,11,14,17,20,23,2c"
             os: ubuntu-latest
-            container: ubuntu:25.04
+            container: ubuntu:24.04
             install:
               - clang-20
               - libc++-20-dev
               - libc++abi-20-dev
             cxxflags: -stdlib=libc++
             linkflags: -stdlib=libc++
+          - toolset: clang
+            compiler: clang++-21
+            cxxstd: "03,11,14,17,20,23,2c"
+            os: ubuntu-latest
+            container: ubuntu:25.10
+            install:
+              - clang-21
+              - libc++-21-dev
+              - libc++abi-21-dev
+            cxxflags: -stdlib=libc++
+            linkflags: -stdlib=libc++
           - name: UBSAN
             toolset: clang
             compiler: clang++-15
@@ -365,14 +362,14 @@ jobs:
               - g++-13
 
           - toolset: clang
-            cxxstd: "03,11,14,17,20,2b"
-            os: macos-13
-          - toolset: clang
-            cxxstd: "03,11,14,17,20,2b"
             os: macos-14
-          - toolset: clang
             cxxstd: "03,11,14,17,20,2b"
+          - toolset: clang
             os: macos-15
+            cxxstd: "03,11,14,17,20,23,2c"
+          - toolset: clang
+            os: macos-26
+            cxxstd: "03,11,14,17,20,23,2c"
 
     timeout-minutes: 45
     runs-on: ${{matrix.os}}
@@ -680,9 +677,9 @@ jobs:
         include:
           - os: ubuntu-22.04
           - os: ubuntu-24.04
-          - os: macos-13
           - os: macos-14
           - os: macos-15
+          - os: macos-26
 
     runs-on: ${{matrix.os}}
     timeout-minutes: 10
@@ -747,9 +744,9 @@ jobs:
         include:
           - os: ubuntu-22.04
           - os: ubuntu-24.04
-          - os: macos-13
           - os: macos-14
           - os: macos-15
+          - os: macos-26
 
     runs-on: ${{matrix.os}}
     timeout-minutes: 10
@@ -824,9 +821,9 @@ jobs:
         include:
           - os: ubuntu-22.04
           - os: ubuntu-24.04
-          - os: macos-13
           - os: macos-14
           - os: macos-15
+          - os: macos-26
 
     runs-on: ${{matrix.os}}
     timeout-minutes: 20

appveyor.yml

@@ -27,17 +27,15 @@ environment:
       ADDRMD: 32,64
       CXXSTD: 14,17
 
-    # clang-win 32 bit fails to link with "unable to load mspdbcore.dll (error code: 126)"
-
-    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       TOOLSET: clang-win
       ADDRMD: 64
-      CXXSTD: 14,17,latest
+      CXXSTD: 14,17
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       TOOLSET: clang-win
       ADDRMD: 64
-      CXXSTD: 14,17,latest
+      CXXSTD: 20,latest
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       ADDPATH: C:\cygwin\bin;

benchmark/Jamfile

@@ -0,0 +1,8 @@
+# Copyright 2020, 2025 Peter Dimov
+# Distributed under the Boost Software License, Version 1.0.
+
+project : default-build release <link>static <cxxstd>17
+    : requirements <library>/boost/core//boost_core ;
+
+exe sv_find_first_of : sv_find_first_of.cpp ;
+exe sv_find_first_not_of : sv_find_first_not_of.cpp ;

benchmark/sv_find_first_not_of.cpp

@@ -0,0 +1,277 @@
+// Copyright 2021, 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/utility/string_view.hpp>
+#include <boost/core/detail/string_view.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/core/type_name.hpp>
+#include <boost/cstdint.hpp>
+#include <string_view>
+#include <chrono>
+#include <iostream>
+
+using namespace std::chrono_literals;
+
+template<class Sv> void test()
+{
+    constexpr char const* q1 = "{";
+    constexpr char const* q2 = "<(";
+    constexpr char const* q3 = " :=";
+    constexpr char const* q4 = " \t\r\n";
+    constexpr char const* q6 = " \t\r\n\f\v";
+    constexpr char const* q10 = "0123456789";
+    constexpr char const* q52 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
+
+    constexpr std::size_t npos = static_cast<std::size_t>( -1 );
+
+    constexpr std::size_t N = 1'000'000'000;
+
+    std::cout << boost::core::type_name<Sv>() << ":\n\n";
+
+    auto t0 = std::chrono::steady_clock::now();
+
+    {
+        constexpr char const* q = q1;
+
+        std::string s1( 1'000'000, q[ std::strlen( q ) - 1 ] );
+        std::string s2( 100, q[ std::strlen( q ) - 1 ] );
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s1.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s1 ).find_first_not_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_not_of( sv, \"" << q << "\" ) in " << s1.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s2.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s2 ).find_first_not_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_not_of( sv, \"" << q << "\" ) in " << s2.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+    }
+
+    {
+        constexpr char const* q = q2;
+
+        std::string s1( 1'000'000, q[ std::strlen( q ) - 1 ] );
+        std::string s2( 100, q[ std::strlen( q ) - 1 ] );
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s1.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s1 ).find_first_not_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_not_of( sv, \"" << q << "\" ) in " << s1.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s2.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s2 ).find_first_not_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_not_of( sv, \"" << q << "\" ) in " << s2.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+    }
+
+    {
+        constexpr char const* q = q3;
+
+        std::string s1( 1'000'000, q[ std::strlen( q ) - 1 ] );
+        std::string s2( 100, q[ std::strlen( q ) - 1 ] );
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s1.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s1 ).find_first_not_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_not_of( sv, \"" << q << "\" ) in " << s1.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s2.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s2 ).find_first_not_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_not_of( sv, \"" << q << "\" ) in " << s2.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+    }
+
+    {
+        constexpr char const* q = q4;
+
+        std::string s1( 1'000'000, q[ std::strlen( q ) - 1 ] );
+        std::string s2( 100, q[ std::strlen( q ) - 1 ] );
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s1.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s1 ).find_first_not_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_not_of( sv, \"" << q << "\" ) in " << s1.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s2.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s2 ).find_first_not_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_not_of( sv, \"" << q << "\" ) in " << s2.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+    }
+
+    {
+        constexpr char const* q = q6;
+
+        std::string s1( 1'000'000, q[ std::strlen( q ) - 1 ] );
+        std::string s2( 100, q[ std::strlen( q ) - 1 ] );
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s1.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s1 ).find_first_not_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_not_of( sv, \"" << q << "\" ) in " << s1.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s2.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s2 ).find_first_not_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_not_of( sv, \"" << q << "\" ) in " << s2.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+    }
+
+    {
+        constexpr char const* q = q10;
+
+        std::string s1( 1'000'000, q[ std::strlen( q ) - 1 ] );
+        std::string s2( 100, q[ std::strlen( q ) - 1 ] );
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s1.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s1 ).find_first_not_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_not_of( sv, \"" << q << "\" ) in " << s1.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s2.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s2 ).find_first_not_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_not_of( sv, \"" << q << "\" ) in " << s2.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+    }
+
+    {
+        constexpr char const* q = q52;
+
+        std::string s1( 1'000'000, q[ std::strlen( q ) - 1 ] );
+        std::string s2( 100, q[ std::strlen( q ) - 1 ] );
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s1.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s1 ).find_first_not_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_not_of( sv, \"" << q << "\" ) in " << s1.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s2.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s2 ).find_first_not_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_not_of( sv, \"" << q << "\" ) in " << s2.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+    }
+
+    auto tn = std::chrono::steady_clock::now();
+
+    std::cout << "\nTotal for " << boost::core::type_name<Sv>() << ": " << ( tn - t0 ) / 1ms << " ms\n\n";
+}
+
+int main()
+{
+    test<std::string_view>();
+    test<boost::string_view>();
+    test<boost::core::string_view>();
+
+    return boost::report_errors();
+}

benchmark/sv_find_first_of.cpp

@@ -0,0 +1,259 @@
+// Copyright 2021, 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/utility/string_view.hpp>
+#include <boost/core/detail/string_view.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/core/type_name.hpp>
+#include <boost/cstdint.hpp>
+#include <string_view>
+#include <chrono>
+#include <iostream>
+
+using namespace std::chrono_literals;
+
+template<class Sv> void test()
+{
+    std::string s1( 1'000'000, '\x00' );
+    std::string s2( 100, '\x00' );
+
+    constexpr char const* q1 = "{";
+    constexpr char const* q2 = "<(";
+    constexpr char const* q3 = " :=";
+    constexpr char const* q4 = " \t\r\n";
+    constexpr char const* q6 = " \t\r\n\f\v";
+    constexpr char const* q10 = "0123456789";
+    constexpr char const* q52 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
+
+    constexpr std::size_t npos = static_cast<std::size_t>( -1 );
+
+    constexpr std::size_t N = 1'000'000'000;
+
+    std::cout << boost::core::type_name<Sv>() << ":\n\n";
+
+    auto t0 = std::chrono::steady_clock::now();
+
+    {
+        constexpr char const* q = q1;
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s1.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s1 ).find_first_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_of( sv, \"" << q << "\" ) in " << s1.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s2.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s2 ).find_first_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_of( sv, \"" << q << "\" ) in " << s2.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+    }
+
+    {
+        constexpr char const* q = q2;
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s1.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s1 ).find_first_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_of( sv, \"" << q << "\" ) in " << s1.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s2.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s2 ).find_first_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_of( sv, \"" << q << "\" ) in " << s2.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+    }
+
+    {
+        constexpr char const* q = q3;
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s1.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s1 ).find_first_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_of( sv, \"" << q << "\" ) in " << s1.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s2.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s2 ).find_first_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_of( sv, \"" << q << "\" ) in " << s2.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+    }
+
+    {
+        constexpr char const* q = q4;
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s1.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s1 ).find_first_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_of( sv, \"" << q << "\" ) in " << s1.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s2.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s2 ).find_first_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_of( sv, \"" << q << "\" ) in " << s2.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+    }
+
+    {
+        constexpr char const* q = q6;
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s1.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s1 ).find_first_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_of( sv, \"" << q << "\" ) in " << s1.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s2.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s2 ).find_first_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_of( sv, \"" << q << "\" ) in " << s2.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+    }
+
+    {
+        constexpr char const* q = q10;
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s1.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s1 ).find_first_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_of( sv, \"" << q << "\" ) in " << s1.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s2.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s2 ).find_first_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_of( sv, \"" << q << "\" ) in " << s2.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+    }
+
+    {
+        constexpr char const* q = q52;
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s1.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s1 ).find_first_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_of( sv, \"" << q << "\" ) in " << s1.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+
+        {
+            auto t1 = std::chrono::steady_clock::now();
+
+            for( std::size_t i = 0; i < N / s2.size(); ++i )
+            {
+                BOOST_TEST_EQ( Sv( s2 ).find_first_of( q ), npos );
+            }
+
+            auto t2 = std::chrono::steady_clock::now();
+
+            std::cout << "find_first_of( sv, \"" << q << "\" ) in " << s2.size() << " bytes: " << ( t2 - t1 ) / 1ms << " ms\n";
+        }
+    }
+
+    auto tn = std::chrono::steady_clock::now();
+
+    std::cout << "\nTotal for " << boost::core::type_name<Sv>() << ": " << ( tn - t0 ) / 1ms << " ms\n\n";
+}
+
+int main()
+{
+    test<std::string_view>();
+    test<boost::string_view>();
+    test<boost::core::string_view>();
+
+    return boost::report_errors();
+}

doc/null_deleter.qbk

@@ -18,7 +18,7 @@
 The header `<boost/core/null_deleter.hpp>` defines the `boost::null_deleter` function object,
 which can be used as a deleter with smart pointers such as `unique_ptr` or `shared_ptr`. The
 deleter doesn't do anything with the pointer provided upon deallocation, which makes it useful
-when the pointed object is deallocated elsewhere.
+when the pointed object doesn't need to be deallocated or is deallocated elsewhere.
 
 [section Example]
 ``

include/boost/core/span.hpp

@@ -23,9 +23,15 @@ class span;
 
 namespace detail {
 
-template<class U, class T>
+template<class U, class T, class = void>
 struct span_convertible {
-    static constexpr bool value = std::is_convertible<U(*)[], T(*)[]>::value;
+    static constexpr bool value = false;
+};
+
+template<class U, class T>
+struct span_convertible<U, T, typename
+    std::enable_if<std::is_convertible<U(*)[], T(*)[]>::value>::type> {
+    static constexpr bool value = true;
 };
 
 template<std::size_t E, std::size_t N>

test/span_constructible_test.cpp

@@ -25,6 +25,20 @@ struct range {
     }
 };
 
+struct buffer {
+    void* data() {
+        return 0;
+    }
+
+    const void* data() const {
+        return 0;
+    }
+
+    std::size_t size() const {
+        return 0;
+    }
+};
+
 struct base { };
 
 struct derived
@@ -138,6 +152,10 @@ void test_range()
         const range<int>&>));
     BOOST_TEST_TRAIT_FALSE((std::is_constructible<boost::span<base>,
         range<derived>&>));
+    BOOST_TEST_TRAIT_FALSE((std::is_constructible<boost::span<int>,
+        buffer>));
+    BOOST_TEST_TRAIT_FALSE((std::is_constructible<boost::span<int>,
+        const buffer&>));
 }
 
 void test_initializer_list()