Remove Clang 3.5, 3.6 from GHA

detail::span_convertible handles void types

.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"

appveyor.yml

@@ -27,17 +27,10 @@ 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
-      TOOLSET: clang-win
-      ADDRMD: 64
-      CXXSTD: 14,17,latest
-
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       TOOLSET: clang-win
       ADDRMD: 64
-      CXXSTD: 14,17,latest
+      CXXSTD: 14,17,20,latest
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       ADDPATH: C:\cygwin\bin;

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()