Fix warnings in boost test matrix

* should fix warnings for gcc-6, clang-3.8 to 4.0, msvc-14.1 and 14.2
* should fix errors for clang-6 with cxxstd=17, clang-8 with cxxstd=2a

.appveyor.yml

@@ -49,6 +49,7 @@ test_script:
       ../../b2 $B2_OPTS toolset=gcc-9 cxxstd=latest examples test//all &&
       ../../b2 $B2_OPTS toolset=clang cxxstd=latest variant=histogram_ubasan test//all
 
-on_failure:
-  # Uncomment the following line to stop VM and enable interactive login
-  - ps: $blockRdp = $true; iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-rdp.ps1'))
+## Uncomment the following to stop VM and enable interactive login.
+## Instructions how to log into the Appveyor VM are automatically printed.
+# on_failure:
+#   - ps: $blockRdp = $true; iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-rdp.ps1'))

include/boost/histogram/detail/args_type.hpp

@@ -8,12 +8,12 @@
 #define BOOST_HISTOGRAM_DETAIL_ARGS_TYPE_HPP
 
 #include <boost/config/workaround.hpp>
-#if BOOST_WORKAROUND(BOOST_GCC, >= 60000)
+#if BOOST_WORKAROUND(BOOST_GCC, >= 65000)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wnoexcept-type"
 #endif
 #include <boost/callable_traits/args.hpp>
-#if BOOST_WORKAROUND(BOOST_GCC, >= 60000)
+#if BOOST_WORKAROUND(BOOST_GCC, >= 65000)
 #pragma GCC diagnostic pop
 #endif
 #include <boost/mp11/list.hpp>    // mp_pop_front

include/boost/histogram/detail/span.hpp

@@ -19,7 +19,9 @@ using std::span;
 } // namespace histogram
 } // namespace boost
 
-#else
+#else // C++17 span not available, so we use our implementation
+
+// to be replaced by boost::span
 
 #include <array>
 #include <boost/assert.hpp>
@@ -221,13 +223,13 @@ public:
 #endif
 
 template <class T>
-span<T> make_span(T* begin, T* end) {
-  return span<T>{begin, end};
+auto make_span(T* begin, T* end) {
+  return dtl::span<T>{begin, end};
 }
 
 template <class T>
-span<T> make_span(T* begin, std::size_t size) {
-  return span<T>{begin, size};
+auto make_span(T* begin, std::size_t size) {
+  return dtl::span<T>{begin, size};
 }
 
 template <class Container, class = decltype(dtl::size(std::declval<Container>()),
@@ -237,8 +239,8 @@ auto make_span(const Container& cont) {
 }
 
 template <class T, std::size_t N>
-span<T> make_span(T (&arr)[N]) {
-  return span<T>(arr, N);
+auto make_span(T (&arr)[N]) {
+  return dtl::span<T>(arr, N);
 }
 
 } // namespace detail

include/boost/histogram/fwd.hpp

@@ -12,8 +12,7 @@
   Forward declarations, tag types and type aliases.
 */
 
-#include <boost/config.hpp>            // BOOST_ATTRIBUTE_NODISCARD
-#include <boost/config/workaround.hpp> // BOOST_WORKAROUND
+#include <boost/config.hpp> // BOOST_ATTRIBUTE_NODISCARD
 #include <boost/core/use_default.hpp>
 #include <vector>
 
@@ -120,18 +119,19 @@ using weighted_profile_storage = dense_storage<accumulators::weighted_mean<>>;
 
 #ifndef BOOST_HISTOGRAM_DOXYGEN_INVOKED
 
-#if BOOST_WORKAROUND(BOOST_CLANG, >= 0)
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wc++17-extensions"
+// hot-fix until patch is accepted upstream in boost.config
+#ifdef __has_cpp_attribute
+#undef BOOST_ATTRIBUTE_NODISCARD
+#if __has_cpp_attribute(nodiscard) && !(defined(__clang__) && (__cplusplus < 201700))
+#define BOOST_ATTRIBUTE_NODISCARD [[nodiscard]]
+#else
+#define BOOST_ATTRIBUTE_NODISCARD
+#endif
 #endif
 
 template <class Axes, class Storage = default_storage>
 class BOOST_ATTRIBUTE_NODISCARD histogram;
 
-#if BOOST_WORKAROUND(BOOST_CLANG, >= 0)
-#pragma GCC diagnostic pop
-#endif
-
 #endif
 } // namespace histogram
 } // namespace boost

test/utility_allocator.hpp

@@ -45,7 +45,7 @@ private:
   BOOST_ATTRIBUTE_UNUSED inline void log_impl() {}
 
   template <class T, class... Ts>
-  inline void log_impl(T&& t, Ts&&... ts) {
+  void log_impl(T&& t, Ts&&... ts) {
     std::cerr << t;
     log_impl(std::forward<Ts>(ts)...);
   }
@@ -81,18 +81,18 @@ struct tracing_allocator {
       } else
         db->log("allocator +", n, " ", boost::histogram::detail::type_name<T>());
       auto& p = db->at<T>();
-      p.first += n;
-      p.second += n;
-      db->first += n * sizeof(T);
-      db->second += n * sizeof(T);
+      p.first += static_cast<int>(n);
+      p.second += static_cast<int>(n);
+      db->first += static_cast<int>(n * sizeof(T));
+      db->second += static_cast<int>(n * sizeof(T));
     }
     return static_cast<T*>(::operator new(n * sizeof(T)));
   }
 
   void deallocate(T* p, std::size_t n) {
     if (db) {
-      db->at<T>().first -= n;
-      db->first -= n * sizeof(T);
+      db->at<T>().first -= static_cast<int>(n);
+      db->first -= static_cast<int>(n * sizeof(T));
       db->log("allocator -", n, " ", boost::histogram::detail::type_name<T>());
     }
     ::operator delete((void*)p);