Switch to from c++2a to c++20 for gcc-10 and clang-10.

libc++ disables std::uncaught_exceptions for Mac OS < 10.12 and
iOS < 10.0, even though it defines __cpp_lib_uncaught_exceptions.

Fixes https://github.com/boostorg/core/issues/80.

.travis.yml

@@ -129,7 +129,7 @@ matrix:
     - os: linux
       dist: bionic
       compiler: g++-10
-      env: TOOLSET=gcc COMPILER=g++-10 CXXSTD=03,11,14,17,2a
+      env: TOOLSET=gcc COMPILER=g++-10 CXXSTD=03,11,14,17,20
       addons:
         apt:
           packages:
@@ -140,7 +140,7 @@ matrix:
     - os: linux
       dist: bionic
       compiler: g++-10
-      env: UBSAN=1 TOOLSET=gcc COMPILER=g++-10 CXXSTD=03,11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1 LINKFLAGS=-fuse-ld=gold
+      env: UBSAN=1 TOOLSET=gcc COMPILER=g++-10 CXXSTD=03,11,14,17,20 UBSAN_OPTIONS=print_stacktrace=1 LINKFLAGS=-fuse-ld=gold
       addons:
         apt:
           packages:
@@ -287,7 +287,7 @@ matrix:
 
     - os: linux
       compiler: clang++-10
-      env: TOOLSET=clang COMPILER=clang++-10 CXXSTD=03,11,14,17,2a
+      env: TOOLSET=clang COMPILER=clang++-10 CXXSTD=03,11,14,17,20
       addons:
         apt:
           packages:
@@ -299,7 +299,7 @@ matrix:
 
     - os: linux
       compiler: clang++-10
-      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-10 CXXSTD=03,11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1
+      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-10 CXXSTD=03,11,14,17,20 UBSAN_OPTIONS=print_stacktrace=1
       addons:
         apt:
           packages:

include/boost/core/allocator_access.hpp

@@ -137,6 +137,11 @@ template<class A>
 struct allocator_difference_type {
     typedef typename A::difference_type type;
 };
+#elif defined(BOOST_MSVC)
+template<class A>
+struct allocator_difference_type {
+    typedef typename std::allocator_traits<A>::difference_type type;
+};
 #else
 template<class A, class = void>
 struct allocator_difference_type {
@@ -156,6 +161,11 @@ template<class A>
 struct allocator_size_type {
     typedef typename A::size_type type;
 };
+#elif defined(BOOST_MSVC)
+template<class A>
+struct allocator_size_type {
+    typedef typename std::allocator_traits<A>::size_type type;
+};
 #else
 template<class A, class = void>
 struct allocator_size_type {

include/boost/core/uncaught_exceptions.hpp

@@ -1,5 +1,5 @@
 /*
- *             Copyright Andrey Semashev 2018.
+ *          Copyright Andrey Semashev 2018 - 2020.
  * Distributed under the Boost Software License, Version 1.0.
  *    (See accompanying file LICENSE_1_0.txt or copy at
  *          https://www.boost.org/LICENSE_1_0.txt)
@@ -26,8 +26,23 @@
 #pragma once
 #endif
 
-// Visual Studio 14 supports N4152 std::uncaught_exceptions()
-#if (defined(__cpp_lib_uncaught_exceptions) && __cpp_lib_uncaught_exceptions >= 201411) || \
+#if defined(__APPLE__)
+#include <Availability.h>
+// Apple systems only support std::uncaught_exceptions starting with specific versions:
+// - Mac OS >= 10.12
+// - iOS >= 10.0
+// - tvOS >= 10.0
+// - watchOS >= 3.0
+// https://github.com/boostorg/core/issues/80
+#if (defined(__cpp_lib_uncaught_exceptions) && __cpp_lib_uncaught_exceptions >= 201411) && \
+    ( \
+        (defined(__MAC_OS_X_VERSION_MIN_REQUIRED) && __MAC_OS_X_VERSION_MIN_REQUIRED >= 101200) || \
+        (defined(__IPHONE_OS_VERSION_MIN_REQUIRED) && __IPHONE_OS_VERSION_MIN_REQUIRED >= 100000) \
+    )
+#define BOOST_CORE_HAS_UNCAUGHT_EXCEPTIONS
+#endif
+// Visual Studio 14.0 supports N4152 std::uncaught_exceptions() but doesn't define __cpp_lib_uncaught_exceptions
+#elif (defined(__cpp_lib_uncaught_exceptions) && __cpp_lib_uncaught_exceptions >= 201411) || \
     (defined(_MSC_VER) && _MSC_VER >= 1900)
 #define BOOST_CORE_HAS_UNCAUGHT_EXCEPTIONS
 #endif
@@ -46,7 +61,11 @@
 #if defined(BOOST_CORE_HAS_CXXABI_H)
 // MinGW GCC 4.4 seem to not work the same way the newer GCC versions do. As a result, __cxa_get_globals based implementation will always return 0.
 // Just disable it for now and fall back to std::uncaught_exception().
-#if !(defined(__MINGW32__) && (defined(__GNUC__) && (__GNUC__ * 100 + __GNUC_MINOR__) < 405))
+// On AIX, xlclang++ does have cxxabi.h but doesn't have __cxa_get_globals (https://github.com/boostorg/core/issues/78).
+#if !( \
+        (defined(__MINGW32__) && (defined(__GNUC__) && (__GNUC__ * 100 + __GNUC_MINOR__) < 405)) || \
+        defined(__ibmxl__) \
+    )
 #include <cxxabi.h>
 #include <cstring>
 #define BOOST_CORE_HAS_CXA_GET_GLOBALS
@@ -76,7 +95,7 @@ extern "C" __cxa_eh_globals* __cxa_get_globals() BOOST_NOEXCEPT_OR_NOTHROW __att
 #endif
 } // namespace __cxxabiv1
 #endif
-#endif // !(defined(__MINGW32__) && (defined(__GNUC__) && (__GNUC__ * 100 + __GNUC_MINOR__) < 405))
+#endif
 #endif // defined(BOOST_CORE_HAS_CXXABI_H)
 
 #if defined(_MSC_VER) && _MSC_VER >= 1400

test/lightweight_test_with_fail.cpp

@@ -13,17 +13,17 @@ template <typename T>
 struct with_tolerance
 {
     with_tolerance( T tolerance ): tolerance( tolerance )
-	{
-	}
+    {
+    }
 
-	bool operator()( T lhs, T rhs ) const
+    bool operator()( T lhs, T rhs ) const
     {
         return std::abs( lhs - rhs ) <= tolerance;
     }
 
 private:
 
-	T tolerance;
+    T tolerance;
 };
 
 void test_tolerance_predicate()