Relax C++11 type_traits requirements.

This replaces the cxx11_hdr_type_traits requirement with a local
configure-time check for the presence of only those type traits
which are used in the library. This enables testing of gcc 4.8 and
4.9, which was previously silently skipped.

build.jam

@@ -16,7 +16,8 @@ constant boost_dependencies :
     /boost/core//boost_core
     /boost/type_traits//boost_type_traits ;
 
-local cxx_requirements = [ requires
+local cxx_requirements =
+    [ requires
         exceptions
         sfinae_expr
         cxx11_constexpr
@@ -28,8 +29,9 @@ local cxx_requirements = [ requires
         cxx11_defaulted_functions
         cxx11_deleted_functions
         cxx11_function_template_default_args
-    cxx11_hdr_type_traits
-] ;
+    ]
+    [ check-target-builds config//has_sufficient_cxx11_type_traits "has <type_traits> sufficient for Boost.Scope" : : <build>no ]
+    ;
 
 project /boost/scope
     : common-requirements

config/Jamfile

@@ -0,0 +1,9 @@
+#
+#             Copyright Andrey Semashev 2025.
+# Distributed under the Boost Software License, Version 1.0.
+#    (See accompanying file LICENSE_1_0.txt or copy at
+#          http://www.boost.org/LICENSE_1_0.txt)
+#
+
+obj has_sufficient_cxx11_type_traits : has_sufficient_cxx11_type_traits.cpp : <library>/boost/config//boost_config ;
+explicit has_sufficient_cxx11_type_traits ;

config/has_sufficient_cxx11_type_traits.cpp

@@ -0,0 +1,38 @@
+/*
+ *             Copyright Andrey Semashev 2025.
+ * Distributed under the Boost Software License, Version 1.0.
+ *    (See accompanying file LICENSE_1_0.txt or copy at
+ *          http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+// The test verifies that the compiler provides a <type_traits> header that is
+// sufficient for Boost.Scope needs.
+
+#include <type_traits>
+
+int main(int, char*[])
+{
+    using std::integral_constant;
+    using std::true_type;
+    using std::false_type;
+    using std::conditional;
+    using std::enable_if;
+
+    using std::decay;
+
+    using std::is_class;
+    using std::is_reference;
+
+    using std::is_constructible;
+    using std::is_nothrow_constructible;
+    using std::is_move_constructible;
+    using std::is_nothrow_move_constructible;
+    using std::is_default_constructible;
+    using std::is_nothrow_default_constructible;
+    using std::is_assignable;
+    using std::is_nothrow_assignable;
+    using std::is_move_assignable;
+    using std::is_nothrow_move_assignable;
+
+    return 0;
+}