Type which are convertible to a number should not participate in arit…

…hmetic operator overloads.

Fixes #608

include/boost/multiprecision/detail/number_base.hpp

@@ -153,7 +153,8 @@ struct is_number_expression<detail::expression<tag, Arg1, Arg2, Arg3, Arg4> > :
 template <class T, class Num>
 struct is_compatible_arithmetic_type
     : public std::integral_constant<bool, 
-          std::is_convertible<T, Num>::value && !std::is_same<T, Num>::value && !is_number_expression<T>::value>
+          std::is_convertible<T, Num>::value && !std::is_same<T, Num>::value && !is_number_expression<T>::value
+          && (std::is_constructible<typename Num::backend_type, T>::value || std::is_assignable<typename Num::backend_type, T>::value)>
 {};
 
 namespace detail {

test/Jamfile.v2

@@ -1240,6 +1240,7 @@ test-suite misc :
                                     [ check-target-builds ../config//has_float128 : <source>quadmath <define>TEST_FLOAT128 ]
                                     <define>TEST_CPP_DEC_FLOAT 
                                     <define>TEST_CPP_BIN_FLOAT ]
+      [ compile git_issue_608.cpp ]
       [ compile git_issue_98.cpp : 
          [ check-target-builds ../config//has_float128 : <define>TEST_FLOAT128 <source>quadmath : ]
          [ check-target-builds ../config//has_gmp : <define>TEST_GMP <source>gmp : ]

test/git_issue_608.cpp

@@ -0,0 +1,15 @@
+///////////////////////////////////////////////////////////////////////////////
+//  Copyright 2024 John Maddock. 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)
+
+#include <functional>
+#include <boost/multiprecision/cpp_bin_float.hpp>
+
+using big_float_type = boost::multiprecision::cpp_bin_float_100;
+
+int main()
+{
+   static_assert(boost::multiprecision::is_compatible_arithmetic_type<std::reference_wrapper<big_float_type>, big_float_type>::value == 0);
+}
+