From 4daa1baf6d5eaaf16bfc6294e1ac3dd0e051de3f Mon Sep 17 00:00:00 2001
From: jzmaddock <john@johnmaddock.co.uk>
Date: Mon, 25 Dec 2017 18:07:48 +0000
Subject: [PATCH] Multiprecision: disable explicit conversion operator if the
 target type is already constructible from *this. Prevents ambiguity between
 the target types constructor and the conversion operator. See
 https://github.com/boostorg/multiprecision/issues/30.

---
 include/boost/multiprecision/number.hpp | 6 +++++-
 test/Jamfile.v2                         | 1 +
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/boost/multiprecision/number.hpp b/include/boost/multiprecision/number.hpp
index dc61d696..f99ef2a9 100644
--- a/include/boost/multiprecision/number.hpp
+++ b/include/boost/multiprecision/number.hpp
@@ -628,8 +628,12 @@ public:
       return this->template convert_to<T>();
    }
 #  else
+#if BOOST_WORKAROUND(BOOST_MSVC, < 1900)
    template <class T>
-   explicit operator T()const
+#else
+   template <class T, class = typename boost::disable_if_c<boost::is_constructible<T, self_type const&>::value || !boost::is_default_constructible<T>::value, T>::type>
+#endif
+   explicit operator T ()const
    {
       return this->template convert_to<T>();
    }
diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index 45e07b9d..c4561d4b 100644
--- a/test/Jamfile.v2
+++ b/test/Jamfile.v2
@@ -863,6 +863,7 @@ run test_float_conversions.cpp no_eh_support ;
 #
 compile bug11922.cpp ;
 run bug12039.cpp no_eh_support ;
+compile git_issue_30.cpp ;
 run test_hash.cpp : : : 
    [ check-target-builds ../config//has_float128 : <define>TEST_FLOAT128 <source>quadmath : ]
    [ check-target-builds ../config//has_gmp : <define>TEST_GMP <source>gmp : ]