Correct eval_increment corner case.

Fixes https://github.com/boostorg/multiprecision/issues/277.

include/boost/multiprecision/cpp_int/add.hpp

@@ -263,7 +263,7 @@ eval_increment(cpp_int_backend<MinBits1, MaxBits1, SignType1, Checked1, Allocato
    else if (result.sign() && result.limbs()[0])
    {
       --result.limbs()[0];
-      if (!result.limbs()[0])
+      if (!result.limbs()[0] && (result.size() == 1))
          result.sign(false);
    }
    else

test/Jamfile.v2

@@ -1024,6 +1024,7 @@ test-suite misc :
       [ run git_issue_175.cpp ]
       [ run git_issue_248.cpp ]
       [ run git_issue_265.cpp : : : [ check-target-builds ../config//has_mpfr : <source>gmp <source>mpfr : <build>no ] ]
+      [ run git_issue_277.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_277.cpp

@@ -0,0 +1,40 @@
+///////////////////////////////////////////////////////////////////////////////
+//  Copyright 2019 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 <boost/multiprecision/cpp_int.hpp>
+#include "test.hpp"
+
+template <class I>
+void test()
+{
+   I val(1);
+   val <<= 512;
+   I t(val);
+   ++t;
+   BOOST_CHECK_EQUAL(t - val, 1);
+   --t;
+   BOOST_CHECK_EQUAL(t, val);
+   --t;
+   BOOST_CHECK_EQUAL(val - t, 1);
+
+   val = -val;
+   t   = val;
+   --t;
+   BOOST_CHECK_EQUAL(t - val, -1);
+   ++t;
+   BOOST_CHECK_EQUAL(t, val);
+   ++t;
+   BOOST_CHECK_EQUAL(val - t, -1);
+}
+
+int main()
+{
+   test<boost::multiprecision::cpp_int>();
+   test<boost::multiprecision::int1024_t>();
+   test<boost::multiprecision::checked_int1024_t>();
+   return boost::report_errors();
+}
+
+