Add expanded reproducer test set

Author: mborland

test/Jamfile.v2

@@ -1281,6 +1281,7 @@ test-suite misc :
       [ run git_issue_636.cpp : : : [ check-target-builds ../config//has_float128 : <source>quadmath : <build>no ] ]
       [ run git_issue_652.cpp ]
       [ run git_issue_734.cpp ]
+      [ run git_issue_759.cpp : : : [ check-target-builds ../config//has_float128 : <source>quadmath : <build>no ] ]
       [ 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_759.cpp

@@ -0,0 +1,68 @@
+///////////////////////////////////////////////////////////////////////////////
+//  Copyright 2026 Matt Borland. 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)
+//
+// See: https://github.com/boostorg/multiprecision/issues/759
+
+#include <complex>
+#include <iostream>
+#include <limits>
+#include <boost/multiprecision/float128.hpp>
+#include "test.hpp"
+
+int main()
+{
+   using REAL = boost::multiprecision::float128;
+
+   const REAL inf_v = std::numeric_limits<REAL>::infinity();
+
+   {
+      using std::abs;
+      const REAL r = abs(inf_v);
+      BOOST_CHECK((boost::multiprecision::isinf)(r));
+   }
+
+   {
+      using std::abs;
+      const std::complex<REAL> z{inf_v, REAL(0)};
+      const REAL r = abs(z);
+      BOOST_CHECK((boost::multiprecision::isinf)(r));
+      BOOST_CHECK(!(boost::multiprecision::isnan)(r));
+   }
+
+   {
+      using std::abs;
+      const std::complex<REAL> z{REAL(0), inf_v};
+      const REAL r = abs(z);
+      BOOST_CHECK((boost::multiprecision::isinf)(r));
+      BOOST_CHECK(!(boost::multiprecision::isnan)(r));
+   }
+
+   {
+      using std::abs;
+      const std::complex<REAL> z{-inf_v, REAL(3)};
+      const REAL r = abs(z);
+      BOOST_CHECK((boost::multiprecision::isinf)(r));
+      BOOST_CHECK(!(boost::multiprecision::isnan)(r));
+   }
+
+   {
+      // Per IEEE 754: hypot(inf, NaN) = inf.
+      using std::abs;
+      const REAL nan_v = std::numeric_limits<REAL>::quiet_NaN();
+      const std::complex<REAL> z{inf_v, nan_v};
+      const REAL r = abs(z);
+      BOOST_CHECK((boost::multiprecision::isinf)(r));
+   }
+
+   {
+      // Sanity check on a finite value: abs(3 + 4i) == 5.
+      using std::abs;
+      const std::complex<REAL> z{REAL(3), REAL(4)};
+      const REAL r = abs(z);
+      BOOST_CHECK_EQUAL(r, REAL(5));
+   }
+
+   return boost::report_errors();
+}