diff --git a/.circleci/config.yml b/.circleci/config.yml
index e20005e1b..f689048f4 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -73,11 +73,35 @@ jobs:
     - run:
         name: Build and test
         command: bash ./tools/circleci_bionic_clang6.sh
+  focal_clang9_msan_00:
+    docker:
+    - image: circleci/buildpack-deps:focal
+    steps:
+    - checkout
+    - run:
+        name: Build and test
+        command: bash ./tools/circleci_focal_clang9_msan.sh
+        environment:
+          TEST_NSPLIT: 2
+          SPLIT_TEST_NUM: 0
+  focal_clang9_msan_01:
+    docker:
+    - image: circleci/buildpack-deps:focal
+    steps:
+    - checkout
+    - run:
+        name: Build and test
+        command: bash ./tools/circleci_focal_clang9_msan.sh
+        environment:
+          TEST_NSPLIT: 2
+          SPLIT_TEST_NUM: 1
 
 workflows:
   version: 2
   all_builds:
     jobs:
+    - focal_clang9_msan_00
+    - focal_clang9_msan_01
     - bionic_clang6
     - bionic_gcc7_conda_coverage
     - bionic_gcc7_conda_release
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 5708118e5..8c2e0203e 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -80,6 +80,11 @@ endfunction()
 
 ADD_MPPP_TESTCASE(concepts)
 ADD_MPPP_TESTCASE(global_header)
+# NOTE: the interop test requires all optional
+# deps to be enabled.
+if(MPPP_WITH_QUADMATH AND MPPP_WITH_MPFR AND MPPP_WITH_MPC)
+  ADD_MPPP_TESTCASE(interop_test)
+endif()
 
 ADD_MPPP_TESTCASE(integer_abs)
 ADD_MPPP_TESTCASE(integer_addsub_ui_si)
diff --git a/test/interop_test.cpp b/test/interop_test.cpp
new file mode 100644
index 000000000..34ac47c9d
--- /dev/null
+++ b/test/interop_test.cpp
@@ -0,0 +1,270 @@
+// Copyright 2016-2020 Francesco Biscani (bluescarni@gmail.com)
+//
+// This file is part of the mp++ library.
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#if defined(__clang__) || defined(__GNUC__)
+
+// NOTE: range warnings in the interop between
+// real128 and __(u)int128_t.
+#pragma GCC diagnostic ignored "-Wconversion"
+
+#endif
+
+#include <complex>
+#include <tuple>
+#include <type_traits>
+
+#include <mp++/mp++.hpp>
+
+#include "catch.hpp"
+#include "test_utils.hpp"
+
+// NOLINTNEXTLINE(google-build-using-namespace)
+using namespace mppp;
+// NOLINTNEXTLINE(google-build-using-namespace)
+using namespace mppp_test;
+
+using mppp_types = std::tuple<integer<1>, rational<1>, real, real128, complex128, complex>;
+
+using cpp_types = std::tuple<int, double, std::complex<double>
+#if defined(MPPP_HAVE_GCC_INT128)
+                             ,
+                             __int128_t, __uint128_t
+#endif
+                             >;
+
+template <typename T>
+struct is_mppp_complex : std::false_type {
+};
+
+template <>
+struct is_mppp_complex<complex128> : std::true_type {
+};
+
+template <>
+struct is_mppp_complex<complex> : std::true_type {
+};
+
+struct mppp_interop_tester {
+    template <typename A, typename B>
+    static void run_ineq_cmp(const A &a, const B &b, std::false_type)
+    {
+        REQUIRE(!(a < b));
+        REQUIRE(a <= b);
+        REQUIRE(!(a > b));
+        REQUIRE(a >= b);
+    }
+    template <typename A, typename B>
+    static void run_ineq_cmp(const A &, const B &, std::true_type)
+    {
+    }
+
+    template <typename T>
+    struct runner {
+        template <typename U>
+        void operator()(const U &) const
+        {
+            // Construct T from U.
+            T x1{U{42}};
+            REQUIRE(x1 == 42);
+
+            // Assign U to T.
+            U y1{43};
+            x1 = y1;
+            REQUIRE(x1 == 43);
+
+            // Convert T to U.
+            // NOTE: this is probably not strictly necessary,
+            // as it is covered by switching around T and U.
+            REQUIRE(static_cast<U>(x1) == 43);
+
+            // Basic binary arithmetic.
+            REQUIRE(x1 + U{4} == 47);
+            REQUIRE(x1 - U{4} == 39);
+            REQUIRE(x1 * U{2} == 86);
+            x1 = 10;
+            REQUIRE(x1 / U{2} == 5);
+
+            // Basic in-place arithmetics.
+            REQUIRE((x1 += U{1}) == 11);
+            REQUIRE((x1 -= U{1}) == 10);
+            REQUIRE((x1 *= U{2}) == 20);
+            REQUIRE((x1 /= U{2}) == 10);
+
+            // Exponentiation.
+            REQUIRE(mppp::pow(x1, U{2}) == 100);
+
+            // Comparison.
+            REQUIRE(x1 == U{10});
+            REQUIRE(x1 != U{11});
+
+            run_ineq_cmp(x1, U{10}, std::integral_constant < bool,
+                         is_mppp_complex<T>::value || is_mppp_complex<U>::value > {});
+        }
+        void operator()(const T &) const {}
+    };
+    template <typename T>
+    void operator()(const T &) const
+    {
+        tuple_for_each(mppp_types{}, runner<T>{});
+    }
+};
+
+TEST_CASE("mp++ interop")
+{
+    tuple_for_each(mppp_types{}, mppp_interop_tester{});
+}
+
+struct mppp_cpp_interop_tester {
+    template <typename A, typename B>
+    static void run_ineq_cmp(const A &a, const B &b, std::false_type)
+    {
+        REQUIRE(!(a < b));
+        REQUIRE(a <= b);
+        REQUIRE(!(a > b));
+        REQUIRE(a >= b);
+    }
+    template <typename A, typename B>
+    static void run_ineq_cmp(const A &, const B &, std::true_type)
+    {
+    }
+
+    template <typename T>
+    struct runner {
+        template <typename U>
+        void operator()(const U &) const
+        {
+            // NOTE: here T is an mp++ type,
+            // U a C++ type.
+
+            // Construct T from U.
+            T x1{U{42}};
+            REQUIRE(x1 == 42);
+
+            // Assign U to T.
+            U y1{43};
+            x1 = y1;
+            REQUIRE(x1 == 43);
+
+            // Convert T to U.
+            REQUIRE(static_cast<U>(x1) == U{43});
+
+            // Basic binary arithmetic.
+            REQUIRE(x1 + U{4} == 47.);
+            REQUIRE(x1 - U{4} == 39.);
+            REQUIRE(x1 * U{2} == 86.);
+            x1 = 10;
+            REQUIRE(x1 / U{2} == 5.);
+
+            // Basic in-place arithmetics.
+            REQUIRE((x1 += U{1}) == 11);
+            REQUIRE((x1 -= U{1}) == 10);
+            REQUIRE((x1 *= U{2}) == 20);
+            REQUIRE((x1 /= U{2}) == 10);
+
+            // Exponentiation.
+            // NOTE: don't check the result due to roundoff errors.
+            (void)mppp::pow(x1, U{1});
+
+            // Comparison.
+            REQUIRE(x1 == U{10});
+            REQUIRE(x1 != U{11});
+
+            run_ineq_cmp(x1, U{10}, std::integral_constant < bool,
+                         is_mppp_complex<T>::value || is_cpp_complex<U>::value > {});
+        }
+        void operator()(const T &) const {}
+    };
+    template <typename T>
+    void operator()(const T &) const
+    {
+        tuple_for_each(cpp_types{}, runner<T>{});
+    }
+};
+
+TEST_CASE("mppp cpp interop")
+{
+    tuple_for_each(mppp_types{}, mppp_cpp_interop_tester{});
+}
+
+struct cpp_mppp_interop_tester {
+    template <typename A, typename B>
+    static void run_ineq_cmp(const A &a, const B &b, std::false_type)
+    {
+        REQUIRE(!(a < b));
+        REQUIRE(a <= b);
+        REQUIRE(!(a > b));
+        REQUIRE(a >= b);
+    }
+    template <typename A, typename B>
+    static void run_ineq_cmp(const A &, const B &, std::true_type)
+    {
+    }
+
+    template <typename T>
+    struct runner {
+        template <typename U>
+        void operator()(const U &) const
+        {
+            // NOTE: here T is a C++ type,
+            // U an mp++ type.
+
+            // Construct T from U.
+            T x1{U{42}};
+            // NOLINTNEXTLINE(clang-diagnostic-implicit-int-float-conversion)
+            REQUIRE(x1 == 42.);
+
+            // Assign U to T.
+            U y1{43};
+            x1 = static_cast<T>(y1);
+            // NOLINTNEXTLINE(clang-diagnostic-implicit-int-float-conversion)
+            REQUIRE(x1 == 43.);
+
+            // Convert T to U.
+            REQUIRE(static_cast<U>(x1) == U{43});
+
+            // Basic binary arithmetic.
+            REQUIRE(x1 + U{4} == 47.);
+            REQUIRE(x1 - U{4} == 39.);
+            REQUIRE(x1 * U{2} == 86.);
+            x1 = 10;
+            REQUIRE(x1 / U{2} == 5.);
+
+            // Basic in-place arithmetics.
+            // NOLINTNEXTLINE(clang-diagnostic-implicit-int-float-conversion)
+            REQUIRE((x1 += U{1}) == 11.);
+            // NOLINTNEXTLINE(clang-diagnostic-implicit-int-float-conversion)
+            REQUIRE((x1 -= U{1}) == 10.);
+            // NOLINTNEXTLINE(clang-diagnostic-implicit-int-float-conversion)
+            REQUIRE((x1 *= U{2}) == 20.);
+            // NOLINTNEXTLINE(clang-diagnostic-implicit-int-float-conversion)
+            REQUIRE((x1 /= U{2}) == 10.);
+
+            // Exponentiation.
+            // NOTE: don't check the result due to roundoff errors.
+            (void)mppp::pow(x1, U{1});
+
+            // Comparison.
+            REQUIRE(x1 == U{10});
+            REQUIRE(x1 != U{11});
+
+            run_ineq_cmp(x1, U{10}, std::integral_constant < bool,
+                         is_cpp_complex<T>::value || is_mppp_complex<U>::value > {});
+        }
+        void operator()(const T &) const {}
+    };
+    template <typename T>
+    void operator()(const T &) const
+    {
+        tuple_for_each(mppp_types{}, runner<T>{});
+    }
+};
+
+TEST_CASE("cpp mppp interop")
+{
+    tuple_for_each(cpp_types{}, cpp_mppp_interop_tester{});
+}
diff --git a/tools/circleci_focal_clang9_msan.sh b/tools/circleci_focal_clang9_msan.sh
new file mode 100644
index 000000000..a43caa6da
--- /dev/null
+++ b/tools/circleci_focal_clang9_msan.sh
@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+
+# Echo each command
+set -x
+
+# Exit on error.
+set -e
+
+# Core deps.
+sudo apt-get install build-essential cmake wget clang ninja-build
+
+# Create the build dir and cd into it.
+mkdir build
+cd build
+MPPP_BUILD_DIR=`pwd`
+
+# libc++ setup. See:
+# https://github.com/google/sanitizers/wiki/MemorySanitizerLibcxxHowTo
+git clone --depth=1 https://github.com/llvm/llvm-project
+cd llvm-project
+mkdir build
+cd build
+LLVM_BUILD_DIR=`pwd`
+cmake -GNinja ../llvm \
+	-DCMAKE_BUILD_TYPE=Release \
+	-DLLVM_ENABLE_PROJECTS="libcxx;libcxxabi" \
+	-DCMAKE_C_COMPILER=clang \
+	-DCMAKE_CXX_COMPILER=clang++ \
+	-DLLVM_USE_SANITIZER=MemoryWithOrigins
+cmake --build . -- cxx cxxabi
+# Back to the MPPP build dir.
+cd $MPPP_BUILD_DIR
+
+# Download and compile locally GMP in debug mode.
+GMP_VERSION="6.2.0"
+wget https://gmplib.org/download/gmp/gmp-${GMP_VERSION}.tar.bz2 -O gmp.tar.bz2
+tar xjvf gmp.tar.bz2
+cd gmp-${GMP_VERSION}
+CC=clang CXX=clang++ ./configure --enable-shared --disable-static --enable-assert --enable-alloca=debug --disable-assembly CFLAGS="-g -fsanitize=memory" --prefix=/home/circleci/.local
+make -j2
+make install
+
+# Compile mppp and run the tests.
+cd ..
+MPPP_MSAN_FLAGS="-fsanitize=memory -stdlib=libc++ -nostdinc++ -isystem ${LLVM_BUILD_DIR}/include/c++/v1 -L${LLVM_BUILD_DIR}/lib -Wl,-rpath,${LLVM_BUILD_DIR}/lib -Wno-unused-command-line-argument"
+CC=clang CXX=clang++ cmake ../ -DCMAKE_BUILD_TYPE=Debug -DMPPP_BUILD_TESTS=yes -DCMAKE_CXX_FLAGS="${MPPP_MSAN_FLAGS}" -DCMAKE_C_FLAGS="${MPPP_MSAN_FLAGS}" -DCMAKE_PREFIX_PATH=/home/circleci/.local -DCMAKE_CXX_STANDARD=17 -DMPPP_TEST_NSPLIT=${TEST_NSPLIT} -DMPPP_TEST_SPLIT_NUM=${SPLIT_TEST_NUM}
+make -j2 VERBOSE=1
+ctest -j4 -V
+
+set +e
+set +x