Initial modularization with both Clang and VC hacks

Note: gave up on VC about half way though.

59 files changed, 740 insertions, 106 deletions

diff --git a/tests/base64/buildfile b/tests/base64/buildfile
index 710499e..535b9dc 100644
--- a/tests/base64/buildfile
+++ b/tests/base64/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/base64/driver.cxx b/tests/base64/driver.cxx
index 040eaf8..fb65a49 100644
--- a/tests/base64/driver.cxx
+++ b/tests/base64/driver.cxx
@@ -2,11 +2,25 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <string>
+#include <vector>
 #include <sstream>
-#include <cassert>
+#endif
 
-#include <libbutl/base64.hxx>
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.base64;
+#else
+#include <libbutl/base64.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/build/root.build b/tests/build/root.build
index 633ccb3..6c98ba9 100644
--- a/tests/build/root.build
+++ b/tests/build/root.build
@@ -2,6 +2,11 @@
 # copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 # license   : MIT; see accompanying LICENSE file
 
+using cxx.guess
+
+if ($force_modules != true && $cxx.id == 'clang')
+  cxx.features.modules = false
+
 cxx.std = experimental
 
 using cxx
diff --git a/tests/cpfile/buildfile b/tests/cpfile/buildfile
index 0d95110..41275fa 100644
--- a/tests/cpfile/buildfile
+++ b/tests/cpfile/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/cpfile/driver.cxx b/tests/cpfile/driver.cxx
index 3a19322..db529ba 100644
--- a/tests/cpfile/driver.cxx
+++ b/tests/cpfile/driver.cxx
@@ -2,14 +2,29 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <ios>
 #include <string>
-#include <cassert>
 #include <system_error>
+#endif
 
-#include <libbutl/path.hxx>
-#include <libbutl/fdstream.hxx>
-#include <libbutl/filesystem.hxx>
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.path;
+import butl.fdstream;
+import butl.filesystem;
+#else
+#include <libbutl/path.mxx>
+#include <libbutl/fdstream.mxx>
+#include <libbutl/filesystem.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/curl/buildfile b/tests/curl/buildfile
index 20617cf..dae375b 100644
--- a/tests/curl/buildfile
+++ b/tests/curl/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs test{testscript}
diff --git a/tests/curl/driver.cxx b/tests/curl/driver.cxx
index b5fc400..326d717 100644
--- a/tests/curl/driver.cxx
+++ b/tests/curl/driver.cxx
@@ -2,12 +2,35 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <iostream>
 #include <system_error>
-
-#include <libbutl/path.hxx>
-#include <libbutl/utility.hxx> // operator<<(ostream, exception)
-#include <libbutl/curl.hxx>
+#endif
+
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.curl;
+import butl.path;
+import butl.process;
+import butl.utility; // operator<<(ostream, exception)
+import butl.fdstream;
+
+import butl.optional;     // @@ MOD Clang should not be necessary.
+import butl.small_vector; // @@ MOD Clang should not be necessary.
+#else
+#include <libbutl/curl.mxx>
+#include <libbutl/path.mxx>
+#include <libbutl/process.mxx>
+#include <libbutl/utility.mxx>
+#include <libbutl/fdstream.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/dir-iterator/buildfile b/tests/dir-iterator/buildfile
index e536536..7f83e4f 100644
--- a/tests/dir-iterator/buildfile
+++ b/tests/dir-iterator/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs test{testscript}
diff --git a/tests/dir-iterator/driver.cxx b/tests/dir-iterator/driver.cxx
index f584c68..1adef6f 100644
--- a/tests/dir-iterator/driver.cxx
+++ b/tests/dir-iterator/driver.cxx
@@ -2,14 +2,30 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
-#include <cstddef> // size_t
 #include <cassert>
+
+#ifndef __cpp_lib_modules
+#include <cstddef> // size_t
 #include <iostream>
+#endif
+
+// Other includes.
 
-#include <libbutl/path.hxx>
-#include <libbutl/path-io.hxx>
-#include <libbutl/utility.hxx>    // operator<<(ostream, exception)
-#include <libbutl/filesystem.hxx>
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.path;
+import butl.path_io;
+import butl.utility;
+import butl.filesystem;
+#else
+#include <libbutl/path.mxx>
+#include <libbutl/path-io.mxx>
+#include <libbutl/utility.mxx>    // operator<<(ostream, exception)
+#include <libbutl/filesystem.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/fdstream/buildfile b/tests/fdstream/buildfile
index 5440bc1..6028f3f 100644
--- a/tests/fdstream/buildfile
+++ b/tests/fdstream/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/fdstream/driver.cxx b/tests/fdstream/driver.cxx
index 42414bc..dd70cff 100644
--- a/tests/fdstream/driver.cxx
+++ b/tests/fdstream/driver.cxx
@@ -2,6 +2,9 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #ifndef _WIN32
 #  include <chrono>
 #  include <thread> // this_thread::sleep_for()
@@ -11,18 +14,35 @@
 #include <string>
 #include <vector>
 #include <iomanip>
-#include <cassert>
 #include <sstream>
 #include <fstream>
 #include <utility>   // move()
 #include <iostream>
 #include <exception>
+#endif
+
+// Other includes.
 
-#include <libbutl/path.hxx>
-#include <libbutl/process.hxx>
-#include <libbutl/fdstream.hxx>
-#include <libbutl/timestamp.hxx>
-#include <libbutl/filesystem.hxx>
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#ifndef _WIN32
+//@@ MOD TODO import std.threading;
+#endif
+#endif
+import butl.path;
+import butl.process;
+import butl.fdstream;
+import butl.timestamp;
+import butl.filesystem;
+#else
+#include <libbutl/path.mxx>
+#include <libbutl/process.mxx>
+#include <libbutl/fdstream.mxx>
+#include <libbutl/timestamp.mxx>
+#include <libbutl/filesystem.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/link/buildfile b/tests/link/buildfile
index 9d4f6b5..17e3eac 100644
--- a/tests/link/buildfile
+++ b/tests/link/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/link/driver.cxx b/tests/link/driver.cxx
index a5e94bb..352cadd 100644
--- a/tests/link/driver.cxx
+++ b/tests/link/driver.cxx
@@ -2,14 +2,28 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
-#include <set>
 #include <cassert>
+
+#ifndef __cpp_lib_modules
+#include <set>
 #include <utility>      // pair
 #include <system_error>
+#endif
 
-#include <libbutl/path.hxx>
-#include <libbutl/fdstream.hxx>
-#include <libbutl/filesystem.hxx>
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+#endif
+import butl.path;
+import butl.fdstream;
+import butl.filesystem;
+#else
+#include <libbutl/path.mxx>
+#include <libbutl/fdstream.mxx>
+#include <libbutl/filesystem.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/manifest-parser/buildfile b/tests/manifest-parser/buildfile
index b57cac6..2a89e29 100644
--- a/tests/manifest-parser/buildfile
+++ b/tests/manifest-parser/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/manifest-parser/driver.cxx b/tests/manifest-parser/driver.cxx
index 037df96..76959e5 100644
--- a/tests/manifest-parser/driver.cxx
+++ b/tests/manifest-parser/driver.cxx
@@ -2,14 +2,27 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <vector>
 #include <string>
 #include <utility> // pair
-#include <cassert>
 #include <sstream>
 #include <iostream>
-
-#include <libbutl/manifest-parser.hxx>
+#endif
+
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.manifest_parser;
+#else
+#include <libbutl/manifest-parser.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/manifest-roundtrip/buildfile b/tests/manifest-roundtrip/buildfile
index a2fb3ac..5cd296f 100644
--- a/tests/manifest-roundtrip/buildfile
+++ b/tests/manifest-roundtrip/buildfile
@@ -4,5 +4,11 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
 exe{driver}: test.roundtrip = manifest
diff --git a/tests/manifest-roundtrip/driver.cxx b/tests/manifest-roundtrip/driver.cxx
index 2068409..1f0b311 100644
--- a/tests/manifest-roundtrip/driver.cxx
+++ b/tests/manifest-roundtrip/driver.cxx
@@ -3,12 +3,29 @@
 // license   : MIT; see accompanying LICENSE file
 
 #include <cassert>
+
+#ifndef __cpp_lib_modules
+#include <string>
 #include <iostream>
+#endif
+
+// Other includes.
 
-#include <libbutl/utility.hxx>             // operator<<(ostream, exception)
-#include <libbutl/fdstream.hxx>
-#include <libbutl/manifest-parser.hxx>
-#include <libbutl/manifest-serializer.hxx>
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.utility;             // operator<<(ostream, exception)
+import butl.fdstream;
+import butl.manifest_parser;
+import butl.manifest_serializer;
+#else
+#include <libbutl/utility.mxx>
+#include <libbutl/fdstream.mxx>
+#include <libbutl/manifest-parser.mxx>
+#include <libbutl/manifest-serializer.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/manifest-serializer/buildfile b/tests/manifest-serializer/buildfile
index d13daab..df633ae 100644
--- a/tests/manifest-serializer/buildfile
+++ b/tests/manifest-serializer/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/manifest-serializer/driver.cxx b/tests/manifest-serializer/driver.cxx
index dad96bd..f373ecf 100644
--- a/tests/manifest-serializer/driver.cxx
+++ b/tests/manifest-serializer/driver.cxx
@@ -2,14 +2,27 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <vector>
 #include <string>
 #include <utility> // pair
-#include <cassert>
 #include <sstream>
 #include <iostream>
-
-#include <libbutl/manifest-serializer.hxx>
+#endif
+
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.manifest_serializer;
+#else
+#include <libbutl/manifest-serializer.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/mventry/buildfile b/tests/mventry/buildfile
index b665b61..d9af3a2 100644
--- a/tests/mventry/buildfile
+++ b/tests/mventry/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs test{testscript}
diff --git a/tests/mventry/driver.cxx b/tests/mventry/driver.cxx
index 5639c2b..eab2ce3 100644
--- a/tests/mventry/driver.cxx
+++ b/tests/mventry/driver.cxx
@@ -2,12 +2,28 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <iostream>
 #include <system_error>
+#endif
+
+// Other includes.
 
-#include <libbutl/path.hxx>
-#include <libbutl/utility.hxx>    // operator<<(ostream, exception)
-#include <libbutl/filesystem.hxx>
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.path;
+import butl.utility;    // operator<<(ostream, exception)
+import butl.filesystem;
+#else
+#include <libbutl/path.mxx>
+#include <libbutl/utility.mxx>
+#include <libbutl/filesystem.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/openssl/buildfile b/tests/openssl/buildfile
index 7713f14..08b0878 100644
--- a/tests/openssl/buildfile
+++ b/tests/openssl/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs test{testscript}
diff --git a/tests/openssl/driver.cxx b/tests/openssl/driver.cxx
index b0e37ae..092b2ba 100644
--- a/tests/openssl/driver.cxx
+++ b/tests/openssl/driver.cxx
@@ -2,14 +2,36 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <vector>
 #include <iostream>
 #include <iterator>
 #include <system_error>
+#endif
+
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.path;
+import butl.utility;  // operator<<(ostream, exception)
+import butl.openssl;
+import butl.process;
+import butl.fdstream; // nullfd
 
-#include <libbutl/path.hxx>
-#include <libbutl/utility.hxx> // operator<<(ostream, exception)
-#include <libbutl/openssl.hxx>
+import butl.optional;     // @@ MOD Clang should not be necessary.
+import butl.small_vector; // @@ MOD Clang should not be necessary.
+#else
+#include <libbutl/path.mxx>
+#include <libbutl/utility.mxx>
+#include <libbutl/openssl.mxx>
+#include <libbutl/fdstream.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/pager/buildfile b/tests/pager/buildfile
index 1c1e9f4..d7d0123 100644
--- a/tests/pager/buildfile
+++ b/tests/pager/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/pager/driver.cxx b/tests/pager/driver.cxx
index 87c811d..6842cef 100644
--- a/tests/pager/driver.cxx
+++ b/tests/pager/driver.cxx
@@ -2,15 +2,28 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <ios>      // ios_base::failure
 #include <vector>
 #include <string>
 #include <utility>  // move()
-#include <cassert>
 #include <sstream>
 #include <iostream>
-
-#include <libbutl/pager.hxx>
+#endif
+
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.pager;
+#else
+#include <libbutl/pager.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/path-entry/buildfile b/tests/path-entry/buildfile
index 58e315c..e6515ae 100644
--- a/tests/path-entry/buildfile
+++ b/tests/path-entry/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs test{testscript}
diff --git a/tests/path-entry/driver.cxx b/tests/path-entry/driver.cxx
index 4f36f61..c69bf9c 100644
--- a/tests/path-entry/driver.cxx
+++ b/tests/path-entry/driver.cxx
@@ -2,11 +2,26 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <iostream>
 #include <system_error>
-
-#include <libbutl/utility.hxx>    // operator<<(ostream, exception)
-#include <libbutl/filesystem.hxx>
+#endif
+
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.utility;    // operator<<(ostream, exception)
+import butl.filesystem;
+#else
+#include <libbutl/utility.mxx>
+#include <libbutl/filesystem.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/path/buildfile b/tests/path/buildfile
index e7f19f2..aa39b2d 100644
--- a/tests/path/buildfile
+++ b/tests/path/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/path/driver.cxx b/tests/path/driver.cxx
index 4d859b3..c2919c0 100644
--- a/tests/path/driver.cxx
+++ b/tests/path/driver.cxx
@@ -3,11 +3,25 @@
 // license   : MIT; see accompanying LICENSE file
 
 #include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <iostream>
 #include <type_traits>
+#endif
+
+// Other includes.
 
-#include <libbutl/path.hxx>
-#include <libbutl/path-io.hxx>
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.path;
+import butl.path_io;
+#else
+#include <libbutl/path.mxx>
+#include <libbutl/path-io.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/prefix-map/buildfile b/tests/prefix-map/buildfile
index 5c2dd69..140fda9 100644
--- a/tests/prefix-map/buildfile
+++ b/tests/prefix-map/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/prefix-map/driver.cxx b/tests/prefix-map/driver.cxx
index abf7e01..3e165b2 100644
--- a/tests/prefix-map/driver.cxx
+++ b/tests/prefix-map/driver.cxx
@@ -2,11 +2,24 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
-#include <string>
 #include <cassert>
-#include <iostream>
 
-#include <libbutl/prefix-map.hxx>
+#ifndef __cpp_lib_modules
+#include <string>
+#include <iostream>
+#endif
+
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.prefix_map;
+#else
+#include <libbutl/prefix-map.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/process-run/buildfile b/tests/process-run/buildfile
index 770357c..5200af7 100644
--- a/tests/process-run/buildfile
+++ b/tests/process-run/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs test{testscript}
diff --git a/tests/process-run/driver.cxx b/tests/process-run/driver.cxx
index 695fdb9..765bd7d 100644
--- a/tests/process-run/driver.cxx
+++ b/tests/process-run/driver.cxx
@@ -2,12 +2,31 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
+#include <string>
 #include <iostream>
+#endif
+
+// Other includes.
 
-#include <libbutl/path.hxx>
-#include <libbutl/process.hxx>
-#include <libbutl/fdstream.hxx>
-#include <libbutl/small-vector.hxx>
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.path;
+import butl.process;
+import butl.optional; // @@ MOD Clang shouldn't be needed.
+import butl.fdstream;
+import butl.small_vector;
+#else
+#include <libbutl/path.mxx>
+#include <libbutl/process.mxx>
+#include <libbutl/fdstream.mxx>
+#include <libbutl/small-vector.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/process/buildfile b/tests/process/buildfile
index ddb4688..2ecc915 100644
--- a/tests/process/buildfile
+++ b/tests/process/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs test{testscript}
diff --git a/tests/process/driver.cxx b/tests/process/driver.cxx
index 0338af2..01f2c30 100644
--- a/tests/process/driver.cxx
+++ b/tests/process/driver.cxx
@@ -4,17 +4,34 @@
 
 #include <stdlib.h> // getenv(), setenv(), _putenv()
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <ios>
 #include <string>
 #include <vector>
-#include <cassert>
-#include <iostream>
 #include <iterator>  // istreambuf_iterator, ostream_iterator
 #include <algorithm> // copy()
+#include <iostream>
+#endif
+
+// Other includes.
 
-#include <libbutl/path.hxx>
-#include <libbutl/process.hxx>
-#include <libbutl/fdstream.hxx>
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.path;
+import butl.process;
+import butl.optional;
+import butl.fdstream;
+#else
+#include <libbutl/path.mxx>
+#include <libbutl/process.mxx>
+#include <libbutl/optional.mxx>
+#include <libbutl/fdstream.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/progress/buildfile b/tests/progress/buildfile
index 19e85c0..d8cb6d6 100644
--- a/tests/progress/buildfile
+++ b/tests/progress/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/progress/driver.cxx b/tests/progress/driver.cxx
index 0ff68f0..c8c2080 100644
--- a/tests/progress/driver.cxx
+++ b/tests/progress/driver.cxx
@@ -4,20 +4,43 @@
 
 #ifndef _WIN32
 #  include <unistd.h> // write()
-
-#  include <thread> // this_thread::sleep_for()
 #else
 #  include <libbutl/win32-utility.hxx>
-
 #  include <io.h> //_write()
 #endif
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
+#include <string>
 #include <cstddef>  // size_t
 #include <iostream>
+#ifndef _WIN32
+#  include <thread> // this_thread::sleep_for()
+#endif
+#endif
 
-#include <libbutl/process.hxx>
-#include <libbutl/fdstream.hxx>    // fdnull(), stderr_fd()
-#include <libbutl/diagnostics.hxx>
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#ifndef _WIN32
+//@@ MOD TODO import std.threading;
+#endif
+#endif
+import butl.process;
+import butl.fdstream;
+import butl.diagnostics;
+
+import butl.optional;     // @@ MOD Clang should not be necessary.
+import butl.small_vector; // @@ MOD Clang should not be necessary.
+#else
+#include <libbutl/process.mxx>
+#include <libbutl/fdstream.mxx>    // fdnull(), stderr_fd()
+#include <libbutl/diagnostics.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/regex/buildfile b/tests/regex/buildfile
index baf4bca..ae7e2c3 100644
--- a/tests/regex/buildfile
+++ b/tests/regex/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs test{testscript}
diff --git a/tests/regex/driver.cxx b/tests/regex/driver.cxx
index 054eb31..d48b716 100644
--- a/tests/regex/driver.cxx
+++ b/tests/regex/driver.cxx
@@ -2,13 +2,27 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
-#include <string>
 #include <cassert>
+
+#ifndef __cpp_lib_modules
+#include <string>
 #include <iostream>
 #include <exception>
+#endif
+
+// Other includes.
 
-#include <libbutl/regex.hxx>
-#include <libbutl/utility.hxx> // operator<<(ostream, exception)
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.regex;
+import butl.utility; // operator<<(ostream, exception)
+#else
+#include <libbutl/regex.mxx>
+#include <libbutl/utility.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/sendmail/buildfile b/tests/sendmail/buildfile
index a83cf40..a854e89 100644
--- a/tests/sendmail/buildfile
+++ b/tests/sendmail/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs test{testscript}
diff --git a/tests/sendmail/driver.cxx b/tests/sendmail/driver.cxx
index e48c6b4..f5c9227 100644
--- a/tests/sendmail/driver.cxx
+++ b/tests/sendmail/driver.cxx
@@ -2,12 +2,34 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <iostream>
 #include <system_error>
+#endif
+
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.path;
+import butl.process;
+import butl.utility;  // operator<<(ostream, exception)
+import butl.sendmail;
+import butl.fdstream;
 
-#include <libbutl/path.hxx>
-#include <libbutl/utility.hxx>  // operator<<(ostream, exception)
-#include <libbutl/sendmail.hxx>
+import butl.optional;     // @@ MOD Clang should not be necessary.
+import butl.small_vector; // @@ MOD Clang should not be necessary.
+#else
+#include <libbutl/path.mxx>
+#include <libbutl/process.mxx>
+#include <libbutl/utility.mxx>
+#include <libbutl/sendmail.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/sha256/buildfile b/tests/sha256/buildfile
index 627b78f..fb490d0 100644
--- a/tests/sha256/buildfile
+++ b/tests/sha256/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/sha256/driver.cxx b/tests/sha256/driver.cxx
index 2d2f963..191756c 100644
--- a/tests/sha256/driver.cxx
+++ b/tests/sha256/driver.cxx
@@ -1,12 +1,25 @@
-// file      : tests/triplet/driver.cxx -*- C++ -*-
+// file      : tests/sha256/driver.cxx -*- C++ -*-
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
-#include <string>
 #include <cassert>
+
+#ifndef __cpp_lib_modules
+#include <string>
 #include <iostream>
+#endif
+
+// Other includes.
 
-#include <libbutl/sha256.hxx>
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.sha256;
+#else
+#include <libbutl/sha256.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/small-vector/buildfile b/tests/small-vector/buildfile
index 33be9ad..16d61f8 100644
--- a/tests/small-vector/buildfile
+++ b/tests/small-vector/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/small-vector/driver.cxx b/tests/small-vector/driver.cxx
index c7cafd4..fe386a4 100644
--- a/tests/small-vector/driver.cxx
+++ b/tests/small-vector/driver.cxx
@@ -2,11 +2,25 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
-#include <string>
 #include <cassert>
-#include <iostream>
 
-#include <libbutl/small-vector.hxx>
+#ifndef __cpp_lib_modules
+#include <string>
+#include <vector>
+#include <iostream>
+#endif
+
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.small_vector;
+#else
+#include <libbutl/small-vector.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/standard-version/buildfile b/tests/standard-version/buildfile
index 0583b62..eb30733 100644
--- a/tests/standard-version/buildfile
+++ b/tests/standard-version/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs test{testscript}
diff --git a/tests/standard-version/driver.cxx b/tests/standard-version/driver.cxx
index fa149ae..c6f9c11 100644
--- a/tests/standard-version/driver.cxx
+++ b/tests/standard-version/driver.cxx
@@ -2,14 +2,28 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <ios>       // ios::failbit, ios::badbit
 #include <string>
-#include <cassert>
 #include <iostream>
 #include <stdexcept> // invalid_argument
-
-#include <libbutl/utility.hxx>          // operator<<(ostream,exception)
-#include <libbutl/standard-version.hxx>
+#endif
+
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.utility;          // operator<<(ostream,exception)
+import butl.standard_version;
+#else
+#include <libbutl/utility.mxx>
+#include <libbutl/standard-version.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/strcase/buildfile b/tests/strcase/buildfile
index 71fec53..efe8442 100644
--- a/tests/strcase/buildfile
+++ b/tests/strcase/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/strcase/driver.cxx b/tests/strcase/driver.cxx
index f7f4bd1..285892d 100644
--- a/tests/strcase/driver.cxx
+++ b/tests/strcase/driver.cxx
@@ -2,10 +2,22 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
-#include <string>
 #include <cassert>
 
-#include <libbutl/utility.hxx>
+#ifndef __cpp_lib_modules
+#include <string>
+#endif
+
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+#endif
+import butl.utility;
+#else
+#include <libbutl/utility.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/string-parser/buildfile b/tests/string-parser/buildfile
index 5969cd0..4613df1 100644
--- a/tests/string-parser/buildfile
+++ b/tests/string-parser/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs test{testscript}
diff --git a/tests/string-parser/driver.cxx b/tests/string-parser/driver.cxx
index 09443f2..f454a6f 100644
--- a/tests/string-parser/driver.cxx
+++ b/tests/string-parser/driver.cxx
@@ -2,17 +2,30 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
-#include <ios>      // ios::failbit, ios::badbit
-#include <vector>
-#include <string>
 #include <cassert>
+
+#ifndef __cpp_lib_modules
+#include <string>
+#include <vector>
 #include <iostream>
+#endif
+
+// Other includes.
 
-#include <libbutl/utility.hxx>       // operator<<(ostream,exception)
-#include <libbutl/string-parser.hxx>
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.utility;       // operator<<(ostream,exception)
+import butl.string_parser;
+#else
+#include <libbutl/utility.mxx>
+#include <libbutl/string-parser.mxx>
+#endif
 
 using namespace std;
-using namespace butl;
+using namespace butl::string_parser;
 
 // Usage: argv[0] [-l] [-u] [-p]
 //
@@ -54,8 +67,7 @@ try
   string l;
   while (getline (cin, l))
   {
-    vector<pair<string, size_t>> v (
-      string_parser::parse_quoted_position (l, unquote));
+    vector<pair<string, size_t>> v (parse_quoted_position (l, unquote));
 
     if (!spl)
     {
diff --git a/tests/tab-parser/buildfile b/tests/tab-parser/buildfile
index 0583b62..eb30733 100644
--- a/tests/tab-parser/buildfile
+++ b/tests/tab-parser/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs test{testscript}
diff --git a/tests/tab-parser/driver.cxx b/tests/tab-parser/driver.cxx
index 238c12a..d5ff5a0 100644
--- a/tests/tab-parser/driver.cxx
+++ b/tests/tab-parser/driver.cxx
@@ -2,12 +2,26 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
-#include <ios>      // ios::failbit, ios::badbit
 #include <cassert>
+
+#ifndef __cpp_lib_modules
+#include <string>
 #include <iostream>
+#endif
+
+// Other includes.
 
-#include <libbutl/utility.hxx>    // operator<<(ostream,exception)
-#include <libbutl/tab-parser.hxx>
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.utility;    // operator<<(ostream,exception)
+import butl.tab_parser;
+#else
+#include <libbutl/utility.mxx>
+#include <libbutl/tab-parser.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/target-triplet/buildfile b/tests/target-triplet/buildfile
index b8ca712..7dec846 100644
--- a/tests/target-triplet/buildfile
+++ b/tests/target-triplet/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/target-triplet/driver.cxx b/tests/target-triplet/driver.cxx
index b446a6f..5e61a46 100644
--- a/tests/target-triplet/driver.cxx
+++ b/tests/target-triplet/driver.cxx
@@ -3,10 +3,24 @@
 // license   : MIT; see accompanying LICENSE file
 
 #include <cassert>
+
+#ifndef __cpp_lib_modules
+#include <string>
 #include <iostream>
 #include <stdexcept> // invalid_argument
-
-#include <libbutl/target-triplet.hxx>
+#endif
+
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.target_triplet;
+#else
+#include <libbutl/target-triplet.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/timestamp/buildfile b/tests/timestamp/buildfile
index 085daf0..c554fc3 100644
--- a/tests/timestamp/buildfile
+++ b/tests/timestamp/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/timestamp/driver.cxx b/tests/timestamp/driver.cxx
index 661deb2..ec4c49d 100644
--- a/tests/timestamp/driver.cxx
+++ b/tests/timestamp/driver.cxx
@@ -4,15 +4,28 @@
 
 #include <time.h> // tzset() (POSIX), _tzset() (Windows)
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <chrono>
 #include <locale>
 #include <clocale>
-#include <cassert>
 #include <sstream>
 #include <iomanip>
 #include <system_error>
+#endif
 
-#include <libbutl/timestamp.hxx>
+// Other includes.
+
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.timestamp;
+#else
+#include <libbutl/timestamp.mxx>
+#endif
 
 using namespace std;
 using namespace butl;
diff --git a/tests/wildcard/buildfile b/tests/wildcard/buildfile
index fc8e94b..90e8b58 100644
--- a/tests/wildcard/buildfile
+++ b/tests/wildcard/buildfile
@@ -4,4 +4,10 @@
 
 import libs = libbutl%lib{butl}
 
+if ($cxx.features.modules && ($force_std_modules == true || $cxx.id != 'msvc'))
+{
+  import libs += libstd-modules%liba{std-modules}
+  cxx.poptions += -D__cpp_lib_modules
+}
+
 exe{driver}: {hxx cxx}{*} $libs test{testscript}
diff --git a/tests/wildcard/driver.cxx b/tests/wildcard/driver.cxx
index 502726f..d95d7cd 100644
--- a/tests/wildcard/driver.cxx
+++ b/tests/wildcard/driver.cxx
@@ -2,18 +2,34 @@
 // copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
 // license   : MIT; see accompanying LICENSE file
 
+#include <cassert>
+
+#ifndef __cpp_lib_modules
 #include <map>
 #include <string>
 #include <vector>
-#include <cassert>
-#include <iostream>
 #include <algorithm> // sort()
 #include <exception>
+#include <iostream>
+#endif
+
+// Other includes.
 
-#include <libbutl/path.hxx>
-#include <libbutl/utility.hxx>    // operator<<(ostream, exception)
-#include <libbutl/optional.hxx>
-#include <libbutl/filesystem.hxx>
+#ifdef __cpp_modules
+#ifdef __cpp_lib_modules
+import std.core;
+import std.io;
+#endif
+import butl.path;
+import butl.utility;    // operator<<(ostream, exception)
+import butl.optional;
+import butl.filesystem;
+#else
+#include <libbutl/path.mxx>
+#include <libbutl/utility.mxx>
+#include <libbutl/optional.mxx>
+#include <libbutl/filesystem.mxx>
+#endif
 
 using namespace std;
 using namespace butl;