refs #27, Squashed commit of the following:

commit aabbf8f
Author: Steffen Schümann <s.schuemann@pobox.com>
Date:   Sat Sep 21 16:22:25 2019 +0200

    refs #27, ghc::filesystem::path::preferred_seperator fix for VS2015

commit 03b437c
Merge: cbe0c61 589219b
Author: Steffen Schümann <s.schuemann@pobox.com>
Date:   Fri Sep 20 13:42:31 2019 +0200

    Merge branch 'master' into feature-27-mingw-preferred_separator

commit cbe0c61
Author: Steffen Schuemann <s.schuemann@pobox.com>
Date:   Sat Aug 24 22:20:10 2019 +0200

    Test for workaround of static constexpr issue

include/ghc/filesystem.hpp

@@ -192,24 +192,38 @@ class GHC_FS_API_CLASS not_implemented_exception : public std::logic_error
     }
 };
 
-// 30.10.8 class path
-class GHC_FS_API_CLASS path
+template<typename char_type>
+class path_helper_base
 {
 public:
+    using value_type = char_type;
 #ifdef GHC_OS_WINDOWS
-#ifdef GHC_WIN_WSTRING_STRING_TYPE
-#define GHC_USE_WCHAR_T
-    using value_type = std::wstring::value_type;
-#else
-    using value_type = std::string::value_type;
-#endif
-    using string_type = std::basic_string<value_type>;
     static constexpr value_type preferred_separator = '\\';
 #else
-    using value_type = std::string::value_type;
-    using string_type = std::basic_string<value_type>;
     static constexpr value_type preferred_separator = '/';
 #endif
+};
+
+template <typename char_type>
+constexpr char_type path_helper_base<char_type>::preferred_separator;
+
+// 30.10.8 class path
+class GHC_FS_API_CLASS path
+#if defined(GHC_OS_WINDOWS) && defined(GHC_WIN_WSTRING_STRING_TYPE)
+#define GHC_USE_WCHAR_T
+    : private path_helper_base<std::wstring::value_type>
+{
+public:
+    using path_helper_base<std::wstring::value_type>::value_type;
+#else
+    : private path_helper_base<std::string::value_type>
+{
+public:
+    using path_helper_base<std::string::value_type>::value_type;
+#endif
+    using string_type = std::basic_string<value_type>;
+    using path_helper_base<value_type>::preferred_separator;
+
     // 30.10.10.1 enumeration format
     /// The path format in wich the constructor argument is given.
     enum format {

test/filesystem_test.cpp

@@ -345,6 +345,15 @@ TEST_CASE("fs::detail::toUtf8", "[filesystem][fs.detail.utf8]")
 }
 #endif
 
+TEST_CASE("30.10.8.1 path::preferred_separator", "[filesystem][path][fs.path.generic]")
+{
+#ifdef GHC_OS_WINDOWS
+    CHECK(fs::path::preferred_separator == '\\');
+#else
+    CHECK(fs::path::preferred_separator == '/');
+#endif
+}
+
 #ifndef GHC_OS_WINDOWS
 TEST_CASE("30.10.8.1 path(\"//host\").has_root_name()", "[filesystem][path][fs.path.generic]")
 {