Add support for device UNC paths in canonical() (#171)

README.md

@@ -655,6 +655,8 @@ to the expected behavior.
   to a host-only UNC path now inserts the missing separator.
 * Fix for [#170](https://github.com/gulrak/filesystem/issues/170),
   `lexically_normal()` now preserves IPv6 components in device UNC paths.
+* Fix for [#171](https://github.com/gulrak/filesystem/issues/171), `canonical()`
+  now supports device UNC paths.
 * Fix for [#185](https://github.com/gulrak/filesystem/issues/185),
   `lexically_normal()` now preserves unresolved parent components in relative
   paths.

include/ghc/filesystem.hpp

@@ -3753,6 +3753,19 @@ GHC_INLINE path canonical(const path& p, std::error_code& ec)
         return path();
     }
     path work = p.is_absolute() ? p : absolute(p, ec);
+#ifdef GHC_OS_WINDOWS
+    const path::impl_string_type deviceUncPrefix(GHC_PLATFORM_LITERAL("\\\\.\\UNC\\"));
+    const path::impl_string_type extendedUncPrefix(GHC_PLATFORM_LITERAL("\\\\?\\UNC\\"));
+    auto hasPrefix = [&work](const path::impl_string_type& prefix) {
+        return work._path.length() > prefix.length() &&
+            std::equal(prefix.begin(), prefix.end(), work._path.begin(), [](path::value_type lhs, path::value_type rhs) {
+                return std::toupper(static_cast<unsigned char>(lhs)) == std::toupper(static_cast<unsigned char>(rhs));
+            });
+    };
+    if (hasPrefix(deviceUncPrefix) || hasPrefix(extendedUncPrefix)) {
+        work = path(GHC_PLATFORM_LITERAL("\\\\") + work._path.substr(deviceUncPrefix.length()), path::native_format);
+    }
+#endif
     path result;
 
     auto fs = status(work, ec);

test/filesystem_test.cpp

@@ -2945,6 +2945,14 @@ TEST_CASE("Windows: path namespace handling", "[filesystem][path][fs.path.win.na
         auto p2 = fs::canonical(p, ec);
         CHECK(!ec);
         CHECK(p2 == p);
+
+        auto p3 = fs::canonical(R"(\\.\UNC\localhost\c$\Windows)", ec);
+        CHECK(!ec);
+        CHECK(p3 == p);
+
+        auto p4 = fs::canonical(R"(\\?\UNC\localhost\c$\Windows)", ec);
+        CHECK(!ec);
+        CHECK(p4 == p);
     }
     
     struct TestInfo