diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index 38a0da0f8..8fbb54dd3 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -142,7 +142,7 @@ NixRepl::NixRepl(
     , getValues(getValues)
     , staticEnv(new StaticEnv(nullptr, state->staticBaseEnv))
     , runNixPtr{runNix}
-    , interacter(make_unique<ReadlineLikeInteracter>(getDataDir() + "/repl-history"))
+    , interacter(make_unique<ReadlineLikeInteracter>((getDataDir() / "repl-history").string()))
 {
 }
 
diff --git a/src/libfetchers/cache.cc b/src/libfetchers/cache.cc
index 67361c765..183f106a5 100644
--- a/src/libfetchers/cache.cc
+++ b/src/libfetchers/cache.cc
@@ -38,7 +38,7 @@ struct CacheImpl : Cache
     {
         auto state(_state.lock());
 
-        auto dbPath = getCacheDir() + "/fetcher-cache-v4.sqlite";
+        auto dbPath = (getCacheDir() / "fetcher-cache-v4.sqlite").string();
         createDirs(dirOf(dbPath));
 
         state->db = SQLite(dbPath);
diff --git a/src/libfetchers/git-utils.cc b/src/libfetchers/git-utils.cc
index f62c3b9b4..fecceefff 100644
--- a/src/libfetchers/git-utils.cc
+++ b/src/libfetchers/git-utils.cc
@@ -212,7 +212,7 @@ static void initRepoAtomically(std::filesystem::path & path, bool bare)
     AutoDelete delTmpDir(tmpDir, true);
     Repository tmpRepo;
 
-    if (git_repository_init(Setter(tmpRepo), tmpDir.c_str(), bare))
+    if (git_repository_init(Setter(tmpRepo), tmpDir.string().c_str(), bare))
         throw Error("creating Git repository %s: %s", path, git_error_last()->message);
     try {
         std::filesystem::rename(tmpDir, path);
diff --git a/src/libfetchers/git.cc b/src/libfetchers/git.cc
index 08cd5d1ca..4f5247861 100644
--- a/src/libfetchers/git.cc
+++ b/src/libfetchers/git.cc
@@ -61,7 +61,7 @@ std::optional<std::string> readHead(const std::filesystem::path & path)
         RunOptions{
             .program = "git",
             // FIXME: use 'HEAD' to avoid returning all refs
-            .args = {"ls-remote", "--symref", path},
+            .args = {"ls-remote", "--symref", path.string()},
             .isInteractive = true,
         });
     if (status != 0)
@@ -88,7 +88,7 @@ bool storeCachedHead(const std::string & actualUrl, bool shallow, const std::str
 {
     std::filesystem::path cacheDir = getCachePath(actualUrl, shallow);
     try {
-        runProgram("git", true, {"-C", cacheDir, "--git-dir", ".", "symbolic-ref", "--", "HEAD", headRef});
+        runProgram("git", true, {"-C", cacheDir.string(), "--git-dir", ".", "symbolic-ref", "--", "HEAD", headRef});
     } catch (ExecError & e) {
         if (
 #ifndef WIN32 // TODO abstract over exit status handling on Windows
@@ -115,7 +115,7 @@ std::optional<std::string> readHeadCached(const std::string & actualUrl, bool sh
     time_t now = time(0);
     struct stat st;
     std::optional<std::string> cachedRef;
-    if (stat(headRefFile.c_str(), &st) == 0) {
+    if (stat(headRefFile.string().c_str(), &st) == 0) {
         cachedRef = readHead(cacheDir);
         if (cachedRef != std::nullopt && *cachedRef != gitInitialBranch && isCacheFileWithinTtl(now, st)) {
             debug("using cached HEAD ref '%s' for repo '%s'", *cachedRef, actualUrl);
@@ -450,7 +450,7 @@ struct GitInputScheme : InputScheme
         if (input.getRev())
             throw UnimplementedError("cloning a specific revision is not implemented");
 
-        args.push_back(destDir);
+        args.push_back(destDir.string());
 
         runProgram("git", true, args, {}, true);
     }
diff --git a/src/libfetchers/mercurial.cc b/src/libfetchers/mercurial.cc
index 65999497c..dd31e224f 100644
--- a/src/libfetchers/mercurial.cc
+++ b/src/libfetchers/mercurial.cc
@@ -281,27 +281,28 @@ struct MercurialInputScheme : InputScheme
         /* If this is a commit hash that we already have, we don't
            have to pull again. */
         if (!(input.getRev() && pathExists(cacheDir)
-              && runProgram(hgOptions({"log", "-R", cacheDir, "-r", input.getRev()->gitRev(), "--template", "1"}))
+              && runProgram(
+                     hgOptions({"log", "-R", cacheDir.string(), "-r", input.getRev()->gitRev(), "--template", "1"}))
                          .second
                      == "1")) {
             Activity act(*logger, lvlTalkative, actUnknown, fmt("fetching Mercurial repository '%s'", actualUrl));
 
             if (pathExists(cacheDir)) {
                 try {
-                    runHg({"pull", "-R", cacheDir, "--", actualUrl});
+                    runHg({"pull", "-R", cacheDir.string(), "--", actualUrl});
                 } catch (ExecError & e) {
                     auto transJournal = cacheDir / ".hg" / "store" / "journal";
                     /* hg throws "abandoned transaction" error only if this file exists */
                     if (pathExists(transJournal)) {
-                        runHg({"recover", "-R", cacheDir});
-                        runHg({"pull", "-R", cacheDir, "--", actualUrl});
+                        runHg({"recover", "-R", cacheDir.string()});
+                        runHg({"pull", "-R", cacheDir.string(), "--", actualUrl});
                     } else {
                         throw ExecError(e.status, "'hg pull' %s", statusToString(e.status));
                     }
                 }
             } else {
                 createDirs(dirOf(cacheDir.string()));
-                runHg({"clone", "--noupdate", "--", actualUrl, cacheDir});
+                runHg({"clone", "--noupdate", "--", actualUrl, cacheDir.string()});
             }
         }
 
@@ -309,7 +310,7 @@ struct MercurialInputScheme : InputScheme
         auto tokens = tokenizeString<std::vector<std::string>>(runHg(
             {"log",
              "-R",
-             cacheDir,
+             cacheDir.string(),
              "-r",
              input.getRev() ? input.getRev()->gitRev() : *input.getRef(),
              "--template",
@@ -329,7 +330,7 @@ struct MercurialInputScheme : InputScheme
         std::filesystem::path tmpDir = createTempDir();
         AutoDelete delTmpDir(tmpDir, true);
 
-        runHg({"archive", "-R", cacheDir, "-r", rev.gitRev(), tmpDir});
+        runHg({"archive", "-R", cacheDir.string(), "-r", rev.gitRev(), tmpDir.string()});
 
         deletePath(tmpDir / ".hg_archival.txt");
 
diff --git a/src/libfetchers/registry.cc b/src/libfetchers/registry.cc
index c81eb6b53..f96ef89b3 100644
--- a/src/libfetchers/registry.cc
+++ b/src/libfetchers/registry.cc
@@ -150,7 +150,7 @@ static std::shared_ptr<Registry> getGlobalRegistry(const Settings & settings, St
                 if (!isAbsolute(path)) {
                     auto storePath = downloadFile(store, settings, path, "flake-registry.json").storePath;
                     if (auto store2 = dynamic_cast<LocalFSStore *>(&store))
-                        store2->addPermRoot(storePath, getCacheDir() + "/flake-registry.json");
+                        store2->addPermRoot(storePath, (getCacheDir() / "flake-registry.json").string());
                     return {store.requireStoreObjectAccessor(storePath)};
                 } else {
                     return SourcePath{getFSSourceAccessor(), CanonPath{path}}.resolveSymlinks();
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index 8a59e3dac..ad1caae2b 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -304,7 +304,7 @@ void printVersion(const std::string & programName)
         std::cout << "System type: " << settings.thisSystem << "\n";
         std::cout << "Additional system types: " << concatStringsSep(", ", settings.extraPlatforms.get()) << "\n";
         std::cout << "Features: " << concatStringsSep(", ", cfg) << "\n";
-        std::cout << "System configuration file: " << settings.nixConfDir + "/nix.conf" << "\n";
+        std::cout << "System configuration file: " << (settings.nixConfDir / "nix.conf") << "\n";
         std::cout << "User configuration files: " << concatStringsSep(":", settings.nixUserConfFiles) << "\n";
         std::cout << "Store directory: " << settings.nixStore << "\n";
         std::cout << "State directory: " << settings.nixStateDir << "\n";
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index 9e72f8577..27d17e1a9 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -116,7 +116,7 @@ void loadConfFile(AbstractConfig & config)
         }
     };
 
-    applyConfigFile(settings.nixConfDir + "/nix.conf");
+    applyConfigFile((settings.nixConfDir / "nix.conf").string());
 
     /* We only want to send overrides to the daemon, i.e. stuff from
        ~/.nix/nix.conf or the command line. */
@@ -145,7 +145,7 @@ std::vector<Path> getUserConfigFiles()
     std::vector<Path> files;
     auto dirs = getConfigDirs();
     for (auto & dir : dirs) {
-        files.insert(files.end(), dir + "/nix.conf");
+        files.insert(files.end(), (dir / "nix.conf").string());
     }
     return files;
 }
diff --git a/src/libstore/nar-info-disk-cache.cc b/src/libstore/nar-info-disk-cache.cc
index 11608a667..b0e56fa74 100644
--- a/src/libstore/nar-info-disk-cache.cc
+++ b/src/libstore/nar-info-disk-cache.cc
@@ -86,7 +86,7 @@ public:
 
     Sync<State> _state;
 
-    NarInfoDiskCacheImpl(Path dbPath = getCacheDir() + "/binary-cache-v7.sqlite")
+    NarInfoDiskCacheImpl(Path dbPath = (getCacheDir() / "binary-cache-v7.sqlite").string())
     {
         auto state(_state.lock());
 
diff --git a/src/libutil/file-system.cc b/src/libutil/file-system.cc
index e81263a56..0ad6d9285 100644
--- a/src/libutil/file-system.cc
+++ b/src/libutil/file-system.cc
@@ -687,7 +687,7 @@ std::filesystem::path createTempDir(const std::filesystem::path & tmpRoot, const
         checkInterrupt();
         std::filesystem::path tmpDir = makeTempPath(tmpRoot, prefix);
         if (mkdir(
-                tmpDir.c_str()
+                tmpDir.string().c_str()
 #ifndef _WIN32 // TODO abstract mkdir perms for Windows
                     ,
                 mode
@@ -734,7 +734,7 @@ AutoCloseFD createAnonymousTempFile()
 
 std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix)
 {
-    Path tmpl(defaultTempDir() + "/" + prefix + ".XXXXXX");
+    Path tmpl(defaultTempDir().string() + "/" + prefix + ".XXXXXX");
     // Strictly speaking, this is UB, but who cares...
     // FIXME: use O_TMPFILE.
     // FIXME: Windows should use FILE_ATTRIBUTE_TEMPORARY | FILE_FLAG_DELETE_ON_CLOSE
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index ba8e573a7..19ea4debd 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -207,7 +207,8 @@ void FdSource::restart()
         throw Error("can't seek to the start of a file");
     buffer.reset();
     read = bufPosOut = bufPosOut = 0;
-    if (lseek(fd, 0, SEEK_SET) == -1)
+    int fd_ = fromDescriptorReadOnly(fd);
+    if (lseek(fd_, 0, SEEK_SET) == -1)
         throw SysError("seeking to the start of a file");
 }
 
diff --git a/src/libutil/users.cc b/src/libutil/users.cc
index e07b5535e..1fa643730 100644
--- a/src/libutil/users.cc
+++ b/src/libutil/users.cc
@@ -90,7 +90,7 @@ std::string expandTilde(std::string_view path)
     // TODO: expand ~user ?
     auto tilde = path.substr(0, 2);
     if (tilde == "~/" || tilde == "~")
-        return getHome() + std::string(path.substr(1));
+        return getHome().string() + std::string(path.substr(1));
     else
         return std::string(path);
 }
diff --git a/src/libutil/windows/users.cc b/src/libutil/windows/users.cc
index cbeadcb81..eb92e7ab6 100644
--- a/src/libutil/windows/users.cc
+++ b/src/libutil/windows/users.cc
@@ -40,7 +40,7 @@ std::filesystem::path getHome()
     static std::filesystem::path homeDir = []() {
         std::filesystem::path homeDir = getEnv("USERPROFILE").value_or("C:\\Users\\Default");
         assert(!homeDir.empty());
-        return canonPath(homeDir);
+        return canonPath(homeDir.string());
     }();
     return homeDir;
 }
diff --git a/src/nix/nix-env/nix-env.cc b/src/nix/nix-env/nix-env.cc
index edfccffa6..a673259f2 100644
--- a/src/nix/nix-env/nix-env.cc
+++ b/src/nix/nix-env/nix-env.cc
@@ -1292,7 +1292,7 @@ static void opSwitchProfile(Globals & globals, Strings opFlags, Strings opArgs)
         throw UsageError("exactly one argument expected");
 
     Path profile = absPath(opArgs.front());
-    Path profileLink = settings.useXDGBaseDirectories ? createNixStateDir() + "/profile" : getHome() + "/.nix-profile";
+    auto profileLink = settings.useXDGBaseDirectories ? createNixStateDir() / "profile" : getHome() / ".nix-profile";
 
     switchLink(profileLink, profile);
 }
diff --git a/src/nix/profile.cc b/src/nix/profile.cc
index 5b0e033e0..822c8046e 100644
--- a/src/nix/profile.cc
+++ b/src/nix/profile.cc
@@ -247,13 +247,13 @@ struct ProfileManifest
             }
         }
 
-        buildProfile(tempDir, std::move(pkgs));
+        buildProfile(tempDir.string(), std::move(pkgs));
 
-        writeFile(tempDir + "/manifest.json", toJSON(*store).dump());
+        writeFile(tempDir / "manifest.json", toJSON(*store).dump());
 
         /* Add the symlink tree to the store. */
         StringSink sink;
-        dumpPath(tempDir, sink);
+        dumpPath(tempDir.string(), sink);
 
         auto narHash = hashString(HashAlgorithm::SHA256, sink.s);
 
diff --git a/src/nix/registry.cc b/src/nix/registry.cc
index d62665a58..6d913adcd 100644
--- a/src/nix/registry.cc
+++ b/src/nix/registry.cc
@@ -43,7 +43,7 @@ public:
     Path getRegistryPath()
     {
         if (registry_path.empty()) {
-            return fetchers::getUserRegistryPath();
+            return fetchers::getUserRegistryPath().string();
         } else {
             return registry_path;
         }