diff --git a/src/libexpr-tests/nix_api_expr.cc b/src/libexpr-tests/nix_api_expr.cc
index 026c89598..fdd09a449 100644
--- a/src/libexpr-tests/nix_api_expr.cc
+++ b/src/libexpr-tests/nix_api_expr.cc
@@ -49,7 +49,7 @@ TEST_F(nix_api_expr_test, nix_eval_state_lookup_path)
 
     auto pathStr = nix_get_path_string(ctx, value);
     assert_ctx_ok();
-    ASSERT_EQ(0, strcmp(pathStr, nixpkgs.c_str()));
+    ASSERT_EQ(0, strcmp(pathStr, nixpkgs.string().c_str()));
 
     nix_gc_decref(nullptr, value);
 }
diff --git a/src/libflake-tests/nix_api_flake.cc b/src/libflake-tests/nix_api_flake.cc
index 84ac8eb40..ec690b812 100644
--- a/src/libflake-tests/nix_api_flake.cc
+++ b/src/libflake-tests/nix_api_flake.cc
@@ -121,8 +121,8 @@ TEST_F(nix_api_store_test, nix_api_load_flake)
     assert_ctx_ok();
     ASSERT_NE(nullptr, parseFlags);
 
-    auto r0 =
-        nix_flake_reference_parse_flags_set_base_directory(ctx, parseFlags, tmpDir.c_str(), tmpDir.string().size());
+    auto r0 = nix_flake_reference_parse_flags_set_base_directory(
+        ctx, parseFlags, tmpDir.string().c_str(), tmpDir.string().size());
     assert_ctx_ok();
     ASSERT_EQ(NIX_OK, r0);
 
@@ -231,8 +231,8 @@ TEST_F(nix_api_store_test, nix_api_load_flake_with_flags)
     assert_ctx_ok();
     ASSERT_NE(nullptr, parseFlags);
 
-    auto r0 =
-        nix_flake_reference_parse_flags_set_base_directory(ctx, parseFlags, tmpDir.c_str(), tmpDir.string().size());
+    auto r0 = nix_flake_reference_parse_flags_set_base_directory(
+        ctx, parseFlags, tmpDir.string().c_str(), tmpDir.string().size());
     assert_ctx_ok();
     ASSERT_EQ(NIX_OK, r0);
 
diff --git a/src/libstore-test-support/include/nix/store/tests/nix_api_store.hh b/src/libstore-test-support/include/nix/store/tests/nix_api_store.hh
index 829972d84..a35d2b1ee 100644
--- a/src/libstore-test-support/include/nix/store/tests/nix_api_store.hh
+++ b/src/libstore-test-support/include/nix/store/tests/nix_api_store.hh
@@ -41,9 +41,9 @@ protected:
     {
 #ifdef _WIN32
         // no `mkdtemp` with MinGW
-        auto tmpl = nix::defaultTempDir() + "/tests_nix-store.";
+        auto tmpl = nix::defaultTempDir() / "tests_nix-store.";
         for (size_t i = 0; true; ++i) {
-            nixDir = tmpl + std::string{i};
+            nixDir = tmpl.string() + std::to_string(i);
             if (std::filesystem::create_directory(nixDir))
                 break;
         }
diff --git a/src/libstore-tests/nar-info-disk-cache.cc b/src/libstore-tests/nar-info-disk-cache.cc
index 98a94b91e..b925a4a1e 100644
--- a/src/libstore-tests/nar-info-disk-cache.cc
+++ b/src/libstore-tests/nar-info-disk-cache.cc
@@ -14,9 +14,9 @@ TEST(NarInfoDiskCacheImpl, create_and_read)
     int prio = 12345;
     bool wantMassQuery = true;
 
-    Path tmpDir = createTempDir();
+    auto tmpDir = createTempDir();
     AutoDelete delTmpDir(tmpDir);
-    Path dbPath(tmpDir + "/test-narinfo-disk-cache.sqlite");
+    auto dbPath(tmpDir / "test-narinfo-disk-cache.sqlite");
 
     int savedId;
     int barId;
@@ -24,7 +24,7 @@ TEST(NarInfoDiskCacheImpl, create_and_read)
     SQLiteStmt getIds;
 
     {
-        auto cache = getTestNarInfoDiskCache(dbPath);
+        auto cache = getTestNarInfoDiskCache(dbPath.string());
 
         // Set up "background noise" and check that different caches receive different ids
         {
@@ -73,7 +73,7 @@ TEST(NarInfoDiskCacheImpl, create_and_read)
     {
         // We can't clear the in-memory cache, so we use a new cache object. This is
         // more realistic anyway.
-        auto cache2 = getTestNarInfoDiskCache(dbPath);
+        auto cache2 = getTestNarInfoDiskCache(dbPath.string());
 
         {
             auto r = cache2->upToDateCacheExists("http://foo");
diff --git a/src/libstore-tests/nix_api_store.cc b/src/libstore-tests/nix_api_store.cc
index 142f648d3..ea600f905 100644
--- a/src/libstore-tests/nix_api_store.cc
+++ b/src/libstore-tests/nix_api_store.cc
@@ -212,9 +212,9 @@ TEST_F(nix_api_store_test, nix_store_real_path)
 TEST_F(nix_api_util_context, nix_store_real_path_relocated)
 {
     auto tmp = nix::createTempDir();
-    std::string storeRoot = tmp / "store";
-    std::string stateDir = tmp / "state";
-    std::string logDir = tmp / "log";
+    auto storeRoot = (tmp / "store").string();
+    auto stateDir = (tmp / "state").string();
+    auto logDir = (tmp / "log").string();
     const char * rootkv[] = {"root", storeRoot.c_str()};
     const char * statekv[] = {"state", stateDir.c_str()};
     const char * logkv[] = {"log", logDir.c_str()};
diff --git a/src/libutil-tests/file-system.cc b/src/libutil-tests/file-system.cc
index d5b810110..1551227cb 100644
--- a/src/libutil-tests/file-system.cc
+++ b/src/libutil-tests/file-system.cc
@@ -307,10 +307,10 @@ TEST(DirectoryIterator, works)
     auto tmpDir = nix::createTempDir();
     nix::AutoDelete delTmpDir(tmpDir, true);
 
-    nix::writeFile(tmpDir + "/somefile", "");
+    nix::writeFile(tmpDir / "somefile", "");
 
     for (auto path : DirectoryIterator(tmpDir)) {
-        ASSERT_EQ(path.path().string(), tmpDir + "/somefile");
+        ASSERT_EQ(path.path(), tmpDir / "somefile");
     }
 }
 
@@ -388,13 +388,14 @@ TEST(openFileEnsureBeneathNoSymlinks, works)
 TEST(createAnonymousTempFile, works)
 {
     auto fd = createAnonymousTempFile();
+    auto fd_ = fromDescriptorReadOnly(fd.get());
     writeFull(fd.get(), "test");
-    lseek(fd.get(), 0, SEEK_SET);
+    lseek(fd_, 0, SEEK_SET);
     FdSource source{fd.get()};
     EXPECT_EQ(source.drain(), "test");
-    lseek(fd.get(), 0, SEEK_END);
+    lseek(fd_, 0, SEEK_END);
     writeFull(fd.get(), "test");
-    lseek(fd.get(), 0, SEEK_SET);
+    lseek(fd_, 0, SEEK_SET);
     EXPECT_EQ(source.drain(), "testtest");
 }
 
@@ -405,8 +406,9 @@ TEST(createAnonymousTempFile, works)
 TEST(FdSource, restartWorks)
 {
     auto fd = createAnonymousTempFile();
+    auto fd_ = fromDescriptorReadOnly(fd.get());
     writeFull(fd.get(), "hello world");
-    lseek(fd.get(), 0, SEEK_SET);
+    lseek(fd_, 0, SEEK_SET);
     FdSource source{fd.get()};
     EXPECT_EQ(source.drain(), "hello world");
     source.restart();