From 288a69b1d2e41afcbb8a975a022e0c5d04435d02 Mon Sep 17 00:00:00 2001
From: Niels Lohmann <mail@nlohmann.me>
Date: Sat, 5 Sep 2026 21:10:04 +0200
Subject: [PATCH] test: add direct coverage for the std::u8string to_json
 overload (#5423)

The ADL to_json overload for std::basic_string<char8_t, ...> was only ever
reached indirectly, via std::filesystem::path::u8string(). Add a test that
constructs a json value directly from a std::u8string, gated the same way as
the overload itself (include/nlohmann/detail/conversions/to_json.hpp): behind
both the std::filesystem::path feature guard and __cpp_lib_char8_t, since the
overload only exists when both are satisfied.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
---
 tests/src/unit-conversions.cpp | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/tests/src/unit-conversions.cpp b/tests/src/unit-conversions.cpp
index 1937affbb..f1cd94aef 100644
--- a/tests/src/unit-conversions.cpp
+++ b/tests/src/unit-conversions.cpp
@@ -1761,6 +1761,33 @@ TEST_CASE("std::filesystem::path")
 }
 #endif
 
+// the ADL to_json overload for std::u8string only exists under the same guard
+// as std::filesystem::path support (it is otherwise only reached indirectly,
+// via std::filesystem::path::u8string()) -- mirror both #if conditions from
+// include/nlohmann/detail/conversions/to_json.hpp exactly
+#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
+#if defined(__cpp_lib_char8_t)
+TEST_CASE("std::u8string")
+{
+    SECTION("ascii")
+    {
+        const std::u8string s = u8"Path";
+        json const j = s;
+
+        CHECK(j.template get<std::string>() == "Path");
+    }
+
+    SECTION("utf-8")
+    {
+        const std::u8string s = u8"P\xc4\x9b\xc5\xa1ina";
+        json const j = s;
+
+        CHECK(j.template get<std::string>() == "P\xc4\x9b\xc5\xa1ina");
+    }
+}
+#endif
+#endif
+
 TEST_CASE("std::optional")
 {
     SECTION("null")