Add char8_t* overload for _json and _json_pointer (#4963)

tests/src/unit-deserialization.cpp

@@ -19,6 +19,11 @@ using nlohmann::json;
 #include <sstream>
 #include <valarray>
 
+#if defined(_WIN32)
+    #define NOMINMAX
+    #include <windows.h> // for GetACP()
+#endif
+
 namespace
 {
 struct SaxEventLogger : public nlohmann::json_sax<json>
@@ -213,6 +218,20 @@ class proxy_iterator
   private:
     iterator* m_it = nullptr;
 };
+
+// JSON_HAS_CPP_20
+#if defined(__cpp_char8_t)
+bool check_utf8()
+{
+#if defined(_WIN32)
+    // Runtime check of the active ANSI code page
+    // 65001 == UTF-8
+    return GetACP() == 65001;
+#else
+    return true;
+#endif
+}
+#endif
 } // namespace
 
 TEST_CASE("deserialization")
@@ -1132,6 +1151,31 @@ TEST_CASE("deserialization")
             CHECK(object_count == 4);
         }
     }
+
+    // build with C++20
+    // JSON_HAS_CPP_20
+#if defined(__cpp_char8_t)
+    SECTION("Using _json with char8_t literals #4945")
+    {
+        // Regular narrow string literal
+        const auto j1 = R"({"key": "value", "num": 42})"_json;
+        CHECK(j1["key"] == "value");
+        CHECK(j1["num"] == 42);
+
+        // UTF-8 prefixed literal (C++20 and later);
+        // MSVC may not set /utf-8, so we need to check
+        if (check_utf8())
+        {
+            const auto j2 = u8R"({"emoji": "😀", "msg": "hello"})"_json;
+            CHECK(j2["emoji"] == "😀");
+            CHECK(j2["msg"] == "hello");
+        }
+
+        const auto j3 = u8R"({"key": "value", "num": 42})"_json;
+        CHECK(j3["key"] == "value");
+        CHECK(j3["num"] == 42);
+    }
+#endif
 }
 
 // select the types to test - char8_t is only available since C++20 if and only

tests/src/unit-json_pointer.cpp

@@ -788,4 +788,18 @@ TEST_CASE("JSON pointers")
             CHECK_FALSE(ptr_oj != ptr);
         }
     }
+
+    // build with C++20
+    // JSON_HAS_CPP_20
+#if defined(__cpp_char8_t)
+    SECTION("Using _json_pointer with char8_t literals #4945")
+    {
+        const json j = R"({"a": {"b": {"c": 123}}})"_json;
+        const auto p1 = "/a/b/c"_json_pointer;
+        CHECK(j[p1] == 123);
+
+        const auto p2 = u8"/a/b/c"_json_pointer;
+        CHECK(j[p2] == 123);
+    }
+#endif
 }