Fix the clang-tidy findings in the tests and headers of the API stack

- make_move_iterator_if_mutable takes the iterator by const reference
  (performance-unnecessary-value-param).
- throw_patch_missing_member builds its message without a local that
  is unused with JSON_NOEXCEPTION (bugprone-unused-local-non-trivial-variable).
- Tests: NOLINT the copies that test the copy constructor and the
  constant mt19937 seed, NOLINT the deliberate "\xC3" "a" literal
  concatenation, use references for the const views in
  unit-throw_messages and return {} from its lambda.

A full clang-tidy 22 run over all 107 test translation units is clean.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-constructor2.cpp

@@ -198,11 +198,11 @@ TEST_CASE("copy constructor and destructor across value types")
         const json src_uint = 7u;
         const json src_float = 2.5;
 
-        json c_null(src_null);
-        json c_bool(src_bool);
-        json c_int(src_int);
-        json c_uint(src_uint);
-        json c_float(src_float);
+        json c_null(src_null); // NOLINT(performance-unnecessary-copy-initialization): tests the copy constructor
+        json c_bool(src_bool); // NOLINT(performance-unnecessary-copy-initialization): tests the copy constructor
+        json c_int(src_int); // NOLINT(performance-unnecessary-copy-initialization): tests the copy constructor
+        json c_uint(src_uint); // NOLINT(performance-unnecessary-copy-initialization): tests the copy constructor
+        json c_float(src_float); // NOLINT(performance-unnecessary-copy-initialization): tests the copy constructor
 
         CHECK(c_null.is_null());
         CHECK(c_bool == true);

tests/src/unit-json_pointer.cpp

@@ -1230,7 +1230,7 @@ TEST_CASE("JSON pointer array index tokens match strtoull semantics")
 
     SECTION("random tokens")
     {
-        std::mt19937 gen(42);
+        std::mt19937 gen(42); // NOLINT(cert-msc32-c,cert-msc51-cpp,bugprone-random-generator-seed): reproducible
         const std::string alphabet = "0123456789012345678901234567890123456789-+ ex";
         for (int i = 0; i < 5000; ++i)
         {

tests/src/unit-serialization.cpp

@@ -1435,7 +1435,7 @@ TEST_CASE("invalid UTF-8 at every position is handled like the reference")
         "\xF4\x90\x80\x80",
         // sequences cut short by ASCII, a control character, a quote, a
         // backslash, DEL, or the lead byte of a well-formed character
-        "\xC3" "a", "\xE2\x82" "\n", "\xE2\x82\"", "\xF0\x9F\x98\\", "\xC3\x7F", "\xC3\xC3\xA4", "\xE2\xF0\x9F\x98\x80",
+        "\xC3" "a", "\xE2\x82" "\n", "\xE2\x82\"", "\xF0\x9F\x98\\", "\xC3\x7F", "\xC3\xC3\xA4", "\xE2\xF0\x9F\x98\x80", // NOLINT(bugprone-suspicious-missing-comma): "\xC3" "a" ends the hex escape
         // truncated sequences
         "\xC3", "\xE2\x82", "\xF0\x9F\x98"
     };

tests/src/unit-throw_messages.cpp

@@ -54,8 +54,8 @@ TEST_CASE("exception messages of out-of-line throw sites")
 
     SECTION("at")
     {
-        const json carr = arr;
-        const json cobj = obj;
+        const json& carr = arr;
+        const json& cobj = obj;
         CHECK_MSG((void)arr.at(static_cast<json::size_type>(10)), "[json.exception.out_of_range.401] array index 10 is out of range");
         CHECK_MSG((void)carr.at(static_cast<json::size_type>(3)), "[json.exception.out_of_range.401] array index 3 is out of range");
         CHECK_MSG((void)num.at(static_cast<json::size_type>(0)), "[json.exception.type_error.304] cannot use at() with number");
@@ -65,7 +65,7 @@ TEST_CASE("exception messages of out-of-line throw sites")
         CHECK_MSG((void)num.at("a"), "[json.exception.type_error.304] cannot use at() with number");
 
         nlohmann::ordered_json oobj = {{"a", 1}};
-        const nlohmann::ordered_json coobj = oobj;
+        const nlohmann::ordered_json& coobj = oobj;
         CHECK_MSG((void)oobj.at("z"), "[json.exception.out_of_range.403] key 'z' not found");
         CHECK_MSG((void)coobj.at("z"), "[json.exception.out_of_range.403] key 'z' not found");
         CHECK_MSG((void)coobj.at(std::string("zz")), "[json.exception.out_of_range.403] key 'zz' not found");
@@ -97,7 +97,7 @@ TEST_CASE("exception messages of out-of-line throw sites")
 #if !defined(JSON_NOEXCEPTION) // patch_msg() catches the exceptions, which JSON_NOEXCEPTION turns into std::abort()
     SECTION("patch")
     {
-        const auto patch_msg = [](const char* doc, const char* patch)
+        const auto patch_msg = [](const char* doc, const char* patch) -> std::string
         {
             try
             {
@@ -110,8 +110,9 @@ TEST_CASE("exception messages of out-of-line throw sites")
                 // and with JSON_DIAGNOSTIC_POSITIONS its position like "(bytes 0-2) "; drop both
                 std::string msg = e.what();
                 for (const char* marker :
-                        {" (/", " (bytes "
-                        })
+            {
+                " (/", " (bytes "
+            })
                 {
                     const auto pos = msg.find(marker);
                     if (pos != std::string::npos)
@@ -121,7 +122,7 @@ TEST_CASE("exception messages of out-of-line throw sites")
                 }
                 return msg;
             }
-            return std::string();
+            return {};
         };
 
         CHECK(patch_msg("{}", "{}") == "[json.exception.parse_error.104] parse error: JSON patch must be an array of objects");