Suppress GCC's false -Warray-bounds error in set_parents() with JSON_DIAGNOSTICS

With JSON_DIAGNOSTICS, GCC 13 to 15 report a false -Warray-bounds error
at -O3 when set_parents() is inlined right after a non-container value
(e.g., a string) was created: the std::map access in the object branch
is checked against the string allocation although m_type rules that
branch out. GCC 15 fixed the pattern from #4819 but not this one.

Suppress -Warray-bounds around set_parents() only, and add a regression
test that is compiled with -O3 -Werror=array-bounds on GCC.

Fixes #5742

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

tests/src/unit-diagnostics-optimized.cpp

@@ -0,0 +1,76 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// Regression test for https://github.com/nlohmann/json/issues/5742: with
+// JSON_DIAGNOSTICS, GCC (13 to at least 15) reports a false -Warray-bounds
+// error in the inlined set_parents() at -O3. The warning depends on GCC's
+// inlining decisions, so the sections cover patterns that trigger it on
+// different GCC versions (#4819: GCC 13 and 14; #5742: GCC 14 and 15).
+// On GCC, this file is compiled with -O3 -Werror=array-bounds (see
+// tests/CMakeLists.txt), so the test fails to build if the warning returns.
+
+#include "doctest_compatibility.h"
+
+#ifdef JSON_DIAGNOSTICS
+    #undef JSON_DIAGNOSTICS
+#endif
+
+#define JSON_DIAGNOSTICS 1
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+#include <algorithm>
+#include <iterator>
+#include <utility>
+#include <vector>
+
+namespace
+{
+enum class diag_color
+{
+    red,
+    green,
+    blue
+};
+
+void to_json(json& j, const diag_color& c)
+{
+    static const std::pair<diag_color, json> m[] = // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
+    {
+        {diag_color::red, "r"},
+        {diag_color::green, "g"},
+        {diag_color::blue, "b"},
+    };
+    const auto* it = std::find_if(std::begin(m), std::end(m), [c](const std::pair<diag_color, json>& p)
+    {
+        return p.first == c;
+    });
+    j = it->second;
+}
+} // namespace
+
+TEST_CASE("diagnostics with optimization")
+{
+    SECTION("issue #4819 - object in vector")
+    {
+        std::vector<json> jsons{};
+        jsons.emplace_back(json({{"key", "value"}}));
+        CHECK(jsons.back()["key"] == "value");
+    }
+
+    SECTION("issue #5742 - string values from a static table")
+    {
+        json j = json::array();
+        j.push_back(diag_color::red);
+        j.push_back(diag_color::green);
+        j.push_back(diag_color::blue);
+        CHECK(j.dump() == R"(["r","g","b"])");
+        CHECK_THROWS_WITH_AS(j[1].get<int>(), "[json.exception.type_error.302] (/1) type must be number, but is string", json::type_error);
+    }
+}