Do not use Qt's keyword macros as identifiers

The prefix tree used by unflatten() (#5836) had a member named slots, which
Qt defines as an empty macro, so including a Qt header before this library
failed to compile. Rename it to table and add a test that includes the
library with Qt's keyword macros (slots, signals, emit, foreach, forever)
defined.

Fixes #5865

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

tests/src/unit-qt_keywords.cpp

@@ -0,0 +1,62 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  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
+
+#include "doctest_compatibility.h"
+
+// Qt defines its keywords as macros (see qtmetamacros.h and qforeach.h) unless
+// QT_NO_KEYWORDS is defined, and it is common to include Qt headers before this
+// library; it must therefore not use these words as identifiers (see #5865)
+#define slots
+#define signals public
+#define emit
+#define Q_FOREACH(variable, container) for (variable : container)
+#define foreach Q_FOREACH
+#define forever for (;;)
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+TEST_CASE("Qt keyword macros")
+{
+    SECTION("parse and dump")
+    {
+        const json j = json::parse(R"({"a": [1, 2.5, "x", null, true]})");
+        CHECK(j.dump() == R"({"a":[1,2.5,"x",null,true]})");
+    }
+
+    SECTION("flatten and unflatten")
+    {
+        const json j = {{"a", {{"b", {1, 2}}, {"c", "d"}}}, {"e", nullptr}};
+        CHECK(j.flatten().unflatten() == j);
+    }
+
+    SECTION("JSON pointer")
+    {
+        json j = {{"a", {{"b", {1, 2}}}}};
+        CHECK(j.at(json::json_pointer("/a/b/1")) == 2);
+        j[json::json_pointer("/a/c")] = 3;
+        CHECK(j.contains(json::json_pointer("/a/c")));
+    }
+
+    SECTION("binary formats")
+    {
+        const json j = {{"a", {1, 2, 3}}, {"b", "c"}};
+        CHECK(json::from_cbor(json::to_cbor(j)) == j);
+        CHECK(json::from_msgpack(json::to_msgpack(j)) == j);
+        CHECK(json::from_ubjson(json::to_ubjson(j)) == j);
+        CHECK(json::from_bjdata(json::to_bjdata(j)) == j);
+        CHECK(json::from_bson(json::to_bson(j)) == j);
+    }
+
+    SECTION("patch and diff")
+    {
+        const json source = {{"a", 1}, {"b", {1, 2}}};
+        const json target = {{"a", 2}, {"c", "d"}};
+        CHECK(source.patch(json::diff(source, target)) == target);
+    }
+}