test: cover JSON_NO_IO, JSON_THROW/TRY/CATCH_USER, JSON_SKIP_LIBRARY_VERSION_CHECK, and JSON_DisableEnumSerialization in CI (#5423)

These four supported configuration macros were never actually compiled
anywhere in the test matrix:

- JSON_NO_IO and the JSON_THROW_USER/JSON_TRY_USER/JSON_CATCH_USER trio
  are exercised together in a new tests/src/unit-no_io_and_user_exceptions.cpp,
  which is automatically picked up by the existing unit-*.cpp test glob and
  thus built across the whole standard test matrix.
- JSON_SKIP_LIBRARY_VERSION_CHECK is exercised by a new, dedicated
  tests/src/skip_library_version_check.cpp, compiled directly by the new
  ci_test_skiplibraryversioncheck target in cmake/ci.cmake: the scenario it
  simulates (mixing two differently-versioned inclusions of the library)
  unavoidably triggers the compiler's own "macro redefined" warning, which
  would fail under the library's own -Weverything/-Werror unit test matrix
  for a reason unrelated to the macro under test.
- JSON_DisableEnumSerialization already had #if-guarded tests in several
  unit-*.cpp files (from #4384), but no CMake target ever actually set the
  JSON_DisableEnumSerialization CMake option, so that guarded code was never
  compiled. Add ci_test_disableenumserialization, mirroring the existing
  ci_test_noimplicitconversions/ci_test_noglobaludls targets. Building the
  full test suite with this option on surfaced one real, narrow gap: get<T>()
  on std::vector<std::byte> (used by unit-regression2.cpp's custom BinaryType
  tests) relies on std::byte being handled via enum serialization, so add the
  same #if-guard convention to the two affected SECTIONs there.

Both new CI targets are added to the ci_cmake_options matrix in
.github/workflows/ubuntu.yml, alongside the existing ci_test_* targets.

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

tests/src/unit-no_io_and_user_exceptions.cpp

@@ -0,0 +1,69 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  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
+
+// This translation unit is a dedicated, small compile-and-run check for two
+// configuration macros that (per #5423) were never exercised anywhere in the
+// test matrix:
+// - JSON_NO_IO, which removes the library's <istream>/<ostream> support
+//   (operator<<, operator>>, and the stream-based overloads of dump()/parse())
+// - the JSON_THROW_USER / JSON_TRY_USER / JSON_CATCH_USER trio, which lets a
+//   user replace the library's internal exception handling
+//
+// Both macros are about excluding/replacing a facility the library would
+// otherwise pull in on its own, and defining one has no bearing on the other,
+// so -- to keep the test matrix small -- they are exercised together in a
+// single dedicated file instead of two.
+//
+// JSON_NO_IO requires this file itself to never rely on <iostream>/<sstream>;
+// only string-based parsing/dumping is used below.
+#define JSON_NO_IO 1
+
+// The user-supplied exception macros below are a *conforming* replacement:
+// they simply forward to the real throw/try/catch keywords (via a counter so
+// the test can assert each macro was actually invoked, not just defined), so
+// every exception-related behavior the library relies on internally --
+// including rethrowing std::out_of_range as json::out_of_range in at() --
+// keeps working exactly as it would with the library's own default macros.
+static int json_throw_user_call_count = 0; // NOLINT(cppcoreguidelines-avoid-non-const-global-variables)
+
+#define JSON_THROW_USER(exception) do { ++json_throw_user_call_count; throw exception; } while (false) // NOLINT(cppcoreguidelines-macro-usage)
+#define JSON_TRY_USER try // NOLINT(cppcoreguidelines-macro-usage)
+#define JSON_CATCH_USER(exception) catch (exception) // NOLINT(cppcoreguidelines-macro-usage)
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using json = nlohmann::json;
+
+TEST_CASE("JSON_NO_IO")
+{
+    // everything that does not touch <istream>/<ostream> must keep working:
+    // parsing from and dumping to std::string
+    const json j = json::parse("{\"a\":[1,2,3],\"b\":true}");
+    CHECK(j.dump() == "{\"a\":[1,2,3],\"b\":true}");
+    CHECK(j.at("a").size() == 3);
+    CHECK(j.at("b").get<bool>() == true);
+}
+
+TEST_CASE("JSON_THROW_USER, JSON_TRY_USER, JSON_CATCH_USER")
+{
+    json_throw_user_call_count = 0;
+
+    // a parse error goes through JSON_THROW directly, i.e., through our
+    // JSON_THROW_USER override
+    CHECK_THROWS_AS(json::parse("this is not JSON"), json::parse_error&);
+    CHECK(json_throw_user_call_count > 0);
+
+    // at() on an out-of-range array index internally catches std::out_of_range
+    // (JSON_TRY_USER/JSON_CATCH_USER) and rethrows it as json::out_of_range
+    // (JSON_THROW_USER again), so this exercises all three macros together
+    const int count_before = json_throw_user_call_count;
+    const json arr = json::array({1, 2, 3});
+    CHECK_THROWS_AS(arr.at(10), json::out_of_range&);
+    CHECK(json_throw_user_call_count > count_before);
+}