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Fix discarded nodiscard results across the test suite for GCC's warn_unused_result

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A plain (void) cast on a call expression suppresses the C++17 [[nodiscard]]
warning but not GCC's warning for functions annotated via the GNU
__attribute__((warn_unused_result)) form -- which is what
JSON_HEDLEY_WARN_UNUSED_RESULT expands to on GCC. Several existing tests
that call a newly-annotated function (dump(), empty()) purely to check
that it throws/does not throw, discarding the result via (void), newly
warned (and failed -Werror builds) once the annotation was broadened.
Route those discards through a small ignore_return_value() helper
instead, which actually consumes the value and suppresses the warning
on both attribute forms.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-06 12:18:38 +02:00
parent 354d5fa723
commit 7cb4d3284e
11 changed files with 63 additions and 22 deletions
Download Patch File Download Diff File Expand all files Collapse all files
tests/src/test_utils.hpp
+9
View File
@@ -15,6 +15,15 @@
namespace utils
{
// Some tests intentionally discard the [[nodiscard]]/JSON_HEDLEY_WARN_UNUSED_RESULT
// return value of a call they only make to exercise its side effects (e.g. checking
// that it does not throw). A plain (void) cast on the call expression does not
// suppress GCC's warning for functions using the GNU __attribute__((warn_unused_result))
// form (as opposed to the C++17 [[nodiscard]] attribute) -- passing the value into an
// ordinary function call does.
template<typename T>
inline void ignore_return_value(T&& /*unused*/) noexcept {}
inline std::vector<std::uint8_t> read_binary_file(const std::string& filename)
{
std::ifstream file(filename, std::ios::binary);
tests/src/unit-class_parser.cpp
+4 -1
View File
@@ -23,6 +23,8 @@ using nlohmann::json;
#include <utility>
#include <vector>
#include "test_utils.hpp"
namespace
{
class SaxEventLogger
@@ -624,7 +626,8 @@ TEST_CASE("parser class")
SECTION("overflow")
{
// overflows during parsing yield an exception
CHECK_THROWS_WITH_AS(parser_helper("1.18973e+4932").empty(), "[json.exception.out_of_range.406] number overflow parsing '1.18973e+4932'", json::out_of_range&);
// empty() is nodiscard; the exception is thrown by parser_helper() itself, before empty() would run
CHECK_THROWS_WITH_AS(utils::ignore_return_value(parser_helper("1.18973e+4932").empty()), "[json.exception.out_of_range.406] number overflow parsing '1.18973e+4932'", json::out_of_range&);
}
SECTION("invalid numbers")
tests/src/unit-class_parser_diagnostic_positions.cpp
+4 -1
View File
@@ -22,6 +22,8 @@ using nlohmann::json;
#include <valarray>
#include "test_utils.hpp"
namespace
{
class SaxEventLogger
@@ -629,7 +631,8 @@ TEST_CASE("parser class")
SECTION("overflow")
{
// overflows during parsing yield an exception
CHECK_THROWS_WITH_AS(parser_helper("1.18973e+4932").empty(), "[json.exception.out_of_range.406] number overflow parsing '1.18973e+4932'", json::out_of_range&);
// empty() is nodiscard; the exception is thrown by parser_helper() itself, before empty() would run
CHECK_THROWS_WITH_AS(utils::ignore_return_value(parser_helper("1.18973e+4932").empty()), "[json.exception.out_of_range.406] number overflow parsing '1.18973e+4932'", json::out_of_range&);
}
SECTION("invalid numbers")
tests/src/unit-regression1.cpp
+3 -1
View File
@@ -29,6 +29,7 @@ using nlohmann::json;
#include <limits>
#include <cstdio>
#include "make_test_data_available.hpp"
#include "test_utils.hpp"
#ifdef JSON_HAS_CPP_17
#include <variant>
@@ -1373,7 +1374,8 @@ TEST_CASE("regression tests 1")
std::array<uint8_t, 28> key1 = {{ 103, 92, 117, 48, 48, 48, 55, 92, 114, 215, 126, 214, 95, 92, 34, 174, 40, 71, 38, 174, 40, 71, 38, 223, 134, 247, 127, 0 }};
std::string const key1_str(reinterpret_cast<char*>(key1.data()));
json const j = key1_str;
CHECK_THROWS_WITH_AS(j.dump(), "[json.exception.type_error.316] invalid UTF-8 byte at index 10: 0x7E", json::type_error&);
// dump() is nodiscard; the exception is thrown by dump() itself before it would return
CHECK_THROWS_WITH_AS(utils::ignore_return_value(j.dump()), "[json.exception.type_error.316] invalid UTF-8 byte at index 10: 0x7E", json::type_error&);
}
#if JSON_USE_IMPLICIT_CONVERSIONS
tests/src/unit-regression2.cpp
+15 -7
View File
@@ -31,6 +31,8 @@ using ordered_json = nlohmann::ordered_json;
#include <type_traits>
#include <utility>
#include "test_utils.hpp"
#ifdef JSON_HAS_CPP_17
#include <any>
#include <variant>
@@ -639,8 +641,12 @@ TEST_CASE("regression tests 2")
s += static_cast<char>(i);
}
dump_test["1"] = s;
// dump() is nodiscard; this only checks that dumping does not throw/crash
(void)dump_test.dump(-1, ' ', true, nlohmann::json::error_handler_t::replace);
// dump() is nodiscard; this only checks that dumping does not throw/crash.
// A (void) cast on the call itself does not suppress GCC's warning for the
// GNU warn_unused_result attribute (unlike a real C++17 [[nodiscard]]), so
// capture the result in a variable and discard that instead.
auto dump_result = dump_test.dump(-1, ' ', true, nlohmann::json::error_handler_t::replace);
(void)dump_result;
}
}
@@ -732,12 +738,14 @@ TEST_CASE("regression tests 2")
{
const std::array<unsigned char, 23> data = {{0x81, 0xA4, 0x64, 0x61, 0x74, 0x61, 0xC4, 0x0F, 0x33, 0x30, 0x30, 0x32, 0x33, 0x34, 0x30, 0x31, 0x30, 0x37, 0x30, 0x35, 0x30, 0x31, 0x30}};
const json j = json::from_msgpack(data.data(), data.size());
// dump() is nodiscard; this only checks that dumping does not throw
CHECK_NOTHROW(
j.dump(4, // Indent
' ', // Indent char
false, // Ensure ascii
json::error_handler_t::strict // Error
));
utils::ignore_return_value(
j.dump(4, // Indent
' ', // Indent char
false, // Ensure ascii
json::error_handler_t::strict // Error
)));
}
SECTION("PR #2181 - regression bug with lvalue")
tests/src/unit-serialization.cpp
+11 -6
View File
@@ -15,6 +15,8 @@ using nlohmann::json;
#include <sstream>
#include <iomanip>
#include "test_utils.hpp"
TEST_CASE("serialization")
{
SECTION("operator<<")
@@ -84,8 +86,9 @@ TEST_CASE("serialization")
{
const json j = "ä\xA9ü";
CHECK_THROWS_WITH_AS(j.dump(), "[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9", json::type_error&);
CHECK_THROWS_WITH_AS(j.dump(1, ' ', false, json::error_handler_t::strict), "[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9", json::type_error&);
// dump() is nodiscard; the exception is thrown by dump() itself before it would return
CHECK_THROWS_WITH_AS(utils::ignore_return_value(j.dump()), "[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9", json::type_error&);
CHECK_THROWS_WITH_AS(utils::ignore_return_value(j.dump(1, ' ', false, json::error_handler_t::strict)), "[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9", json::type_error&);
CHECK(j.dump(-1, ' ', false, json::error_handler_t::ignore) == "\"äü\"");
CHECK(j.dump(-1, ' ', false, json::error_handler_t::replace) == "\"ä\xEF\xBF\xBDü\"");
CHECK(j.dump(-1, ' ', true, json::error_handler_t::replace) == "\"\\u00e4\\ufffd\\u00fc\"");
@@ -95,8 +98,9 @@ TEST_CASE("serialization")
{
const json j = "123\xC2";
CHECK_THROWS_WITH_AS(j.dump(), "[json.exception.type_error.316] incomplete UTF-8 string; last byte: 0xC2", json::type_error&);
CHECK_THROWS_AS(j.dump(1, ' ', false, json::error_handler_t::strict), json::type_error&);
// dump() is nodiscard; the exception is thrown by dump() itself before it would return
CHECK_THROWS_WITH_AS(utils::ignore_return_value(j.dump()), "[json.exception.type_error.316] incomplete UTF-8 string; last byte: 0xC2", json::type_error&);
CHECK_THROWS_AS(utils::ignore_return_value(j.dump(1, ' ', false, json::error_handler_t::strict)), json::type_error&);
CHECK(j.dump(-1, ' ', false, json::error_handler_t::ignore) == "\"123\"");
CHECK(j.dump(-1, ' ', false, json::error_handler_t::replace) == "\"123\xEF\xBF\xBD\"");
CHECK(j.dump(-1, ' ', true, json::error_handler_t::replace) == "\"123\\ufffd\"");
@@ -106,8 +110,9 @@ TEST_CASE("serialization")
{
const json j = "123\xF1\xB0\x34\x35\x36";
CHECK_THROWS_WITH_AS(j.dump(), "[json.exception.type_error.316] invalid UTF-8 byte at index 5: 0x34", json::type_error&);
CHECK_THROWS_AS(j.dump(1, ' ', false, json::error_handler_t::strict), json::type_error&);
// dump() is nodiscard; the exception is thrown by dump() itself before it would return
CHECK_THROWS_WITH_AS(utils::ignore_return_value(j.dump()), "[json.exception.type_error.316] invalid UTF-8 byte at index 5: 0x34", json::type_error&);
CHECK_THROWS_AS(utils::ignore_return_value(j.dump(1, ' ', false, json::error_handler_t::strict)), json::type_error&);
CHECK(j.dump(-1, ' ', false, json::error_handler_t::ignore) == "\"123456\"");
CHECK(j.dump(-1, ' ', false, json::error_handler_t::replace) == "\"123\xEF\xBF\xBD\x34\x35\x36\"");
CHECK(j.dump(-1, ' ', true, json::error_handler_t::replace) == "\"123\\ufffd456\"");
tests/src/unit-unicode1.cpp
+5 -2
View File
@@ -17,6 +17,7 @@ using nlohmann::json;
#include <sstream>
#include <iomanip>
#include "make_test_data_available.hpp"
#include "test_utils.hpp"
TEST_CASE("Unicode (1/5)" * doctest::skip())
{
@@ -240,7 +241,8 @@ void roundtrip(bool success_expected, const std::string& s)
if (success_expected)
{
// serialization succeeds
CHECK_NOTHROW(j.dump());
// dump() is nodiscard; this only checks that dumping does not throw
CHECK_NOTHROW(utils::ignore_return_value(j.dump()));
// exclude parse test for U+0000
if (s[0] != '\0')
@@ -259,7 +261,8 @@ void roundtrip(bool success_expected, const std::string& s)
else
{
// serialization fails
CHECK_THROWS_AS(j.dump(), json::type_error&);
// dump() is nodiscard; the exception is thrown by dump() itself before it would return
CHECK_THROWS_AS(utils::ignore_return_value(j.dump()), json::type_error&);
// parsing JSON text fails
CHECK_THROWS_AS(_ = json::parse(ps), json::parse_error&);
tests/src/unit-unicode2.cpp
+3 -1
View File
@@ -19,6 +19,7 @@ using nlohmann::json;
#include <iostream>
#include <iomanip>
#include "make_test_data_available.hpp"
#include "test_utils.hpp"
// this test suite uses static variables with non-trivial destructors
DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
@@ -97,7 +98,8 @@ void check_utf8dump(bool success_expected, int byte1, int byte2 = -1, int byte3
else
{
// strict mode must throw if success is not expected
CHECK_THROWS_AS(j.dump(), json::type_error&);
// dump() is nodiscard; the exception is thrown by dump() itself before it would return
CHECK_THROWS_AS(utils::ignore_return_value(j.dump()), json::type_error&);
// ignore and replace must create different dumps
CHECK(s_ignored != s_replaced);
tests/src/unit-unicode3.cpp
+3 -1
View File
@@ -19,6 +19,7 @@ using nlohmann::json;
#include <iostream>
#include <iomanip>
#include "make_test_data_available.hpp"
#include "test_utils.hpp"
// this test suite uses static variables with non-trivial destructors
DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
@@ -97,7 +98,8 @@ void check_utf8dump(bool success_expected, int byte1, int byte2 = -1, int byte3
else
{
// strict mode must throw if success is not expected
CHECK_THROWS_AS(j.dump(), json::type_error&);
// dump() is nodiscard; the exception is thrown by dump() itself before it would return
CHECK_THROWS_AS(utils::ignore_return_value(j.dump()), json::type_error&);
// ignore and replace must create different dumps
CHECK(s_ignored != s_replaced);
tests/src/unit-unicode4.cpp
+3 -1
View File
@@ -19,6 +19,7 @@ using nlohmann::json;
#include <iostream>
#include <iomanip>
#include "make_test_data_available.hpp"
#include "test_utils.hpp"
// this test suite uses static variables with non-trivial destructors
DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
@@ -97,7 +98,8 @@ void check_utf8dump(bool success_expected, int byte1, int byte2 = -1, int byte3
else
{
// strict mode must throw if success is not expected
CHECK_THROWS_AS(j.dump(), json::type_error&);
// dump() is nodiscard; the exception is thrown by dump() itself before it would return
CHECK_THROWS_AS(utils::ignore_return_value(j.dump()), json::type_error&);
// ignore and replace must create different dumps
CHECK(s_ignored != s_replaced);
tests/src/unit-unicode5.cpp
+3 -1
View File
@@ -19,6 +19,7 @@ using nlohmann::json;
#include <iostream>
#include <iomanip>
#include "make_test_data_available.hpp"
#include "test_utils.hpp"
// this test suite uses static variables with non-trivial destructors
DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
@@ -97,7 +98,8 @@ void check_utf8dump(bool success_expected, int byte1, int byte2 = -1, int byte3
else
{
// strict mode must throw if success is not expected
CHECK_THROWS_AS(j.dump(), json::type_error&);
// dump() is nodiscard; the exception is thrown by dump() itself before it would return
CHECK_THROWS_AS(utils::ignore_return_value(j.dump()), json::type_error&);
// ignore and replace must create different dumps
CHECK(s_ignored != s_replaced);