Broaden JSON_HEDLEY_WARN_UNUSED_RESULT coverage to pure query functions (#5477)

* Broaden JSON_HEDLEY_WARN_UNUSED_RESULT coverage to pure query functions

Add JSON_HEDLEY_WARN_UNUSED_RESULT to the unambiguous, const,
side-effect-free observer functions whose return value is the entire
purpose of the call:

- dump()
- type(), type_name()
- all is_* predicates (is_primitive, is_structured, is_null,
  is_boolean, is_number, is_number_integer, is_number_unsigned,
  is_number_float, is_object, is_array, is_string, is_binary,
  is_discarded)
- empty(), size(), max_size()
- count(...) (both overloads) and contains(...) (all overloads,
  including the deprecated json_pointer<BasicJsonType> overload)

This mirrors the direction the standard library has taken with
[[nodiscard]] on the analogous std::vector/std::map members, and
catches real bugs such as `j.empty();` (meant `j.clear();`) or
`j.contains(k);` with the result thrown away.

Deliberately out of scope (left for a separate, later policy
decision, per the issue): at(), value(), get*(), flatten(),
unflatten(), patch(), merge_patch(), begin()/end(), comparison
operators, erase(), and emplace().

Compiling the full test suite (tests/src/unit-*.cpp) with
-Wunused-result -Werror uncovered one real hit: a regression test in
unit-regression2.cpp called dump() purely to check it does not throw,
discarding the result. Fixed by explicitly casting to void, since the
call is intentionally result-less there.

Fixes #5410

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

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

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>

* Use utils::ignore_return_value() for the issue #1445 dump() discard too

Addresses review feedback from @gregmarr on PR #5477: this call site was
still using the older "capture in a variable, then (void) it" pattern
from before this PR introduced utils::ignore_return_value(), instead of
the helper now used at every other discarded-nodiscard-result call site
this PR touches.

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

---------

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

tests/src/unit-unicode1.cpp

@@ -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&);