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-regression2.cpp

@@ -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>
@@ -639,7 +641,8 @@ TEST_CASE("regression tests 2")
                 s += static_cast<char>(i);
             }
             dump_test["1"] = s;
-            dump_test.dump(-1, ' ', true, nlohmann::json::error_handler_t::replace);
+            // dump() is nodiscard; this only checks that dumping does not throw/crash
+            utils::ignore_return_value(dump_test.dump(-1, ' ', true, nlohmann::json::error_handler_t::replace));
         }
     }
 
@@ -731,12 +734,14 @@ 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")