Fix long double inf test under Valgrind. Fixes #5175 (#5176)

* Fix for printing long doubles bug in dump_float

When you use long double as a floating point type with the current version of this file and try to dump json it prints trash instead of actual number. This if-else fixes the problem. On using long double you just need to add an 'L' modifier before 'g' in format string.

Signed-off-by: Kirill Lokotkov <klokotkov@ya.ru>

* C++11 compatibility

Signed-off-by: Kirill Lokotkov <klokotkov@ya.ru>

* Shorter solution

Signed-off-by: Kirill Lokotkov <klokotkov@ya.ru>

* Applied amalgamate

Signed-off-by: rusloker <klokotkov@ya.ru>

* Add unit tests for `dump()` with `long double` in custom `basic_json`

Signed-off-by: rusloker <klokotkov@ya.ru>

* Fix UB in `snprintf_float` by using `%.*Lg` for `long double`

Signed-off-by: rusloker <klokotkov@ya.ru>

* Use `std::array` for `values` in serialization unit tests to improve type safety

Signed-off-by: rusloker <klokotkov@ya.ru>

* Fix brace initialization for `std::array` in serialization unit tests

Signed-off-by: rusloker <klokotkov@ya.ru>

* Remove comments in `snprintf_float` regarding `%Lg` usage

Signed-off-by: rusloker <klokotkov@ya.ru>

* Skip `long double` infinity dump assertions under Valgrind

Signed-off-by: rusloker <klokotkov@ya.ru>

* Clarify Valgrind bug-tracker reference in `long double` test

Signed-off-by: rusloker <klokotkov@ya.ru>

* Satisfy clang-tidy in `long double` infinity probe

Signed-off-by: rusloker <klokotkov@ya.ru>

---------

Signed-off-by: Kirill Lokotkov <klokotkov@ya.ru>
Signed-off-by: rusloker <klokotkov@ya.ru>

Author: RUSLoker

tests/src/unit-serialization.cpp

@@ -344,8 +344,22 @@ TEST_CASE("dump for basic_json with long double number_float_t")
     SECTION("NaN and infinity dump as null")
     {
         CHECK(long_double_json(std::numeric_limits<long double>::quiet_NaN()).dump() == "null");
-        CHECK(long_double_json(std::numeric_limits<long double>::infinity()).dump() == "null");
-        CHECK(long_double_json(-std::numeric_limits<long double>::infinity()).dump() == "null");
+
+        // Probe the platform's runtime behavior — `volatile` forces a runtime
+        // call rather than constexpr-folding to a known answer at compile time.
+        // Skip the infinity assertions if std::isfinite() doesn't actually
+        // recognize long double infinity on this platform (notably, Valgrind
+        // 3.22's x87 80-bit emulation reports +/-inf as a large finite value).
+        // TODO(rusloker): remove this guard once Valgrind's 80-bit long double
+        // support ships (Valgrind bug https://bugs.kde.org/show_bug.cgi?id=197915,
+        // ASSIGNED since 2009 — the Valgrind project tracks its bugs on
+        // bugs.kde.org) and the minimum supported Valgrind version contains it.
+        const volatile long double inf_probe = std::numeric_limits<long double>::infinity();
+        if (!std::isfinite(inf_probe))
+        {
+            CHECK(long_double_json(std::numeric_limits<long double>::infinity()).dump() == "null");
+            CHECK(long_double_json(-std::numeric_limits<long double>::infinity()).dump() == "null");
+        }
     }
 
     SECTION("dump output matches double for exactly-representable values")