From 3f0f5e0ae4355c90a791fd36e059bd5e0c2249fb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tim=20Jen=C3=9Fen?= <tim.jenssen@qt.io>
Date: Fri, 14 Mar 2025 12:32:39 +0100
Subject: [PATCH] QmlDesigner: proper std::to_chars float fallback

On older toolchains (e.g., RHEL 8, macOS < 13.3), std::to_chars
for floating-point types is may be partially implemented.

Task-number: QDS-14932
Change-Id: I63b17d640efa240e223f506333538a54b121de2c
Reviewed-by: Marco Bubke <marco.bubke@qt.io>
---
 src/libs/nanotrace/staticstring.h | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/src/libs/nanotrace/staticstring.h b/src/libs/nanotrace/staticstring.h
index d787bd2fe30..0cd85ae5fca 100644
--- a/src/libs/nanotrace/staticstring.h
+++ b/src/libs/nanotrace/staticstring.h
@@ -5,14 +5,17 @@
 
 #include <utils/smallstringview.h>
 
-#if !(defined(__cpp_lib_to_chars) && (__cpp_lib_to_chars >= 201611L))
-#  include <QLocale>
-#endif
-
 #include <array>
 #include <charconv>
 #include <limits>
 
+#if !defined(__cpp_lib_to_chars) || (__cpp_lib_to_chars < 201611L) || \
+    (defined(__GNUC__) && __GNUC__ < 11) || \
+    (defined(_MSC_VER) && _MSC_VER < 1930) || \
+    (defined(__clang_major__) && __clang_major__ < 14)
+#define NO_STD_FLOAT_TO_CHARS 1
+#endif
+
 namespace NanotraceHR {
 
 template<std::size_t Capacity>
@@ -58,10 +61,14 @@ public:
     template<typename Type, typename std::enable_if_t<std::is_arithmetic_v<Type>, bool> = true>
     void append(Type number)
     {
-#if !(defined(__cpp_lib_to_chars) && (__cpp_lib_to_chars >= 201611L))
+#if NO_STD_FLOAT_TO_CHARS
+        // Fallback using snprintf with sufficient precision.
         if constexpr (std::is_floating_point_v<Type>) {
-            QLocale locale{QLocale::Language::C};
-            append(locale.toString(number).toStdString());
+            char buffer[std::numeric_limits<Type>::max_digits10 + 2];
+            auto size = std::snprintf(buffer, sizeof(buffer), "%.9f", number);
+
+            if (size >= 0)
+                append({buffer, size});
             return;
         }
 #endif