From 60734e063eec18f066d599e7e29591d96928e25d Mon Sep 17 00:00:00 2001
From: HedgehogInTheCPP <a.rainman@gmail.com>
Date: Fri, 21 Aug 2026 19:56:09 +0300
Subject: [PATCH] Improvements detection of features, part 5.

---
 CMakeLists.txt                                | 15 ++++----
 include/fast_float/constexpr_feature_detect.h | 37 +++++++------------
 2 files changed, 21 insertions(+), 31 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index f0f3907..f8e5d2f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -17,7 +17,7 @@ else(FASTFLOAT_TEST)
 endif(FASTFLOAT_TEST)
 
 option(FASTFLOAT_SANITIZE "Sanitize addresses" OFF)
-if (NOT CMAKE_BUILD_TYPE)
+if(NOT CMAKE_BUILD_TYPE)
   if(FASTFLOAT_SANITIZE)
     set(CMAKE_BUILD_TYPE Debug CACHE STRING "Choose the type of build." FORCE)
   else()
@@ -67,16 +67,15 @@ if(FASTFLOAT_SANITIZE)
       -fsanitize=undefined
       -fno-sanitize-recover=all
   )
-  if (CMAKE_COMPILER_IS_GNUCC)
+  if(CMAKE_COMPILER_IS_GNUCC)
     target_link_libraries(fast_float INTERFACE -fuse-ld=gold)
   endif()
 endif()
 
-target_compile_options(
-  fast_float
-  INTERFACE
-  $<$<AND:$<CXX_COMPILER_ID:MSVC>,$<VERSION_GREATER_EQUAL:$<CXX_COMPILER_VERSION>,19.10>>:/permissive->
-)
+if(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC"
+   AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 19.10)
+  target_compile_options(fast_float INTERFACE /permissive-)
+endif()
 
 if(FASTFLOAT_INSTALL)
   include(CMakePackageConfigHelpers)
@@ -85,7 +84,7 @@ if(FASTFLOAT_INSTALL)
   set(FASTFLOAT_PROJECT_CONFIG "${CMAKE_CURRENT_BINARY_DIR}/module/FastFloatConfig.cmake")
   set(FASTFLOAT_CONFIG_INSTALL_DIR "${CMAKE_INSTALL_DATAROOTDIR}/cmake/FastFloat")
 
-  if(${CMAKE_VERSION} VERSION_LESS "3.14")
+  if(CMAKE_VERSION VERSION_LESS "3.14")
     write_basic_package_version_file(
       "${FASTFLOAT_VERSION_CONFIG}"
       VERSION ${PROJECT_VERSION}
diff --git a/include/fast_float/constexpr_feature_detect.h b/include/fast_float/constexpr_feature_detect.h
index 9a3b754..e4ce70d 100644
--- a/include/fast_float/constexpr_feature_detect.h
+++ b/include/fast_float/constexpr_feature_detect.h
@@ -8,16 +8,12 @@
 #endif
 
 // C++14 constexpr
-#if defined(_MSC_VER)
-#if defined(_MSVC_LANG) && _MSVC_LANG >= 201402L
-#define FASTFLOAT_CONSTEXPR14 constexpr
-#else
-#define FASTFLOAT_CONSTEXPR14
-#endif
-#elif defined(__cpp_constexpr) && __cpp_constexpr >= 201304L
+#if defined(__cpp_constexpr) && __cpp_constexpr >= 201304L
 #define FASTFLOAT_CONSTEXPR14 constexpr
 #elif defined(__cplusplus) && __cplusplus >= 201402L
 #define FASTFLOAT_CONSTEXPR14 constexpr
+#elif defined(_MSC_VER) && defined(_MSVC_LANG) && _MSVC_LANG >= 201402L
+#define FASTFLOAT_CONSTEXPR14 constexpr
 #else
 #define FASTFLOAT_CONSTEXPR14
 #endif
@@ -58,6 +54,7 @@
 #define FASTFLOAT_INLINE_VARIABLE static constexpr
 #endif
 
+// Before C++17, constexpr variables may need an out-of-class definition.
 #if (defined(__cplusplus) && __cplusplus >= 201703L) ||                        \
     (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
 #define FASTFLOAT_DETAIL_MUST_DEFINE_CONSTEXPR_VARIABLE 0
@@ -72,33 +69,27 @@
 #define FASTFLOAT_HAS_BIT_CAST 0
 #endif
 
+// C++20 std::is_constant_evaluated
 #if defined(__cpp_lib_is_constant_evaluated) &&                                \
     __cpp_lib_is_constant_evaluated >= 201811L
 #define FASTFLOAT_HAS_IS_CONSTANT_EVALUATED 1
-#else
-#define FASTFLOAT_HAS_IS_CONSTANT_EVALUATED 0
-#endif
-
-// C++20 consteval
-#if defined(__cpp_consteval) && __cpp_consteval >= 201811L
 #define FASTFLOAT_CONSTEVAL consteval
 #else
+#define FASTFLOAT_HAS_IS_CONSTANT_EVALUATED 0
 #define FASTFLOAT_CONSTEVAL FASTFLOAT_CONSTEXPR14
 #endif
 
-// C++20 constexpr function support
-#if defined(_MSC_VER)
-#if defined(_MSVC_LANG) && _MSVC_LANG >= 202002L
-#define FASTFLOAT_CONSTEXPR20 constexpr
-#else
-#define FASTFLOAT_CONSTEXPR20
-#endif
-#elif defined(__cpp_constexpr) && __cpp_constexpr >= 201907L
-#define FASTFLOAT_CONSTEXPR20 constexpr
-#elif defined(__cplusplus) && __cplusplus >= 202002L
+// C++20 constexpr library support, including std::is_constant_evaluated(),
+// std::bit_cast(), and constexpr algorithms. Therefore all required library
+// feature macros must be present.
+#if FASTFLOAT_HAS_IS_CONSTANT_EVALUATED && FASTFLOAT_HAS_BIT_CAST &&           \
+    defined(__cpp_lib_constexpr_algorithms) &&                                 \
+    __cpp_lib_constexpr_algorithms >= 201806L
 #define FASTFLOAT_CONSTEXPR20 constexpr
+#define FASTFLOAT_IS_CONSTEXPR 1
 #else
 #define FASTFLOAT_CONSTEXPR20
+#define FASTFLOAT_IS_CONSTEXPR 0
 #endif
 
 // C++20 std::byteswap