* improvements in compiler function detection.

Author: IRainman

include/fast_float/constexpr_feature_detect.h

@@ -7,14 +7,32 @@
 #endif
 #endif
 
-// Testing for https://wg21.link/N3652, adopted in C++14
-#if defined(__cpp_constexpr) && __cpp_constexpr >= 201304
+// C++14 constexpr
+#if defined(__cpp_constexpr) && __cpp_constexpr >= 201304L
+#define FASTFLOAT_CONSTEXPR14 constexpr
+#elif __cplusplus >= 201402L
+#define FASTFLOAT_CONSTEXPR14 constexpr
+#elif defined(_MSC_VER) && _MSC_VER >= 1910 && _MSVC_LANG >= 201402L
 #define FASTFLOAT_CONSTEXPR14 constexpr
 #else
 #define FASTFLOAT_CONSTEXPR14
 #endif
 
+// C++14 variable templates
+#if defined(__cpp_variable_templates) && __cpp_variable_templates >= 201304L
+#define FASTFLOAT_HAS_VARIABLE_TEMPLATES 1
+#elif __cplusplus >= 201402L
+#define FASTFLOAT_HAS_VARIABLE_TEMPLATES 1
+#elif defined(_MSC_FULL_VER) && _MSC_FULL_VER >= 190023918L &&                 \
+    _MSVC_LANG >= 201402L
+#define FASTFLOAT_HAS_VARIABLE_TEMPLATES 1
+#else
+#define FASTFLOAT_HAS_VARIABLE_TEMPLATES 0
+#endif
+
+// C++20 std::bit_cast
 #if defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L
+#include <bit>
 #define FASTFLOAT_HAS_BIT_CAST 1
 #else
 #define FASTFLOAT_HAS_BIT_CAST 0
@@ -35,12 +53,39 @@
 #define FASTFLOAT_HAS_BYTESWAP 0
 #endif
 
+// C++17 if constexpr
 #if defined(__cpp_if_constexpr) && __cpp_if_constexpr >= 201606L
 #define FASTFLOAT_IF_CONSTEXPR17(x) if constexpr (x)
+#elif defined(__cpp_constexpr) && __cpp_constexpr >= 201603L
+#define FASTFLOAT_IF_CONSTEXPR17(x) if constexpr (x)
+#elif __cplusplus >= 201703L
+#define FASTFLOAT_IF_CONSTEXPR17(x) if constexpr (x)
+#elif defined(_MSC_VER) && _MSC_VER >= 1911 && _MSVC_LANG >= 201703L
+#define FASTFLOAT_IF_CONSTEXPR17(x) if constexpr (x)
 #else
 #define FASTFLOAT_IF_CONSTEXPR17(x) if (x)
 #endif
 
+// C++17 inline variables
+#if defined(__cpp_inline_variables) && __cpp_inline_variables >= 201606L
+#define FASTFLOAT_INLINE_VARIABLE inline constexpr
+#elif __cplusplus >= 201703L
+#define FASTFLOAT_INLINE_VARIABLE inline constexpr
+#elif defined(_MSC_VER) && _MSC_VER >= 1912 && _MSVC_LANG >= 201703L
+#define FASTFLOAT_INLINE_VARIABLE inline constexpr
+#else
+#define FASTFLOAT_INLINE_VARIABLE static constexpr
+#endif
+
+#if defined(__cpp_lib_is_constant_evaluated) &&                                \
+    __cpp_lib_is_constant_evaluated >= 201811L
+#define FASTFLOAT_HAS_IS_CONSTANT_EVALUATED 1
+#define FASTFLOAT_CONSTEVAL consteval
+#else
+#define FASTFLOAT_HAS_IS_CONSTANT_EVALUATED 0
+#define FASTFLOAT_CONSTEVAL FASTFLOAT_CONSTEXPR14
+#endif
+
 // Testing for relevant C++20 constexpr library features
 #if FASTFLOAT_HAS_IS_CONSTANT_EVALUATED && FASTFLOAT_HAS_BIT_CAST &&           \
     defined(__cpp_lib_constexpr_algorithms) &&                                 \
@@ -58,6 +103,12 @@
 #define FASTFLOAT_DETAIL_MUST_DEFINE_CONSTEXPR_VARIABLE 1
 #endif
 
+#if defined(__has_builtin)
+#define FASTFLOAT_HAS_BUILTIN(x) __has_builtin(x)
+#else
+#define FASTFLOAT_HAS_BUILTIN(x) false
+#endif
+
 // For support attribute [[assume]] is declared in P1774
 #if defined(__cpp_attrubute_assume)
 #define FASTFLOAT_ASSUME(expr) [[assume(expr)]]