diff --git a/include/fast_float/constexpr_feature_detect.h b/include/fast_float/constexpr_feature_detect.h
index a32035b..84c1041 100644
--- a/include/fast_float/constexpr_feature_detect.h
+++ b/include/fast_float/constexpr_feature_detect.h
@@ -9,14 +9,14 @@
 
 // C++14 constexpr
 #if defined(_MSC_VER)
-#if _MSVC_LANG >= 201402L
+#if defined(_MSVC_LANG) && _MSVC_LANG >= 201402L
 #define FASTFLOAT_CONSTEXPR14 constexpr
 #else
 #define FASTFLOAT_CONSTEXPR14
 #endif
 #elif defined(__cpp_constexpr) && __cpp_constexpr >= 201304L
 #define FASTFLOAT_CONSTEXPR14 constexpr
-#elif __cplusplus >= 201402L
+#elif defined(__cplusplus) && __cplusplus >= 201402L
 #define FASTFLOAT_CONSTEXPR14 constexpr
 #else
 #define FASTFLOAT_CONSTEXPR14
@@ -25,15 +25,46 @@
 // C++14 variable templates
 #if defined(__cpp_variable_templates) && __cpp_variable_templates >= 201304L
 #define FASTFLOAT_HAS_VARIABLE_TEMPLATES 1
-#elif __cplusplus >= 201402L
+#elif defined(__cplusplus) && __cplusplus >= 201402L
 #define FASTFLOAT_HAS_VARIABLE_TEMPLATES 1
 #elif defined(_MSC_FULL_VER) && _MSC_FULL_VER >= 190023918L &&                 \
-    _MSVC_LANG >= 201402L
+    defined(_MSVC_LANG) && _MSVC_LANG >= 201402L
 #define FASTFLOAT_HAS_VARIABLE_TEMPLATES 1
 #else
 #define FASTFLOAT_HAS_VARIABLE_TEMPLATES 0
 #endif
 
+// C++17 if constexpr
+#if defined(__cpp_if_constexpr) && __cpp_if_constexpr >= 201606L
+#define FASTFLOAT_CONSTEXPR17 constexpr
+#elif defined(__cpp_constexpr) && __cpp_constexpr >= 201603L
+#define FASTFLOAT_CONSTEXPR17 constexpr
+#elif defined(__cplusplus) && __cplusplus >= 201703L
+#define FASTFLOAT_CONSTEXPR17 constexpr
+#elif defined(_MSC_VER) && defined(_MSVC_LANG) && _MSVC_LANG >= 201703L
+#define FASTFLOAT_CONSTEXPR17 constexpr
+#else
+#define FASTFLOAT_CONSTEXPR17
+#endif
+
+// C++17 inline variables
+#if defined(__cpp_inline_variables) && __cpp_inline_variables >= 201606L
+#define FASTFLOAT_INLINE_VARIABLE inline constexpr
+#elif defined(__cplusplus) && __cplusplus >= 201703L
+#define FASTFLOAT_INLINE_VARIABLE inline constexpr
+#elif defined(_MSC_VER) && defined(_MSVC_LANG) && _MSVC_LANG >= 201703L
+#define FASTFLOAT_INLINE_VARIABLE inline constexpr
+#else
+#define FASTFLOAT_INLINE_VARIABLE static constexpr
+#endif
+
+#if (defined(__cplusplus) && __cplusplus >= 201703L) ||                        \
+    (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
+#define FASTFLOAT_DETAIL_MUST_DEFINE_CONSTEXPR_VARIABLE 0
+#else
+#define FASTFLOAT_DETAIL_MUST_DEFINE_CONSTEXPR_VARIABLE 1
+#endif
+
 // C++20 std::bit_cast
 #if defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L
 #define FASTFLOAT_HAS_BIT_CAST 1
@@ -41,70 +72,82 @@
 #define FASTFLOAT_HAS_BIT_CAST 0
 #endif
 
-// Detect consteval, C++20 constexpr extensions and std::is_constant_evaluated.
-#if !defined(__cpp_lib_is_constant_evaluated)
-#define FASTFLOAT__USE_CONSTEVAL 0
-#elif __cplusplus < 201709L
-#define FASTFLOAT_USE_CONSTEVAL 0
-#elif defined(__apple_build_version__) && __apple_build_version__ < 14000029L
-#define FASTFLOAT_USE_CONSTEVAL 0 // consteval is broken in Apple clang < 14.
-#elif _MSC_VER && _MSC_VER < 1940
-#define FASTFLOAT_USE_CONSTEVAL                                                \
-  0 // consteval is broken in some MSVC2022 versions.
-#elif defined(__cpp_consteval)
-#define FASTFLOAT_USE_CONSTEVAL 1
-#elif defined(__cpp_lib_is_constant_evaluated) &&                              \
+// std::is_constant_evaluated()
+//
+// This is a library facility and must be detected independently from
+// consteval. Do not use it to infer the C++ language mode.
+#if defined(__cpp_lib_is_constant_evaluated) &&                                \
     __cpp_lib_is_constant_evaluated >= 201811L
-#define FASTFLOAT_USE_CONSTEVAL 1
-#elif FMT_GCC_VERSION >= 1002 || FMT_CLANG_VERSION >= 1101
-#define FASTFLOAT_USE_CONSTEVAL 1
+#define FASTFLOAT_HAS_IS_CONSTANT_EVALUATED 1
 #else
-#define FASTFLOAT_USE_CONSTEVAL 0
-#endif
-#if FASTFLOAT_USE_CONSTEVAL
-#define FASTFLOAT_CONSTEVAL consteval
-#define FASTFLOAT_CONSTEXPR20 constexpr
-#else
-#define FASTFLOAT_CONSTEVAL
-#define FASTFLOAT_CONSTEXPR20
+#define FASTFLOAT_HAS_IS_CONSTANT_EVALUATED 0
 #endif
 
-#if defined(__cpp_lib_byteswap)
+// C++20 language mode.
+//
+// _MSVC_LANG is required for MSVC because __cplusplus historically does not
+// report the selected language mode there.
+#if defined(_MSC_VER)
+#if defined(_MSVC_LANG) && _MSVC_LANG >= 202002L
+#define FASTFLOAT_CXX20 1
+#else
+#define FASTFLOAT_CXX20 0
+#endif
+#else
+#if defined(__cplusplus) && __cplusplus >= 202002L
+#define FASTFLOAT_CXX20 1
+#else
+#define FASTFLOAT_CXX20 0
+#endif
+#endif
+
+// C++20 consteval.
+//
+// consteval is a language feature. It must be detected independently from
+// std::is_constant_evaluated() and from the C++20 constexpr library.
+#if FASTFLOAT_CXX20
+#if defined(__apple_build_version__) && __apple_build_version__ < 14000029L
+#define FASTFLOAT_USE_CONSTEVAL 0 // consteval is broken in Apple clang < 14.
+#elif defined(_MSC_VER) && !defined(__clang__) && _MSC_VER < 1940
+#define FASTFLOAT_USE_CONSTEVAL                                                \
+  0 // consteval is broken in some MSVC2022 versions.
+#elif defined(__cpp_consteval) && __cpp_consteval >= 201811L
+#define FASTFLOAT_USE_CONSTEVAL 1
+#else
+#define FASTFLOAT_USE_CONSTEVAL 0
+#endif
+#else
+#define FASTFLOAT_USE_CONSTEVAL 0
+#endif
+
+#if FASTFLOAT_USE_CONSTEVAL
+#define FASTFLOAT_CONSTEVAL consteval
+#else
+#define FASTFLOAT_CONSTEVAL
+#endif
+
+// C++20 constexpr.
+//
+// This is deliberately independent from consteval. The functions using
+// FASTFLOAT_CONSTEXPR20 also require the corresponding C++20 constexpr
+// library facilities.
+#if FASTFLOAT_CXX20 && 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
+#if defined(__cpp_lib_byteswap) && __cpp_lib_byteswap >= 202110L
 #define FASTFLOAT_HAS_BYTESWAP 1
 #else
 #define FASTFLOAT_HAS_BYTESWAP 0
 #endif
 
-// C++17 if constexpr
-#if defined(__cpp_if_constexpr) && __cpp_if_constexpr >= 201606L
-#define FASTFLOAT_CONSTEXPR17 constexpr
-#elif defined(__cpp_constexpr) && __cpp_constexpr >= 201603L
-#define FASTFLOAT_CONSTEXPR17 constexpr
-#elif __cplusplus >= 201703L
-#define FASTFLOAT_CONSTEXPR17 constexpr
-#elif defined(_MSC_VER) && _MSC_VER >= 1911 && _MSVC_LANG >= 201703L
-#define FASTFLOAT_CONSTEXPR17 constexpr
-#else
-#define FASTFLOAT_CONSTEXPR17
-#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 __cplusplus >= 201703L || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
-#define FASTFLOAT_DETAIL_MUST_DEFINE_CONSTEXPR_VARIABLE 0
-#else
-#define FASTFLOAT_DETAIL_MUST_DEFINE_CONSTEXPR_VARIABLE 1
-#endif
-
 #if defined(__has_builtin)
 #define FASTFLOAT_HAS_BUILTIN(x) __has_builtin(x)
 #else