Basics of formatting at compile-time based on compile-time API (#2019)

include/fmt/core.h

@@ -96,6 +96,12 @@
 #  define FMT_CONSTEXPR_DECL
 #endif
 
+#if __cplusplus >= 202002L
+#  define FMT_CONSTEXPR20 constexpr
+#else
+#  define FMT_CONSTEXPR20 inline
+#endif
+
 #ifndef FMT_OVERRIDE
 #  if FMT_HAS_FEATURE(cxx_override_control) || \
       (FMT_GCC_VERSION >= 408 && FMT_HAS_GXX_CXX11) || FMT_MSC_VER >= 1900
@@ -283,6 +289,14 @@ struct monostate {};
 
 namespace detail {
 
+constexpr bool is_constant_evaluated() FMT_DETECTED_NOEXCEPT {
+#ifdef __cpp_lib_is_constant_evaluated
+  return std::is_constant_evaluated();
+#else
+  return false;
+#endif
+}
+
 // A helper function to suppress "conditional expression is constant" warnings.
 template <typename T> constexpr T const_check(T value) { return value; }