[libc++] Implement <numbers>

Summary: Constants have 33 significant decimal digits for IEEE 754 128-bit floating-point numbers.

Reviewers: ldionne, #libc, EricWF, zoecarver, curdeius

Reviewed By: ldionne, #libc, curdeius

Differential Revision: https://reviews.llvm.org/D77505

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 4f6c4b473c4a57ec597a201dee483204454c8a6d
diff --git a/include/numbers b/include/numbers
new file mode 100644
index 0000000..cbf54cd
--- /dev/null
+++ b/include/numbers
@@ -0,0 +1,141 @@
+// -*- C++ -*-
+//===---------------------------- numbers ---------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_NUMBERS
+#define _LIBCPP_NUMBERS
+
+/*
+    numbers synopsis
+
+namespace std::numbers {
+  template<class T> inline constexpr T e_v          = unspecified;
+  template<class T> inline constexpr T log2e_v      = unspecified;
+  template<class T> inline constexpr T log10e_v     = unspecified;
+  template<class T> inline constexpr T pi_v         = unspecified;
+  template<class T> inline constexpr T inv_pi_v     = unspecified;
+  template<class T> inline constexpr T inv_sqrtpi_v = unspecified;
+  template<class T> inline constexpr T ln2_v        = unspecified;
+  template<class T> inline constexpr T ln10_v       = unspecified;
+  template<class T> inline constexpr T sqrt2_v      = unspecified;
+  template<class T> inline constexpr T sqrt3_v      = unspecified;
+  template<class T> inline constexpr T inv_sqrt3_v  = unspecified;
+  template<class T> inline constexpr T egamma_v     = unspecified;
+  template<class T> inline constexpr T phi_v        = unspecified;
+
+  template<floating_point T> inline constexpr T e_v<T>          = see below;
+  template<floating_point T> inline constexpr T log2e_v<T>      = see below;
+  template<floating_point T> inline constexpr T log10e_v<T>     = see below;
+  template<floating_point T> inline constexpr T pi_v<T>         = see below;
+  template<floating_point T> inline constexpr T inv_pi_v<T>     = see below;
+  template<floating_point T> inline constexpr T inv_sqrtpi_v<T> = see below;
+  template<floating_point T> inline constexpr T ln2_v<T>        = see below;
+  template<floating_point T> inline constexpr T ln10_v<T>       = see below;
+  template<floating_point T> inline constexpr T sqrt2_v<T>      = see below;
+  template<floating_point T> inline constexpr T sqrt3_v<T>      = see below;
+  template<floating_point T> inline constexpr T inv_sqrt3_v<T>  = see below;
+  template<floating_point T> inline constexpr T egamma_v<T>     = see below;
+  template<floating_point T> inline constexpr T phi_v<T>        = see below;
+
+  inline constexpr double e          = e_v<double>;
+  inline constexpr double log2e      = log2e_v<double>;
+  inline constexpr double log10e     = log10e_v<double>;
+  inline constexpr double pi         = pi_v<double>;
+  inline constexpr double inv_pi     = inv_pi_v<double>;
+  inline constexpr double inv_sqrtpi = inv_sqrtpi_v<double>;
+  inline constexpr double ln2        = ln2_v<double>;
+  inline constexpr double ln10       = ln10_v<double>;
+  inline constexpr double sqrt2      = sqrt2_v<double>;
+  inline constexpr double sqrt3      = sqrt3_v<double>;
+  inline constexpr double inv_sqrt3  = inv_sqrt3_v<double>;
+  inline constexpr double egamma     = egamma_v<double>;
+  inline constexpr double phi        = phi_v<double>;
+}
+*/
+
+#include <__config>
+
+#if _LIBCPP_STD_VER > 17
+
+#include <type_traits>
+#include <version>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace numbers {
+
+template <class T>
+inline constexpr bool __false = false;
+
+template <class T>
+struct __illformed
+{
+  static_assert(__false<T>, "A program that instantiates a primary template of a mathematical constant variable template is ill-formed.");
+};
+
+template <class T> inline constexpr T e_v =          __illformed<T>{};
+template <class T> inline constexpr T log2e_v =      __illformed<T>{};
+template <class T> inline constexpr T log10e_v =     __illformed<T>{};
+template <class T> inline constexpr T pi_v =         __illformed<T>{};
+template <class T> inline constexpr T inv_pi_v =     __illformed<T>{};
+template <class T> inline constexpr T inv_sqrtpi_v = __illformed<T>{};
+template <class T> inline constexpr T ln2_v =        __illformed<T>{};
+template <class T> inline constexpr T ln10_v =       __illformed<T>{};
+template <class T> inline constexpr T sqrt2_v =      __illformed<T>{};
+template <class T> inline constexpr T sqrt3_v =      __illformed<T>{};
+template <class T> inline constexpr T inv_sqrt3_v =  __illformed<T>{};
+template <class T> inline constexpr T egamma_v =     __illformed<T>{};
+template <class T> inline constexpr T phi_v =        __illformed<T>{};
+
+template <class T>
+concept __floating_point = std::is_floating_point_v<T>;
+
+template <__floating_point T> inline constexpr T e_v<T>          = 2.718281828459045235360287471352662;
+template <__floating_point T> inline constexpr T log2e_v<T>      = 1.442695040888963407359924681001892;
+template <__floating_point T> inline constexpr T log10e_v<T>     = 0.434294481903251827651128918916605;
+template <__floating_point T> inline constexpr T pi_v<T>         = 3.141592653589793238462643383279502;
+template <__floating_point T> inline constexpr T inv_pi_v<T>     = 0.318309886183790671537767526745028;
+template <__floating_point T> inline constexpr T inv_sqrtpi_v<T> = 0.564189583547756286948079451560772;
+template <__floating_point T> inline constexpr T ln2_v<T>        = 0.693147180559945309417232121458176;
+template <__floating_point T> inline constexpr T ln10_v<T>       = 2.302585092994045684017991454684364;
+template <__floating_point T> inline constexpr T sqrt2_v<T>      = 1.414213562373095048801688724209698;
+template <__floating_point T> inline constexpr T sqrt3_v<T>      = 1.732050807568877293527446341505872;
+template <__floating_point T> inline constexpr T inv_sqrt3_v<T>  = 0.577350269189625764509148780501957;
+template <__floating_point T> inline constexpr T egamma_v<T>     = 0.577215664901532860606512090082402;
+template <__floating_point T> inline constexpr T phi_v<T>        = 1.618033988749894848204586834365638;
+
+inline constexpr double e          = e_v<double>;
+inline constexpr double log2e      = log2e_v<double>;
+inline constexpr double log10e     = log10e_v<double>;
+inline constexpr double pi         = pi_v<double>;
+inline constexpr double inv_pi     = inv_pi_v<double>;
+inline constexpr double inv_sqrtpi = inv_sqrtpi_v<double>;
+inline constexpr double ln2        = ln2_v<double>;
+inline constexpr double ln10       = ln10_v<double>;
+inline constexpr double sqrt2      = sqrt2_v<double>;
+inline constexpr double sqrt3      = sqrt3_v<double>;
+inline constexpr double inv_sqrt3  = inv_sqrt3_v<double>;
+inline constexpr double egamma     = egamma_v<double>;
+inline constexpr double phi        = phi_v<double>;
+
+} // namespace numbers
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif //_LIBCPP_STD_VER > 17
+
+#endif // _LIBCPP_NUMBERS