From 9aaa4edda19a201f393d6eac457c4c484c5aab16 Mon Sep 17 00:00:00 2001
From: jzmaddock <john@johnmaddock.co.uk>
Date: Fri, 24 Jan 2020 18:33:21 +0000
Subject: [PATCH] Add helper macro BOOST_IF_CONSTEXPR.

---
 doc/macro_reference.qbk                |  1 +
 include/boost/config/detail/suffix.hpp |  8 ++++++++
 test/helper_macro_test.cpp             | 11 +++++++++++
 3 files changed, 20 insertions(+)

diff --git a/doc/macro_reference.qbk b/doc/macro_reference.qbk
index 277b9d36..75c1dc9f 100644
--- a/doc/macro_reference.qbk
+++ b/doc/macro_reference.qbk
@@ -978,6 +978,7 @@ provide compliant C++17 support.
 [table
 [[Macro ][ Description ]]
 [[`BOOST_INLINE_VARIABLE`][This macro expands to `inline` on compilers that support C++17 inline variables and to nothing otherwise. Users may need to check for `BOOST_NO_CXX17_INLINE_VARIABLES` for further adjustments to the code.]]
+[[`BOOST_IF_CONSTEXPR`][Expands to `if constexpr` when supported, or `if` otherwise.]]
 ]
 
 [endsect]
diff --git a/include/boost/config/detail/suffix.hpp b/include/boost/config/detail/suffix.hpp
index 9131cba4..4dfb0422 100644
--- a/include/boost/config/detail/suffix.hpp
+++ b/include/boost/config/detail/suffix.hpp
@@ -1002,6 +1002,14 @@ namespace std{ using ::type_info; }
 #else
 #define BOOST_INLINE_VARIABLE
 #endif
+//
+// C++17 if constexpr
+//
+#if !defined(BOOST_NO_CXX17_IF_CONSTEXPR)
+#  define BOOST_IF_CONSTEXPR if constexpr
+#else
+#  define BOOST_IF_CONSTEXPR if
+#endif
 
 //
 // Unused variable/typedef workarounds:
diff --git a/test/helper_macro_test.cpp b/test/helper_macro_test.cpp
index efdb20f9..18b9268a 100644
--- a/test/helper_macro_test.cpp
+++ b/test/helper_macro_test.cpp
@@ -51,6 +51,12 @@ struct no_unique
    BOOST_ATTRIBUTE_NO_UNIQUE_ADDRESS empty b;
 };
 
+template <bool b>
+struct trait
+{
+   enum { value = b };
+};
+
 
 int main()
 {
@@ -66,6 +72,11 @@ int main()
          always_throw();
       nodiscard_struct s;
       no_unique no_un;
+
+      BOOST_IF_CONSTEXPR(trait<true>::value)
+      {
+         result += 2;
+      }
    }
    catch(int)
    {