Add helper macro BOOST_IF_CONSTEXPR.

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]

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:

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)
    {