From f13c9c83fe8670dc5e9e620dc521d8f7df45248d Mon Sep 17 00:00:00 2001
From: Moritz Bunkus <moritz@bunkus.org>
Date: Wed, 13 May 2020 16:34:17 +0200
Subject: [PATCH] configure: add test for C++17 feature "constexpr if"

---
 ac/c++-features.m4 | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/ac/c++-features.m4 b/ac/c++-features.m4
index bdaf3fb66..c46c6b576 100644
--- a/ac/c++-features.m4
+++ b/ac/c++-features.m4
@@ -506,6 +506,29 @@ AC_DEFUN([AX_CXX17_STD_GCD],[
   fi
 ])
 
+AC_DEFUN([AX_CXX17_CONSTEXPR_IF],[
+  AC_CACHE_CHECK([for support for C++17 feature "constexpr if"], [ax_cv_cxx17_constexpr_if],[
+
+    CXXFLAGS_SAVED=$CXXFLAGS
+    CXXFLAGS="$CXXFLAGS $STD_CXX"
+    export CXXFLAGS
+
+    AC_LANG_PUSH(C++)
+    AC_TRY_COMPILE(
+      [],
+      [if constexpr (true) return 42; else return 54;],
+      [ax_cv_cxx17_constexpr_if="yes"],
+      [ax_cv_cxx17_constexpr_if="no"])
+    AC_LANG_POP
+
+    CXXFLAGS="$CXXFLAGS_SAVED"
+  ])
+
+  if ! test x"$ax_cv_cxx17_constexpr_if" = xyes ; then
+    missing_cxx_features="$missing_cxx_features\n  * constexpr if (C++17)"
+  fi
+])
+
 dnl AC_DEFUN([AX_CXX17_DEF_NAME],[
 dnl   AC_CACHE_CHECK([for support for C++17 feature "human"], [ax_cv_cxx17_def_name],[
 dnl
@@ -548,6 +571,7 @@ AX_CXX17_NESTED_NAMESPACE_DEFINITION
 AX_CXX17_STRUCTURED_BINDINGS
 AX_CXX17_STD_OPTIONAL
 AX_CXX17_STD_GCD
+AX_CXX17_CONSTEXPR_IF
 
 if test x"$missing_cxx_features" != x ; then
   printf "The following features of the C++11/C++14/C++17 standards are not supported by $CXX:$missing_cxx_features\n"