From 368d08f74744ed5d5ef5ef747e86bddbfaa47312 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ludovic=20Court=C3=A8s?= <ludo@gnu.org>
Date: Tue, 19 May 2015 09:09:47 +0200
Subject: [PATCH] build: Make sure $CXX supports C++11.

* m4/guix.m4 (GUIX_CHECK_CXX11, GUIX_ASSERT_CXX11): New macros.
* config-daemon.ac: Use 'AC_LANG([C++])' and 'GUIX_ASSERT_CXX11'.C
* doc/guix.texi (Requirements): Mention C++11 support.
---
 config-daemon.ac |  3 +++
 doc/guix.texi    |  7 ++++---
 m4/guix.m4       | 39 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 46 insertions(+), 3 deletions(-)

diff --git a/config-daemon.ac b/config-daemon.ac
index fb80c754c9..a6cf29ca42 100644
--- a/config-daemon.ac
+++ b/config-daemon.ac
@@ -5,9 +5,12 @@ AC_MSG_RESULT([$guix_build_daemon])
 
 dnl C++ environment.  This macro must be used unconditionnaly.
 AC_PROG_CXX
+AC_LANG([C++])
 
 if test "x$guix_build_daemon" = "xyes"; then
 
+  GUIX_ASSERT_CXX11
+
   AC_PROG_RANLIB
   AC_CONFIG_HEADER([nix/config.h])
 
diff --git a/doc/guix.texi b/doc/guix.texi
index 6964a4ec18..91c86dc30a 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -368,9 +368,10 @@ Unless @code{--disable-daemon} was passed to @command{configure}, the
 following packages are also needed:
 
 @itemize
-@item @url{http://sqlite.org, SQLite 3}
-@item @url{http://www.bzip.org, libbz2}
-@item @url{http://gcc.gnu.org, GCC's g++}
+@item @url{http://sqlite.org, SQLite 3};
+@item @url{http://www.bzip.org, libbz2};
+@item @url{http://gcc.gnu.org, GCC's g++}, with support for the
+C++11 standard.
 @end itemize
 
 When a working installation of @url{http://nixos.org/nix/, the Nix package
diff --git a/m4/guix.m4 b/m4/guix.m4
index 445ce857dd..fa5a4023ba 100644
--- a/m4/guix.m4
+++ b/m4/guix.m4
@@ -218,3 +218,42 @@ AC_DEFUN([GUIX_CHECK_FILE_NAME_LIMITS], [
     AC_MSG_ERROR([store directory '$storedir' would lead to overly long hash-bang lines])
   fi
 ])
+
+dnl GUIX_CHECK_CXX11
+dnl
+dnl Check whether the C++ compiler can compile a typical C++11 program.
+AC_DEFUN([GUIX_CHECK_CXX11], [
+  AC_REQUIRE([AC_PROG_CXX])
+  AC_CACHE_CHECK([whether $CXX supports C++11],
+    [ac_cv_guix_cxx11_support],
+    [save_CXXFLAGS="$CXXFLAGS"
+     CXXFLAGS="-std=c++11 $CXXFLAGS"
+     AC_COMPILE_IFELSE([
+      AC_LANG_SOURCE([
+	#include <functional>
+
+	std::function<int(int)>
+	return_plus_lambda (int x)
+	{
+	  auto result = [[&]](int y) {
+	    return x + y;
+	  };
+
+	  return result;
+	}
+      ])],
+      [ac_cv_guix_cxx11_support=yes],
+      [ac_cv_guix_cxx11_support=no])
+    CXXFLAGS="$save_CXXFLAGS"
+  ])
+])
+
+dnl GUIX_ASSERT_CXX11
+dnl
+dnl Error out if the C++ compiler cannot compile C++11 code.
+AC_DEFUN([GUIX_ASSERT_CXX11], [
+  GUIX_CHECK_CXX11
+  if test "x$ac_cv_guix_cxx11_support" != "xyes"; then
+    AC_MSG_ERROR([C++ compiler '$CXX' does not support the C++11 standard])
+  fi
+])