From 280bac1e049bd90896eed57582fdb2601bc2176a Mon Sep 17 00:00:00 2001
From: Stefan Seefeld <stefan@seefeld.name>
Date: Sun, 2 Jul 2017 12:34:30 -0400
Subject: [PATCH] Fix compile-time failure.

---
 include/boost/gil/premultiply.hpp | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/include/boost/gil/premultiply.hpp b/include/boost/gil/premultiply.hpp
index 5b18beb9..89e21922 100644
--- a/include/boost/gil/premultiply.hpp
+++ b/include/boost/gil/premultiply.hpp
@@ -32,15 +32,28 @@ struct channel_premultiply {
 	DstP & dst_;
 };
 
+namespace detail
+{
+template <typename SrcP, typename DstP>
+void assign_alpha_if(mpl::true_, SrcP const &src, DstP &dst)
+{
+  get_color (dst,alpha_t()) = alpha_or_max (src);
+};
+template <typename SrcP, typename DstP>
+void assign_alpha_if(mpl::false_, SrcP const &src, DstP &dst)
+{
+  // nothing to do
+};
+}
 struct premultiply {
 	template <typename SrcP, typename DstP>
 	void operator()(const SrcP& src, DstP& dst) const {
 		typedef typename color_space_type<SrcP>::type src_colour_space_t;
 		typedef typename color_space_type<DstP>::type dst_colour_space_t;
 		typedef typename mpl:: remove <src_colour_space_t, alpha_t>:: type src_colour_channels;
+		typedef mpl::bool_<mpl::contains<dst_colour_space_t, alpha_t>::value> has_alpha_t;
 		mpl:: for_each <src_colour_channels> ( channel_premultiply <SrcP, DstP> (src, dst) );
-		if (mpl:: contains <dst_colour_space_t, alpha_t>:: value)
-			get_color (dst,alpha_t()) = alpha_or_max (src);
+		detail::assign_alpha_if(has_alpha_t(), src, dst);
 	}
 };