Fix conflict with std::fill_n and boost::range::fill_n (Trac 7189)

Add minimal test for the std::fill and boost::array or std::array as
pixel type.

include/boost/gil/algorithm.hpp

@@ -355,7 +355,7 @@ void fill(boost::gil::iterator_from_2d<IL> first, boost::gil::iterator_from_2d<I
         std::ptrdiff_t n=last-first;
         while (n>0) {
             std::ptrdiff_t numToDo=std::min<const std::ptrdiff_t>(n,(std::ptrdiff_t)(first.width()-first.x_pos()));
-            fill_n(first.x(), numToDo, val);
+            std::fill_n(first.x(), numToDo, val);
             first+=numToDo;
             n-=numToDo;
         }

test/Jamfile

@@ -71,6 +71,7 @@ run image.cpp sample_image.cpp error_if.cpp : : gil_reference_checksums.txt ;
 run channel.cpp error_if.cpp ;
 run pixel.cpp error_if.cpp ;
 run pixel_iterator.cpp error_if.cpp ;
+build-project algorithm ;
 build-project channel ;
 build-project image_view ;
 

test/algorithm/Jamfile

@@ -15,3 +15,4 @@ project
     ;
 
 run for_each_pixel.cpp ;
+run std_fill.cpp ;

test/algorithm/std_fill.cpp

@@ -0,0 +1,37 @@
+//
+// Copyright 2018 Mateusz Loskot <mateusz at loskot dot net>
+//
+// Distributed under the Boost Software License, Version 1.0
+// See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt
+//
+#include <boost/gil/algorithm.hpp>
+#include <boost/gil/image.hpp>
+#include <boost/gil/image_view.hpp>
+
+#include <boost/array.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/range/algorithm/fill_n.hpp>
+
+#include <array>
+#include <cstdint>
+
+namespace gil = boost::gil;
+
+template <typename ArrayPixel>
+void test_array_as_range()
+{
+    static_assert(ArrayPixel().size() == 2, "two-element array expected");
+
+    gil::image<ArrayPixel> img(1, 1);
+    std::fill(gil::view(img).begin(), gil::view(img).end(), ArrayPixel{0, 1});
+    BOOST_TEST(*gil::view(img).at(0,0) == (ArrayPixel{0, 1}));
+}
+
+int main()
+{
+    test_array_as_range<boost::array<int, 2>>();
+    test_array_as_range<std::array<int, 2>>();
+
+    return boost::report_errors();
+}