patch 8.0.0616: not always setting 'background' correctly after :hi Normal

Problem:    When setting the cterm background with ":hi Normal" the value of
            'background' may be set wrongly.
Solution:   Check that the color is less than 16.  Don't set 'background' when
            it was set explicitly. (Lemonboy, closes #1710)

src/syntax.c

@@ -7834,18 +7834,25 @@ do_highlight(
 			must_redraw = CLEAR;
 			if (color >= 0)
 			{
+			    int dark = -1;
+
 			    if (termcap_active)
 				term_bg_color(color);
 			    if (t_colors < 16)
-				i = (color == 0 || color == 4);
+				dark = (color == 0 || color == 4);
-			    else
+			    /* Limit the heuristic to the standard 16 colors */
-				i = (color < 7 || color == 8);
+			    else if (color < 16)
+				dark = (color < 7 || color == 8);
 			    /* Set the 'background' option if the value is
 			     * wrong. */
-			    if (i != (*p_bg == 'd'))
+			    if (dark != -1
+				    && dark != (*p_bg == 'd')
+				    && !option_was_set((char_u *)"bg"))
+			    {
 				set_option_value((char_u *)"bg", 0L,
-					i ?  (char_u *)"dark"
+				       (char_u *)(dark ? "dark" : "light"), 0);
-					  : (char_u *)"light", 0);
+				reset_option_was_set((char_u *)"bg");
+			    }
 			}
 		    }
 		}

src/testdir/test_syntax.vim

@@ -401,3 +401,26 @@ func Test_highlight_invalid_arg()
   call assert_fails('hi XXX xxx=White', 'E423:')
 endfunc
 
+func Test_bg_detection()
+  if has('gui_running')
+    return
+  endif
+  " auto-detection of &bg, make sure sure it isn't set anywhere before
+  " this test
+  hi Normal ctermbg=0
+  call assert_equal('dark', &bg)
+  hi Normal ctermbg=4
+  call assert_equal('dark', &bg)
+  hi Normal ctermbg=12
+  call assert_equal('light', &bg)
+  hi Normal ctermbg=15
+  call assert_equal('light', &bg)
+
+  " manually-set &bg takes precendence over auto-detection
+  set bg=light
+  hi Normal ctermbg=4
+  call assert_equal('light', &bg)
+  set bg=dark
+  hi Normal ctermbg=12
+  call assert_equal('dark', &bg)
+endfunc

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    616,
 /**/
     615,
 /**/