patch 8.2.2075: error for const argument to mapnew()

Problem:    Error for const argument to mapnew().
Solution:   Don't give an error. (closes #7400)

src/list.c

@@ -2065,7 +2065,7 @@ filter_map(typval_T *argvars, typval_T *rettv, filtermap_T filtermap)
 
 		    --todo;
 		    di = HI2DI(hi);
-		    if (filtermap != FILTERMAP_FILTER
+		    if (filtermap == FILTERMAP_MAP
 					 && (value_check_lock(di->di_tv.v_lock,
 							   arg_errmsg, TRUE)
 				|| var_check_ro(di->di_flags,
@@ -2225,12 +2225,12 @@ filter_map(typval_T *argvars, typval_T *rettv, filtermap_T filtermap)
 	    }
 	    else
 	    {
-		// Materialized list from range(): loop over the items
+		// Materialized list: loop over the items
 		for (li = l->lv_first; li != NULL; li = nli)
 		{
 		    typval_T newtv;
 
-		    if (filtermap != FILTERMAP_FILTER && value_check_lock(
+		    if (filtermap == FILTERMAP_MAP && value_check_lock(
 					   li->li_tv.v_lock, arg_errmsg, TRUE))
 			break;
 		    nli = li->li_next;

src/testdir/test_filter_map.vim

@@ -123,13 +123,19 @@ func Test_mapnew_dict()
   let dout = mapnew(din, {k, v -> string(v)})
   call assert_equal(#{one: 1, two: 2}, din)
   call assert_equal(#{one: '1', two: '2'}, dout)
+
+  const dconst = #{one: 1, two: 2, three: 3}
+  call assert_equal(#{one: 2, two: 3, three: 4}, mapnew(dconst, {_, v -> v + 1}))
 endfunc
 
 func Test_mapnew_list()
   let lin = [1, 2, 3]
   let lout = mapnew(lin, {k, v -> string(v)})
   call assert_equal([1, 2, 3], lin)
   call assert_equal(['1', '2', '3'], lout)
+
+  const lconst = [1, 2, 3]
+  call assert_equal([2, 3, 4], mapnew(lconst, {_, v -> v + 1}))
 endfunc
 
 func Test_mapnew_blob()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2075,
 /**/
     2074,
 /**/