patch 8.2.1397: Vim9: return type of maparg() not adjusted for fourth arg

Problem:    Vim9: return type of maparg() not adjusted for fourth argument.
Solution:   Check if fourth argument is present. (closes #6645)

src/evalfunc.c

@@ -402,6 +402,15 @@ ret_getreg(int argcount, type_T **argtypes UNUSED)
     return &t_string;
 }
 
+    static type_T *
+ret_maparg(int argcount, type_T **argtypes UNUSED)
+{
+    // Assume that if the fourth argument is passed it's non-zero
+    if (argcount == 4)
+	return &t_dict_any;
+    return &t_string;
+}
+
 static type_T *ret_f_function(int argcount, type_T **argtypes);
 
 /*
@@ -729,7 +738,7 @@ static funcentry_T global_functions[] =
 #endif
 			},
     {"map",		2, 2, FEARG_1,	  ret_any,	f_map},
-    {"maparg",		1, 4, FEARG_1,	  ret_string,	f_maparg},
+    {"maparg",		1, 4, FEARG_1,	  ret_maparg,	f_maparg},
     {"mapcheck",	1, 3, FEARG_1,	  ret_string,	f_mapcheck},
     {"mapset",		3, 3, FEARG_1,	  ret_void,	f_mapset},
     {"match",		2, 4, FEARG_1,	  ret_any,	f_match},

src/testdir/test_maparg.vim

@@ -81,6 +81,19 @@ func Test_maparg()
   abclear
 endfunc
 
+def Test_vim9_maparg()
+  nmap { w
+  let one: string = maparg('{')
+  assert_equal('w', one)
+  let two: string = maparg('{', 'n')
+  assert_equal('w', two)
+  let three: string = maparg('{', 'n', 0)
+  assert_equal('w', three)
+  let four: dict<any> = maparg('{', 'n', 0, 1)
+  call assert_equal(['{', 'w', 'n'], [four.lhs, four.rhs, four.mode])
+  nunmap {
+enddef
+
 func Test_mapcheck()
   call assert_equal('', mapcheck('a'))
   call assert_equal('', mapcheck('abc'))

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1397,
 /**/
     1396,
 /**/