parser: fix mutable map args

vlib/builtin/map_test.v

@@ -158,6 +158,16 @@ fn test_string_arr() {
 	assert m['a'][1] == 'two'
 }
 
+fn mut_map(m mut map[string]int) {
+	m['a'] = 10
+}
+
+fn test_mut_arg() {
+	mut m := map[string]int
+	mut_map(mut m)
+	assert m['a'] == 10
+}
+
 /*
 fn test_ref() {
 	m := { 'one': 1 }

vlib/compiler/gen_c.v

@@ -400,7 +400,11 @@ fn (p mut Parser) gen_array_set(typ string, is_ptr, is_map bool,fn_ph, assign_po
 	mut cao_tmp := p.cgen.cur_line
 	mut func := ''
 	if is_map {
+		if is_ptr {
+			func = 'map_set('
+		} else {
 			func = 'map_set(&'
+		}
 		// CAO on map is a bit more complicated as it loads
 		// the value inside a pointer instead of returning it.
 	}

vlib/compiler/parser.v

@@ -1831,7 +1831,7 @@ fn (p mut Parser) index_expr(typ_ string, fn_ph int) string {
 	if is_indexer {
 		is_fixed_arr := typ[0] == `[`
 		if !is_str && !is_arr && !is_map && !is_ptr && !is_fixed_arr && !is_variadic_arg {
-			p.error('Cant [] non-array/string/map. Got type "$typ"')
+			p.error('invalid operation: type `$typ` does not support indexing')
 		}
 		p.check(.lsbr)
 		// Get element type (set `typ` to it)
@@ -1864,8 +1864,8 @@ fn (p mut Parser) index_expr(typ_ string, fn_ph int) string {
 			// typ = 'byte'
 			typ = typ.replace('*', '')
 			// modify(mut []string) fix
-			if !is_arr {
+			if !is_arr && !is_map {
-				p.gen('[/*ptr*/')
+				p.gen('[/*ptr!*/')
 				close_bracket = true
 			}
 		}