parser: fix anonymous fns parameter checks, behaving differently than named fns (fix #18779) (#18785)

vlib/v/parser/fn.v

@@ -764,6 +764,7 @@ fn (mut p Parser) anon_fn() ast.AnonFn {
 			name: arg.name
 			typ: arg.typ
 			is_mut: arg.is_mut
+			is_auto_deref: arg.is_mut || arg.is_auto_rec
 			pos: arg.pos
 			is_used: true
 			is_arg: true

vlib/v/tests/anon_fn_test.v

@@ -21,3 +21,23 @@ fn test_anon_assign_struct() {
 	}
 	assert w.fn_()
 }
+
+//
+
+fn fnormal(mut acc []string, e int) []string {
+	acc << e.str()
+	return acc
+}
+
+fn test_anon_fn_returning_a_mut_parameter_should_act_the_same_as_normal_fn_returning_a_mut_parameter() {
+	fanon := fn (mut acc []string, e int) []string {
+		acc << e.str()
+		return acc
+	}
+	assert '${fanon}' == '${fnormal}'
+	mut a := ['a', 'b', 'c']
+	mut b := a.clone()
+	x := fanon(mut a, 123)
+	y := fnormal(mut b, 123)
+	assert a == b
+}