checker: fix fn returning alias of pointer (fix #17861) (#17864)

Author: yuyi98

vlib/v/checker/return.v

@@ -229,8 +229,9 @@ fn (mut c Checker) return_stmt(mut node ast.Return) {
 			c.error('fn `${c.table.cur_fn.name}` expects you to return a non reference type `${c.table.type_to_str(exp_type)}`, but you are returning `${c.table.type_to_str(got_typ)}` instead',
 				pos)
 		}
-		if (exp_type.is_ptr() || exp_type.is_pointer())
-			&& (!got_typ.is_ptr() && !got_typ.is_pointer()) && got_typ != ast.int_literal_type
+		unaliased_got_typ := c.table.unaliased_type(got_typ)
+		if (exp_type.is_ptr() || exp_type.is_pointer()) && !got_typ.is_real_pointer()
+			&& !unaliased_got_typ.is_real_pointer() && got_typ != ast.int_literal_type
 			&& !c.pref.translated && !c.file.is_translated {
 			pos := node.exprs[expr_idxs[i]].pos()
 			if node.exprs[expr_idxs[i]].is_auto_deref_var() {

vlib/v/tests/fn_return_alias_of_ptr_test.v

@@ -0,0 +1,16 @@
+type HANDLE = voidptr
+
+struct ExampleStruct {
+	handle HANDLE
+}
+
+fn get_ptr(arg ExampleStruct) voidptr {
+	return arg.handle
+}
+
+fn test_fn_return_alias_of_ptr() {
+	h := ExampleStruct{}
+	r := get_ptr(h)
+	println(r)
+	assert r == unsafe { nil }
+}