parser: allow passing `mut a AliasOfPointerType`

vlib/v/parser/fn.v

@@ -824,13 +824,21 @@ fn (mut p Parser) fn_args() ([]table.Param, bool, bool) {
 
 fn (mut p Parser) check_fn_mutable_arguments(typ table.Type, pos token.Position) {
 	sym := p.table.get_type_symbol(typ)
-	if sym.kind !in [.array, .array_fixed, .interface_, .map, .placeholder, .struct_, .sum_type]
+	if sym.kind in [.array, .array_fixed, .interface_, .map, .placeholder, .struct_, .sum_type] {
-		&& !typ.is_ptr() && !typ.is_pointer() {
+		return
-		p.error_with_pos(
-			'mutable arguments are only allowed for arrays, interfaces, maps, pointers and structs\n' +
-			'return values instead: `fn foo(mut n $sym.name) {` => `fn foo(n $sym.name) $sym.name {`',
-			pos)
 	}
+	if typ.is_ptr() || typ.is_pointer() {
+		return
+	}
+	if sym.kind == .alias {
+		atyp := (sym.info as table.Alias).parent_type
+		p.check_fn_mutable_arguments(atyp, pos)
+		return
+	}
+	p.error_with_pos(
+		'mutable arguments are only allowed for arrays, interfaces, maps, pointers, structs or their aliases\n' +
+		'return values instead: `fn foo(mut n $sym.name) {` => `fn foo(n $sym.name) $sym.name {`',
+		pos)
 }
 
 fn (mut p Parser) check_fn_shared_arguments(typ table.Type, pos token.Position) {

vlib/v/tests/type_alias_of_pointer_types_test.v

@@ -37,3 +37,38 @@ fn test_alias_of_pointer_types() {
 	assert sizeof(PPZZInt) == sizeof(voidptr)
 	assert sizeof(PPZZMyStructInt) == sizeof(voidptr)
 }
+
+fn test_calling_a_function_expecting_a_mut_alias() {
+	eprintln('------------------------')
+	mut s := &MyStructInt{456}
+	mut ps := PZZMyStructInt(s)
+	dump(voidptr(s))
+	dump(voidptr(ps))
+	eprintln('------------------------')
+	dump(&MyStructInt(ps))
+	res := mut_alias(mut ps)
+	dump(&MyStructInt(ps))
+	// the alias `ps` is now changed and points to another object
+	assert res == 123
+	assert s.x == 456 // should remain the same
+	assert (&MyStructInt(ps)).x == 789
+	assert u64(voidptr(s)) != u64(voidptr(ps))
+	dump(voidptr(s))
+	dump(voidptr(ps))
+	eprintln('------------------------')
+}
+
+// do not delete this, its generated code eases comparisons with mut_alias
+fn mut_struct(mut p ZZMyStructInt) int {
+	dump(ptr_str(voidptr(p)))
+	return 999
+}
+
+fn mut_alias(mut ps PZZMyStructInt) int {
+	//	dump(ptr_str(voidptr(ps)))
+	another := &MyStructInt{789}
+	//	dump(ptr_str(voidptr(another)))
+	ps = PZZMyStructInt(another)
+	//	dump(ptr_str(voidptr(ps)))
+	return 123
+}