diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 5cdf7c7293..6c09926407 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -786,9 +786,9 @@ pub fn (mut c Checker) infix_expr(mut infix_expr ast.InfixExpr) ast.Type {
 			}
 		}
 		.key_in, .not_in {
-			match right.kind {
+			match right_final.kind {
 				.array {
-					elem_type := right.array_info().elem_type
+					elem_type := right_final.array_info().elem_type
 					// if left_default.kind != right_sym.kind {
 					c.check_expected(left_type, elem_type) or {
 						c.error('left operand to `$infix_expr.op` does not match the array element type: $err.msg',
@@ -796,7 +796,7 @@ pub fn (mut c Checker) infix_expr(mut infix_expr ast.InfixExpr) ast.Type {
 					}
 				}
 				.map {
-					map_info := right.map_info()
+					map_info := right_final.map_info()
 					c.check_expected(left_type, map_info.key_type) or {
 						c.error('left operand to `$infix_expr.op` does not match the map key type: $err.msg',
 							left_right_pos)
diff --git a/vlib/v/gen/c/array.v b/vlib/v/gen/c/array.v
index 9f1d2e8353..3fb1d519d8 100644
--- a/vlib/v/gen/c/array.v
+++ b/vlib/v/gen/c/array.v
@@ -454,10 +454,11 @@ fn (mut g Gen) gen_array_prepend(node ast.CallExpr) {
 
 fn (mut g Gen) gen_array_contains_method(left_type ast.Type) string {
 	mut left_sym := g.table.get_type_symbol(left_type)
+	left_final_sym := g.table.get_final_type_symbol(left_type)
 	mut left_type_str := g.typ(left_type).replace('*', '')
 	fn_name := '${left_type_str}_contains'
 	if !left_sym.has_method('contains') {
-		left_info := left_sym.info as ast.Array
+		left_info := left_final_sym.info as ast.Array
 		mut elem_type_str := g.typ(left_info.elem_type)
 		elem_sym := g.table.get_type_symbol(left_info.elem_type)
 		if elem_sym.kind == .function {
diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index d3badbda31..8fbac4be7b 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -3640,7 +3640,7 @@ fn (mut g Gen) infix_expr(node ast.InfixExpr) {
 		&& left_sym.kind in [.array, .array_fixed, .alias, .map, .struct_, .sum_type] {
 		g.infix_gen_equality(node, left_type, left_sym, right_sym)
 	} else if op_is_key_in_or_not_in {
-		g.infix_in_or_not_in(node, left_sym, right_sym)
+		g.infix_in_or_not_in(node, left_final_sym, right_final_sym)
 	} else if node.op == .left_shift && left_final_sym.kind == .array {
 		// arr << val
 		tmp := g.new_tmp_var()
diff --git a/vlib/v/tests/in_expression_test.v b/vlib/v/tests/in_expression_test.v
index 9a4d363087..356fed7f46 100644
--- a/vlib/v/tests/in_expression_test.v
+++ b/vlib/v/tests/in_expression_test.v
@@ -107,6 +107,23 @@ fn test_in_expression_with_string() {
 	assert a == false
 }
 
+type MapAlias = map[string]int
+type ArrayAlias = []int
+
+fn test_in_expression_in_alias() {
+	arr := ArrayAlias([0, 1])
+	assert 0 in arr
+	assert 100 !in arr
+
+	m := MapAlias(map{
+		'one':   1
+		'two':   2
+		'three': 3
+	})
+	assert 'one' in m
+	assert 'four' !in m
+}
+
 fn test_in_expression_in_map() {
 	m := map{
 		'one':   1